This is gdb.info, produced by makeinfo version 4.7 from ./gdb.texinfo. INFO-DIR-SECTION Software development START-INFO-DIR-ENTRY * Gdb: (gdb). The GNU debugger. END-INFO-DIR-ENTRY This file documents the GNU debugger GDB. This is the Ninth Edition, of `Debugging with GDB: the GNU Source-Level Debugger' for GDB Version 6.3. Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being "Free Software" and "Free Software Needs Free Documentation", with the Front-Cover Texts being "A GNU Manual," and with the Back-Cover Texts as in (a) below. (a) The Free Software Foundation's Back-Cover Text is: "You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development."  File: gdb.info, Node: Top, Next: Summary, Prev: (dir), Up: (dir) Debugging with GDB ****************** This file describes GDB, the GNU symbolic debugger. This is the Ninth Edition, for GDB Version 6.3. Copyright (C) 1988-2004 Free Software Foundation, Inc. * Menu: * Summary:: Summary of GDB * Sample Session:: A sample GDB session * Invocation:: Getting in and out of GDB * Commands:: GDB commands * Running:: Running programs under GDB * Stopping:: Stopping and continuing * Stack:: Examining the stack * Source:: Examining source files * Data:: Examining data * Macros:: Preprocessor Macros * Tracepoints:: Debugging remote targets non-intrusively * Overlays:: Debugging programs that use overlays * Languages:: Using GDB with different languages * Symbols:: Examining the symbol table * Altering:: Altering execution * GDB Files:: GDB files * Targets:: Specifying a debugging target * Remote Debugging:: Debugging remote programs * Configurations:: Configuration-specific information * Controlling GDB:: Controlling GDB * Sequences:: Canned sequences of commands * TUI:: GDB Text User Interface * Interpreters:: Command Interpreters * Emacs:: Using GDB under GNU Emacs * Annotations:: GDB's annotation interface. * GDB/MI:: GDB's Machine Interface. * GDB Bugs:: Reporting bugs in GDB * Formatting Documentation:: How to format and print GDB documentation * Command Line Editing:: Command Line Editing * Using History Interactively:: Using History Interactively * Installing GDB:: Installing GDB * Maintenance Commands:: Maintenance Commands * Remote Protocol:: GDB Remote Serial Protocol * Agent Expressions:: The GDB Agent Expression Mechanism * Copying:: GNU General Public License says how you can copy and share GDB * GNU Free Documentation License:: The license for this documentation * Index:: Index  File: gdb.info, Node: Summary, Next: Sample Session, Prev: Top, Up: Top Summary of GDB ************** The purpose of a debugger such as GDB is to allow you to see what is going on "inside" another program while it executes--or what another program was doing at the moment it crashed. GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act: * Start your program, specifying anything that might affect its behavior. * Make your program stop on specified conditions. * Examine what has happened, when your program has stopped. * Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another. You can use GDB to debug programs written in C and C++. For more information, see *Note Supported languages: Support. For more information, see *Note C and C++: C. Support for Modula-2 is partial. For information on Modula-2, see *Note Modula-2: Modula-2. Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. GDB does not support entering expressions, printing values, or similar features using Pascal syntax. GDB can be used to debug programs written in Fortran, although it may be necessary to refer to some variables with a trailing underscore. GDB can be used to debug programs written in Objective-C, using either the Apple/NeXT or the GNU Objective-C runtime. * Menu: * Free Software:: Freely redistributable software * Contributors:: Contributors to GDB  File: gdb.info, Node: Free Software, Next: Contributors, Up: Summary Free software ============= GDB is "free software", protected by the GNU General Public License (GPL). The GPL gives you the freedom to copy or adapt a licensed program--but every person getting a copy also gets with it the freedom to modify that copy (which means that they must get access to the source code), and the freedom to distribute further copies. Typical software companies use copyrights to limit your freedoms; the Free Software Foundation uses the GPL to preserve these freedoms. Fundamentally, the General Public License is a license which says that you have these freedoms and that you cannot take these freedoms away from anyone else. Free Software Needs Free Documentation ====================================== The biggest deficiency in the free software community today is not in the software--it is the lack of good free documentation that we can include with the free software. Many of our most important programs do not come with free reference manuals and free introductory texts. Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap. We have many such gaps today. Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this come about? Because the authors of those manuals published them with restrictive terms--no copying, no modification, source files not available--which exclude them from the free software world. That wasn't the first time this sort of thing happened, and it was far from the last. Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free. Free documentation, like free software, is a matter of freedom, not price. The problem with the non-free manual is not that publishers charge a price for printed copies--that in itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use of the manual. Free manuals are available in source code form, and give you permission to copy and modify. Non-free manuals do not allow this. The criteria of freedom for a free manual are roughly the same as for free software. Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper. Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too--so they can provide accurate and clear documentation for the modified program. A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community. Some kinds of limits on the way modification is handled are acceptable. For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified. Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual. However, it must be possible to modify all the _technical_ content of the manual, and then distribute the result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it. Please spread the word about this issue. Our community continues to lose manuals to proprietary publishing. If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community. If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license. Remember that this decision requires your approval--you don't have to let the publisher decide. Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want. If the publisher you are dealing with refuses, please try other publishers. If you're not sure whether a proposed license is free, write to . You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try to reward the publishers that have paid or pay the authors to work on it. The Free Software Foundation maintains a list of free documentation published by other publishers, at `http://www.fsf.org/doc/other-free-books.html'.  File: gdb.info, Node: Contributors, Prev: Free Software, Up: Summary Contributors to GDB =================== Richard Stallman was the original author of GDB, and of many other GNU programs. Many others have contributed to its development. This section attempts to credit major contributors. One of the virtues of free software is that everyone is free to contribute to it; with regret, we cannot actually acknowledge everyone here. The file `ChangeLog' in the GDB distribution approximates a blow-by-blow account. Changes much prior to version 2.0 are lost in the mists of time. _Plea:_ Additions to this section are particularly welcome. If you or your friends (or enemies, to be evenhanded) have been unfairly omitted from this list, we would like to add your names! So that they may not regard their many labors as thankless, we particularly thank those who shepherded GDB through major releases: Andrew Cagney (releases 6.1, 6.0, 5.3, 5.2, 5.1 and 5.0); Jim Blandy (release 4.18); Jason Molenda (release 4.17); Stan Shebs (release 4.14); Fred Fish (releases 4.16, 4.15, 4.13, 4.12, 4.11, 4.10, and 4.9); Stu Grossman and John Gilmore (releases 4.8, 4.7, 4.6, 4.5, and 4.4); John Gilmore (releases 4.3, 4.2, 4.1, 4.0, and 3.9); Jim Kingdon (releases 3.5, 3.4, and 3.3); and Randy Smith (releases 3.2, 3.1, and 3.0). Richard Stallman, assisted at various times by Peter TerMaat, Chris Hanson, and Richard Mlynarik, handled releases through 2.8. Michael Tiemann is the author of most of the GNU C++ support in GDB, with significant additional contributions from Per Bothner and Daniel Berlin. James Clark wrote the GNU C++ demangler. Early work on C++ was by Peter TerMaat (who also did much general update work leading to release 3.0). GDB uses the BFD subroutine library to examine multiple object-file formats; BFD was a joint project of David V. Henkel-Wallace, Rich Pixley, Steve Chamberlain, and John Gilmore. David Johnson wrote the original COFF support; Pace Willison did the original support for encapsulated COFF. Brent Benson of Harris Computer Systems contributed DWARF 2 support. Adam de Boor and Bradley Davis contributed the ISI Optimum V support. Per Bothner, Noboyuki Hikichi, and Alessandro Forin contributed MIPS support. Jean-Daniel Fekete contributed Sun 386i support. Chris Hanson improved the HP9000 support. Noboyuki Hikichi and Tomoyuki Hasei contributed Sony/News OS 3 support. David Johnson contributed Encore Umax support. Jyrki Kuoppala contributed Altos 3068 support. Jeff Law contributed HP PA and SOM support. Keith Packard contributed NS32K support. Doug Rabson contributed Acorn Risc Machine support. Bob Rusk contributed Harris Nighthawk CX-UX support. Chris Smith contributed Convex support (and Fortran debugging). Jonathan Stone contributed Pyramid support. Michael Tiemann contributed SPARC support. Tim Tucker contributed support for the Gould NP1 and Gould Powernode. Pace Willison contributed Intel 386 support. Jay Vosburgh contributed Symmetry support. Marko Mlinar contributed OpenRISC 1000 support. Andreas Schwab contributed M68K GNU/Linux support. Rich Schaefer and Peter Schauer helped with support of SunOS shared libraries. Jay Fenlason and Roland McGrath ensured that GDB and GAS agree about several machine instruction sets. Patrick Duval, Ted Goldstein, Vikram Koka and Glenn Engel helped develop remote debugging. Intel Corporation, Wind River Systems, AMD, and ARM contributed remote debugging modules for the i960, VxWorks, A29K UDI, and RDI targets, respectively. Brian Fox is the author of the readline libraries providing command-line editing and command history. Andrew Beers of SUNY Buffalo wrote the language-switching code, the Modula-2 support, and contributed the Languages chapter of this manual. Fred Fish wrote most of the support for Unix System Vr4. He also enhanced the command-completion support to cover C++ overloaded symbols. Hitachi America (now Renesas America), Ltd. sponsored the support for H8/300, H8/500, and Super-H processors. NEC sponsored the support for the v850, Vr4xxx, and Vr5xxx processors. Mitsubishi (now Renesas) sponsored the support for D10V, D30V, and M32R/D processors. Toshiba sponsored the support for the TX39 Mips processor. Matsushita sponsored the support for the MN10200 and MN10300 processors. Fujitsu sponsored the support for SPARClite and FR30 processors. Kung Hsu, Jeff Law, and Rick Sladkey added support for hardware watchpoints. Michael Snyder added support for tracepoints. Stu Grossman wrote gdbserver. Jim Kingdon, Peter Schauer, Ian Taylor, and Stu Grossman made nearly innumerable bug fixes and cleanups throughout GDB. The following people at the Hewlett-Packard Company contributed support for the PA-RISC 2.0 architecture, HP-UX 10.20, 10.30, and 11.0 (narrow mode), HP's implementation of kernel threads, HP's aC++ compiler, and the Text User Interface (nee Terminal User Interface): Ben Krepp, Richard Title, John Bishop, Susan Macchia, Kathy Mann, Satish Pai, India Paul, Steve Rehrauer, and Elena Zannoni. Kim Haase provided HP-specific information in this manual. DJ Delorie ported GDB to MS-DOS, for the DJGPP project. Robert Hoehne made significant contributions to the DJGPP port. Cygnus Solutions has sponsored GDB maintenance and much of its development since 1991. Cygnus engineers who have worked on GDB fulltime include Mark Alexander, Jim Blandy, Per Bothner, Kevin Buettner, Edith Epstein, Chris Faylor, Fred Fish, Martin Hunt, Jim Ingham, John Gilmore, Stu Grossman, Kung Hsu, Jim Kingdon, John Metzler, Fernando Nasser, Geoffrey Noer, Dawn Perchik, Rich Pixley, Zdenek Radouch, Keith Seitz, Stan Shebs, David Taylor, and Elena Zannoni. In addition, Dave Brolley, Ian Carmichael, Steve Chamberlain, Nick Clifton, JT Conklin, Stan Cox, DJ Delorie, Ulrich Drepper, Frank Eigler, Doug Evans, Sean Fagan, David Henkel-Wallace, Richard Henderson, Jeff Holcomb, Jeff Law, Jim Lemke, Tom Lord, Bob Manson, Michael Meissner, Jason Merrill, Catherine Moore, Drew Moseley, Ken Raeburn, Gavin Romig-Koch, Rob Savoye, Jamie Smith, Mike Stump, Ian Taylor, Angela Thomas, Michael Tiemann, Tom Tromey, Ron Unrau, Jim Wilson, and David Zuhn have made contributions both large and small. Jim Blandy added support for preprocessor macros, while working for Red Hat.  File: gdb.info, Node: Sample Session, Next: Invocation, Prev: Summary, Up: Top 1 A Sample GDB Session ********************** You can use this manual at your leisure to read all about GDB. However, a handful of commands are enough to get started using the debugger. This chapter illustrates those commands. One of the preliminary versions of GNU `m4' (a generic macro processor) exhibits the following bug: sometimes, when we change its quote strings from the default, the commands used to capture one macro definition within another stop working. In the following short `m4' session, we define a macro `foo' which expands to `0000'; we then use the `m4' built-in `defn' to define `bar' as the same thing. However, when we change the open quote string to `' and the close quote string to `', the same procedure fails to define a new synonym `baz': $ cd gnu/m4 $ ./m4 define(foo,0000) foo 0000 define(bar,defn(`foo')) bar 0000 changequote(,) define(baz,defn(foo)) baz C-d m4: End of input: 0: fatal error: EOF in string Let us use GDB to try to see what is going on. $ gdb m4 GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is absolutely no warranty for GDB; type "show warranty" for details. GDB 6.3, Copyright 1999 Free Software Foundation, Inc... (gdb) GDB reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly. We now tell GDB to use a narrower display width than usual, so that examples fit in this manual. (gdb) set width 70 We need to see how the `m4' built-in `changequote' works. Having looked at the source, we know the relevant subroutine is `m4_changequote', so we set a breakpoint there with the GDB `break' command. (gdb) break m4_changequote Breakpoint 1 at 0x62f4: file builtin.c, line 879. Using the `run' command, we start `m4' running under GDB control; as long as control does not reach the `m4_changequote' subroutine, the program runs as usual: (gdb) run Starting program: /work/Editorial/gdb/gnu/m4/m4 define(foo,0000) foo 0000 To trigger the breakpoint, we call `changequote'. GDB suspends execution of `m4', displaying information about the context where it stops. changequote(,) Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) at builtin.c:879 879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3)) Now we use the command `n' (`next') to advance execution to the next line of the current function. (gdb) n 882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\ : nil, `set_quotes' looks like a promising subroutine. We can go into it by using the command `s' (`step') instead of `next'. `step' goes to the next line to be executed in _any_ subroutine, so it steps into `set_quotes'. (gdb) s set_quotes (lq=0x34c78 "", rq=0x34c88 "") at input.c:530 530 if (lquote != def_lquote) The display that shows the subroutine where `m4' is now suspended (and its arguments) is called a stack frame display. It shows a summary of the stack. We can use the `backtrace' command (which can also be spelled `bt'), to see where we are in the stack as a whole: the `backtrace' command displays a stack frame for each active subroutine. (gdb) bt #0 set_quotes (lq=0x34c78 "", rq=0x34c88 "") at input.c:530 #1 0x6344 in m4_changequote (argc=3, argv=0x33c70) at builtin.c:882 #2 0x8174 in expand_macro (sym=0x33320) at macro.c:242 #3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30) at macro.c:71 #4 0x79dc in expand_input () at macro.c:40 #5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195 We step through a few more lines to see what happens. The first two times, we can use `s'; the next two times we use `n' to avoid falling into the `xstrdup' subroutine. (gdb) s 0x3b5c 532 if (rquote != def_rquote) (gdb) s 0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \ def_lquote : xstrdup(lq); (gdb) n 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup(rq); (gdb) n 538 len_lquote = strlen(rquote); The last line displayed looks a little odd; we can examine the variables `lquote' and `rquote' to see if they are in fact the new left and right quotes we specified. We use the command `p' (`print') to see their values. (gdb) p lquote $1 = 0x35d40 "" (gdb) p rquote $2 = 0x35d50 "" `lquote' and `rquote' are indeed the new left and right quotes. To look at some context, we can display ten lines of source surrounding the current line with the `l' (`list') command. (gdb) l 533 xfree(rquote); 534 535 lquote = (lq == nil || *lq == '\0') ? def_lquote\ : xstrdup (lq); 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup (rq); 537 538 len_lquote = strlen(rquote); 539 len_rquote = strlen(lquote); 540 } 541 542 void Let us step past the two lines that set `len_lquote' and `len_rquote', and then examine the values of those variables. (gdb) n 539 len_rquote = strlen(lquote); (gdb) n 540 } (gdb) p len_lquote $3 = 9 (gdb) p len_rquote $4 = 7 That certainly looks wrong, assuming `len_lquote' and `len_rquote' are meant to be the lengths of `lquote' and `rquote' respectively. We can set them to better values using the `p' command, since it can print the value of any expression--and that expression can include subroutine calls and assignments. (gdb) p len_lquote=strlen(lquote) $5 = 7 (gdb) p len_rquote=strlen(rquote) $6 = 9 Is that enough to fix the problem of using the new quotes with the `m4' built-in `defn'? We can allow `m4' to continue executing with the `c' (`continue') command, and then try the example that caused trouble initially: (gdb) c Continuing. define(baz,defn(foo)) baz 0000 Success! The new quotes now work just as well as the default ones. The problem seems to have been just the two typos defining the wrong lengths. We allow `m4' exit by giving it an EOF as input: C-d Program exited normally. The message `Program exited normally.' is from GDB; it indicates `m4' has finished executing. We can end our GDB session with the GDB `quit' command. (gdb) quit  File: gdb.info, Node: Invocation, Next: Commands, Prev: Sample Session, Up: Top 2 Getting In and Out of GDB *************************** This chapter discusses how to start GDB, and how to get out of it. The essentials are: * type `gdb' to start GDB. * type `quit' or `C-d' to exit. * Menu: * Invoking GDB:: How to start GDB * Quitting GDB:: How to quit GDB * Shell Commands:: How to use shell commands inside GDB * Logging output:: How to log GDB's output to a file  File: gdb.info, Node: Invoking GDB, Next: Quitting GDB, Up: Invocation 2.1 Invoking GDB ================ Invoke GDB by running the program `gdb'. Once started, GDB reads commands from the terminal until you tell it to exit. You can also run `gdb' with a variety of arguments and options, to specify more of your debugging environment at the outset. The command-line options described here are designed to cover a variety of situations; in some environments, some of these options may effectively be unavailable. The most usual way to start GDB is with one argument, specifying an executable program: gdb PROGRAM You can also start with both an executable program and a core file specified: gdb PROGRAM CORE You can, instead, specify a process ID as a second argument, if you want to debug a running process: gdb PROGRAM 1234 would attach GDB to process `1234' (unless you also have a file named `1234'; GDB does check for a core file first). Taking advantage of the second command-line argument requires a fairly complete operating system; when you use GDB as a remote debugger attached to a bare board, there may not be any notion of "process", and there is often no way to get a core dump. GDB will warn you if it is unable to attach or to read core dumps. You can optionally have `gdb' pass any arguments after the executable file to the inferior using `--args'. This option stops option processing. gdb --args gcc -O2 -c foo.c This will cause `gdb' to debug `gcc', and to set `gcc''s command-line arguments (*note Arguments::) to `-O2 -c foo.c'. You can run `gdb' without printing the front material, which describes GDB's non-warranty, by specifying `-silent': gdb -silent You can further control how GDB starts up by using command-line options. GDB itself can remind you of the options available. Type gdb -help to display all available options and briefly describe their use (`gdb -h' is a shorter equivalent). All options and command line arguments you give are processed in sequential order. The order makes a difference when the `-x' option is used. * Menu: * File Options:: Choosing files * Mode Options:: Choosing modes  File: gdb.info, Node: File Options, Next: Mode Options, Up: Invoking GDB 2.1.1 Choosing files -------------------- When GDB starts, it reads any arguments other than options as specifying an executable file and core file (or process ID). This is the same as if the arguments were specified by the `-se' and `-c' (or `-p' options respectively. (GDB reads the first argument that does not have an associated option flag as equivalent to the `-se' option followed by that argument; and the second argument that does not have an associated option flag, if any, as equivalent to the `-c'/`-p' option followed by that argument.) If the second argument begins with a decimal digit, GDB will first attempt to attach to it as a process, and if that fails, attempt to open it as a corefile. If you have a corefile whose name begins with a digit, you can prevent GDB from treating it as a pid by prefixing it with `./', eg. `./12345'. If GDB has not been configured to included core file support, such as for most embedded targets, then it will complain about a second argument and ignore it. Many options have both long and short forms; both are shown in the following list. GDB also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with `--' rather than `-', though we illustrate the more usual convention.) `-symbols FILE' `-s FILE' Read symbol table from file FILE. `-exec FILE' `-e FILE' Use file FILE as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump. `-se FILE' Read symbol table from file FILE and use it as the executable file. `-core FILE' `-c FILE' Use file FILE as a core dump to examine. `-c NUMBER' `-pid NUMBER' `-p NUMBER' Connect to process ID NUMBER, as with the `attach' command. If there is no such process, GDB will attempt to open a core file named NUMBER. `-command FILE' `-x FILE' Execute GDB commands from file FILE. *Note Command files: Command Files. `-directory DIRECTORY' `-d DIRECTORY' Add DIRECTORY to the path to search for source files. `-m' `-mapped' _Warning: this option depends on operating system facilities that are not supported on all systems._ If memory-mapped files are available on your system through the `mmap' system call, you can use this option to have GDB write the symbols from your program into a reusable file in the current directory. If the program you are debugging is called `/tmp/fred', the mapped symbol file is `/tmp/fred.syms'. Future GDB debugging sessions notice the presence of this file, and can quickly map in symbol information from it, rather than reading the symbol table from the executable program. The `.syms' file is specific to the host machine where GDB is run. It holds an exact image of the internal GDB symbol table. It cannot be shared across multiple host platforms. `-r' `-readnow' Read each symbol file's entire symbol table immediately, rather than the default, which is to read it incrementally as it is needed. This makes startup slower, but makes future operations faster. You typically combine the `-mapped' and `-readnow' options in order to build a `.syms' file that contains complete symbol information. (*Note Commands to specify files: Files, for information on `.syms' files.) A simple GDB invocation to do nothing but build a `.syms' file for future use is: gdb -batch -nx -mapped -readnow programname  File: gdb.info, Node: Mode Options, Prev: File Options, Up: Invoking GDB 2.1.2 Choosing modes -------------------- You can run GDB in various alternative modes--for example, in batch mode or quiet mode. `-nx' `-n' Do not execute commands found in any initialization files. Normally, GDB executes the commands in these files after all the command options and arguments have been processed. *Note Command files: Command Files. `-quiet' `-silent' `-q' "Quiet". Do not print the introductory and copyright messages. These messages are also suppressed in batch mode. `-batch' Run in batch mode. Exit with status `0' after processing all the command files specified with `-x' (and all commands from initialization files, if not inhibited with `-n'). Exit with nonzero status if an error occurs in executing the GDB commands in the command files. Batch mode may be useful for running GDB as a filter, for example to download and run a program on another computer; in order to make this more useful, the message Program exited normally. (which is ordinarily issued whenever a program running under GDB control terminates) is not issued when running in batch mode. `-nowindows' `-nw' "No windows". If GDB comes with a graphical user interface (GUI) built in, then this option tells GDB to only use the command-line interface. If no GUI is available, this option has no effect. `-windows' `-w' If GDB includes a GUI, then this option requires it to be used if possible. `-cd DIRECTORY' Run GDB using DIRECTORY as its working directory, instead of the current directory. `-fullname' `-f' GNU Emacs sets this option when it runs GDB as a subprocess. It tells GDB to output the full file name and line number in a standard, recognizable fashion each time a stack frame is displayed (which includes each time your program stops). This recognizable format looks like two `\032' characters, followed by the file name, line number and character position separated by colons, and a newline. The Emacs-to-GDB interface program uses the two `\032' characters as a signal to display the source code for the frame. `-epoch' The Epoch Emacs-GDB interface sets this option when it runs GDB as a subprocess. It tells GDB to modify its print routines so as to allow Epoch to display values of expressions in a separate window. `-annotate LEVEL' This option sets the "annotation level" inside GDB. Its effect is identical to using `set annotate LEVEL' (*note Annotations::). The annotation LEVEL controls how much information GDB prints together with its prompt, values of expressions, source lines, and other types of output. Level 0 is the normal, level 1 is for use when GDB is run as a subprocess of GNU Emacs, level 3 is the maximum annotation suitable for programs that control GDB, and level 2 has been deprecated. The annotation mechanism has largely been superseeded by GDB/MI (*note GDB/MI::). `--args' Change interpretation of command line so that arguments following the executable file are passed as command line arguments to the inferior. This option stops option processing. `-baud BPS' `-b BPS' Set the line speed (baud rate or bits per second) of any serial interface used by GDB for remote debugging. `-tty DEVICE' `-t DEVICE' Run using DEVICE for your program's standard input and output. `-tui' Activate the "Text User Interface" when starting. The Text User Interface manages several text windows on the terminal, showing source, assembly, registers and GDB command outputs (*note GDB Text User Interface: TUI.). Alternatively, the Text User Interface can be enabled by invoking the program `gdbtui'. Do not use this option if you run GDB from Emacs (*note Using GDB under GNU Emacs: Emacs.). `-interpreter INTERP' Use the interpreter INTERP for interface with the controlling program or device. This option is meant to be set by programs which communicate with GDB using it as a back end. *Note Command Interpreters: Interpreters. `--interpreter=mi' (or `--interpreter=mi2') causes GDB to use the "GDB/MI interface" (*note The GDB/MI Interface: GDB/MI.) included since GDBN version 6.0. The previous GDB/MI interface, included in GDB version 5.3 and selected with `--interpreter=mi1', is deprecated. Earlier GDB/MI interfaces are no longer supported. `-write' Open the executable and core files for both reading and writing. This is equivalent to the `set write on' command inside GDB (*note Patching::). `-statistics' This option causes GDB to print statistics about time and memory usage after it completes each command and returns to the prompt. `-version' This option causes GDB to print its version number and no-warranty blurb, and exit.  File: gdb.info, Node: Quitting GDB, Next: Shell Commands, Prev: Invoking GDB, Up: Invocation 2.2 Quitting GDB ================ `quit [EXPRESSION]' `q' To exit GDB, use the `quit' command (abbreviated `q'), or type an end-of-file character (usually `C-d'). If you do not supply EXPRESSION, GDB will terminate normally; otherwise it will terminate using the result of EXPRESSION as the error code. An interrupt (often `C-c') does not exit from GDB, but rather terminates the action of any GDB command that is in progress and returns to GDB command level. It is safe to type the interrupt character at any time because GDB does not allow it to take effect until a time when it is safe. If you have been using GDB to control an attached process or device, you can release it with the `detach' command (*note Debugging an already-running process: Attach.).  File: gdb.info, Node: Shell Commands, Next: Logging output, Prev: Quitting GDB, Up: Invocation 2.3 Shell commands ================== If you need to execute occasional shell commands during your debugging session, there is no need to leave or suspend GDB; you can just use the `shell' command. `shell COMMAND STRING' Invoke a standard shell to execute COMMAND STRING. If it exists, the environment variable `SHELL' determines which shell to run. Otherwise GDB uses the default shell (`/bin/sh' on Unix systems, `COMMAND.COM' on MS-DOS, etc.). The utility `make' is often needed in development environments. You do not have to use the `shell' command for this purpose in GDB: `make MAKE-ARGS' Execute the `make' program with the specified arguments. This is equivalent to `shell make MAKE-ARGS'.  File: gdb.info, Node: Logging output, Prev: Shell Commands, Up: Invocation 2.4 Logging output ================== You may want to save the output of GDB commands to a file. There are several commands to control GDB's logging. `set logging on' Enable logging. `set logging off' Disable logging. `set logging file FILE' Change the name of the current logfile. The default logfile is `gdb.txt'. `set logging overwrite [on|off]' By default, GDB will append to the logfile. Set `overwrite' if you want `set logging on' to overwrite the logfile instead. `set logging redirect [on|off]' By default, GDB output will go to both the terminal and the logfile. Set `redirect' if you want output to go only to the log file. `show logging' Show the current values of the logging settings.  File: gdb.info, Node: Commands, Next: Running, Prev: Invocation, Up: Top 3 GDB Commands ************** You can abbreviate a GDB command to the first few letters of the command name, if that abbreviation is unambiguous; and you can repeat certain GDB commands by typing just . You can also use the key to get GDB to fill out the rest of a word in a command (or to show you the alternatives available, if there is more than one possibility). * Menu: * Command Syntax:: How to give commands to GDB * Completion:: Command completion * Help:: How to ask GDB for help  File: gdb.info, Node: Command Syntax, Next: Completion, Up: Commands 3.1 Command syntax ================== A GDB command is a single line of input. There is no limit on how long it can be. It starts with a command name, which is followed by arguments whose meaning depends on the command name. For example, the command `step' accepts an argument which is the number of times to step, as in `step 5'. You can also use the `step' command with no arguments. Some commands do not allow any arguments. GDB command names may always be truncated if that abbreviation is unambiguous. Other possible command abbreviations are listed in the documentation for individual commands. In some cases, even ambiguous abbreviations are allowed; for example, `s' is specially defined as equivalent to `step' even though there are other commands whose names start with `s'. You can test abbreviations by using them as arguments to the `help' command. A blank line as input to GDB (typing just ) means to repeat the previous command. Certain commands (for example, `run') will not repeat this way; these are commands whose unintentional repetition might cause trouble and which you are unlikely to want to repeat. The `list' and `x' commands, when you repeat them with , construct new arguments rather than repeating exactly as typed. This permits easy scanning of source or memory. GDB can also use in another way: to partition lengthy output, in a way similar to the common utility `more' (*note Screen size: Screen Size.). Since it is easy to press one too many in this situation, GDB disables command repetition after any command that generates this sort of display. Any text from a `#' to the end of the line is a comment; it does nothing. This is useful mainly in command files (*note Command files: Command Files.). The `C-o' binding is useful for repeating a complex sequence of commands. This command accepts the current line, like `RET', and then fetches the next line relative to the current line from the history for editing.  File: gdb.info, Node: Completion, Next: Help, Prev: Command Syntax, Up: Commands 3.2 Command completion ====================== GDB can fill in the rest of a word in a command for you, if there is only one possibility; it can also show you what the valid possibilities are for the next word in a command, at any time. This works for GDB commands, GDB subcommands, and the names of symbols in your program. Press the key whenever you want GDB to fill out the rest of a word. If there is only one possibility, GDB fills in the word, and waits for you to finish the command (or press to enter it). For example, if you type (gdb) info bre GDB fills in the rest of the word `breakpoints', since that is the only `info' subcommand beginning with `bre': (gdb) info breakpoints You can either press at this point, to run the `info breakpoints' command, or backspace and enter something else, if `breakpoints' does not look like the command you expected. (If you were sure you wanted `info breakpoints' in the first place, you might as well just type immediately after `info bre', to exploit command abbreviations rather than command completion). If there is more than one possibility for the next word when you press , GDB sounds a bell. You can either supply more characters and try again, or just press a second time; GDB displays all the possible completions for that word. For example, you might want to set a breakpoint on a subroutine whose name begins with `make_', but when you type `b make_' GDB just sounds the bell. Typing again displays all the function names in your program that begin with those characters, for example: (gdb) b make_ GDB sounds bell; press again, to see: make_a_section_from_file make_environ make_abs_section make_function_type make_blockvector make_pointer_type make_cleanup make_reference_type make_command make_symbol_completion_list (gdb) b make_ After displaying the available possibilities, GDB copies your partial input (`b make_' in the example) so you can finish the command. If you just want to see the list of alternatives in the first place, you can press `M-?' rather than pressing twice. `M-?' means ` ?'. You can type this either by holding down a key designated as the shift on your keyboard (if there is one) while typing `?', or as followed by `?'. Sometimes the string you need, while logically a "word", may contain parentheses or other characters that GDB normally excludes from its notion of a word. To permit word completion to work in this situation, you may enclose words in `'' (single quote marks) in GDB commands. The most likely situation where you might need this is in typing the name of a C++ function. This is because C++ allows function overloading (multiple definitions of the same function, distinguished by argument type). For example, when you want to set a breakpoint you may need to distinguish whether you mean the version of `name' that takes an `int' parameter, `name(int)', or the version that takes a `float' parameter, `name(float)'. To use the word-completion facilities in this situation, type a single quote `'' at the beginning of the function name. This alerts GDB that it may need to consider more information than usual when you press or `M-?' to request word completion: (gdb) b 'bubble( M-? bubble(double,double) bubble(int,int) (gdb) b 'bubble( In some cases, GDB can tell that completing a name requires using quotes. When this happens, GDB inserts the quote for you (while completing as much as it can) if you do not type the quote in the first place: (gdb) b bub GDB alters your input line to the following, and rings a bell: (gdb) b 'bubble( In general, GDB can tell that a quote is needed (and inserts it) if you have not yet started typing the argument list when you ask for completion on an overloaded symbol. For more information about overloaded functions, see *Note C++ expressions: C plus plus expressions. You can use the command `set overload-resolution off' to disable overload resolution; see *Note GDB features for C++: Debugging C plus plus.  File: gdb.info, Node: Help, Prev: Completion, Up: Commands 3.3 Getting help ================ You can always ask GDB itself for information on its commands, using the command `help'. `help' `h' You can use `help' (abbreviated `h') with no arguments to display a short list of named classes of commands: (gdb) help List of classes of commands: aliases -- Aliases of other commands breakpoints -- Making program stop at certain points data -- Examining data files -- Specifying and examining files internals -- Maintenance commands obscure -- Obscure features running -- Running the program stack -- Examining the stack status -- Status inquiries support -- Support facilities tracepoints -- Tracing of program execution without stopping the program user-defined -- User-defined commands Type "help" followed by a class name for a list of commands in that class. Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb) `help CLASS' Using one of the general help classes as an argument, you can get a list of the individual commands in that class. For example, here is the help display for the class `status': (gdb) help status Status inquiries. List of commands: info -- Generic command for showing things about the program being debugged show -- Generic command for showing things about the debugger Type "help" followed by command name for full documentation. Command name abbreviations are allowed if unambiguous. (gdb) `help COMMAND' With a command name as `help' argument, GDB displays a short paragraph on how to use that command. `apropos ARGS' The `apropos ARGS' command searches through all of the GDB commands, and their documentation, for the regular expression specified in ARGS. It prints out all matches found. For example: apropos reload results in: set symbol-reloading -- Set dynamic symbol table reloading multiple times in one run show symbol-reloading -- Show dynamic symbol table reloading multiple times in one run `complete ARGS' The `complete ARGS' command lists all the possible completions for the beginning of a command. Use ARGS to specify the beginning of the command you want completed. For example: complete i results in: if ignore info inspect This is intended for use by GNU Emacs. In addition to `help', you can use the GDB commands `info' and `show' to inquire about the state of your program, or the state of GDB itself. Each command supports many topics of inquiry; this manual introduces each of them in the appropriate context. The listings under `info' and under `show' in the Index point to all the sub-commands. *Note Index::. `info' This command (abbreviated `i') is for describing the state of your program. For example, you can list the arguments given to your program with `info args', list the registers currently in use with `info registers', or list the breakpoints you have set with `info breakpoints'. You can get a complete list of the `info' sub-commands with `help info'. `set' You can assign the result of an expression to an environment variable with `set'. For example, you can set the GDB prompt to a $-sign with `set prompt $'. `show' In contrast to `info', `show' is for describing the state of GDB itself. You can change most of the things you can `show', by using the related command `set'; for example, you can control what number system is used for displays with `set radix', or simply inquire which is currently in use with `show radix'. To display all the settable parameters and their current values, you can use `show' with no arguments; you may also use `info set'. Both commands produce the same display. Here are three miscellaneous `show' subcommands, all of which are exceptional in lacking corresponding `set' commands: `show version' Show what version of GDB is running. You should include this information in GDB bug-reports. If multiple versions of GDB are in use at your site, you may need to determine which version of GDB you are running; as GDB evolves, new commands are introduced, and old ones may wither away. Also, many system vendors ship variant versions of GDB, and there are variant versions of GDB in GNU/Linux distributions as well. The version number is the same as the one announced when you start GDB. `show copying' Display information about permission for copying GDB. `show warranty' Display the GNU "NO WARRANTY" statement, or a warranty, if your version of GDB comes with one.  File: gdb.info, Node: Running, Next: Stopping, Prev: Commands, Up: Top 4 Running Programs Under GDB **************************** When you run a program under GDB, you must first generate debugging information when you compile it. You may start GDB with its arguments, if any, in an environment of your choice. If you are doing native debugging, you may redirect your program's input and output, debug an already running process, or kill a child process. * Menu: * Compilation:: Compiling for debugging * Starting:: Starting your program * Arguments:: Your program's arguments * Environment:: Your program's environment * Working Directory:: Your program's working directory * Input/Output:: Your program's input and output * Attach:: Debugging an already-running process * Kill Process:: Killing the child process * Threads:: Debugging programs with multiple threads * Processes:: Debugging programs with multiple processes  File: gdb.info, Node: Compilation, Next: Starting, Up: Running 4.1 Compiling for debugging =========================== In order to debug a program effectively, you need to generate debugging information when you compile it. This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code. To request debugging information, specify the `-g' option when you run the compiler. Most compilers do not include information about preprocessor macros in the debugging information if you specify the `-g' flag alone, because this information is rather large. Version 3.1 of GCC, the GNU C compiler, provides macro information if you specify the options `-gdwarf-2' and `-g3'; the former option requests debugging information in the Dwarf 2 format, and the latter requests "extra information". In the future, we hope to find more compact ways to represent macro information, so that it can be included with `-g' alone. Many C compilers are unable to handle the `-g' and `-O' options together. Using those compilers, you cannot generate optimized executables containing debugging information. GCC, the GNU C compiler, supports `-g' with or without `-O', making it possible to debug optimized code. We recommend that you _always_ use `-g' whenever you compile a program. You may think your program is correct, but there is no sense in pushing your luck. When you debug a program compiled with `-g -O', remember that the optimizer is rearranging your code; the debugger shows you what is really there. Do not be too surprised when the execution path does not exactly match your source file! An extreme example: if you define a variable, but never use it, GDB never sees that variable--because the compiler optimizes it out of existence. Some things do not work as well with `-g -O' as with just `-g', particularly on machines with instruction scheduling. If in doubt, recompile with `-g' alone, and if this fixes the problem, please report it to us as a bug (including a test case!). *Note Variables::, for more information about debugging optimized code. Older versions of the GNU C compiler permitted a variant option `-gg' for debugging information. GDB no longer supports this format; if your GNU C compiler has this option, do not use it.  File: gdb.info, Node: Starting, Next: Arguments, Prev: Compilation, Up: Running 4.2 Starting your program ========================= `run' `r' Use the `run' command to start your program under GDB. You must first specify the program name (except on VxWorks) with an argument to GDB (*note Getting In and Out of GDB: Invocation.), or by using the `file' or `exec-file' command (*note Commands to specify files: Files.). If you are running your program in an execution environment that supports processes, `run' creates an inferior process and makes that process run your program. (In environments without processes, `run' jumps to the start of your program.) The execution of a program is affected by certain information it receives from its superior. GDB provides ways to specify this information, which you must do _before_ starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into four categories: The _arguments._ Specify the arguments to give your program as the arguments of the `run' command. If a shell is available on your target, the shell is used to pass the arguments, so that you may use normal conventions (such as wildcard expansion or variable substitution) in describing the arguments. In Unix systems, you can control which shell is used with the `SHELL' environment variable. *Note Your program's arguments: Arguments. The _environment._ Your program normally inherits its environment from GDB, but you can use the GDB commands `set environment' and `unset environment' to change parts of the environment that affect your program. *Note Your program's environment: Environment. The _working directory._ Your program inherits its working directory from GDB. You can set the GDB working directory with the `cd' command in GDB. *Note Your program's working directory: Working Directory. The _standard input and output._ Your program normally uses the same device for standard input and standard output as GDB is using. You can redirect input and output in the `run' command line, or you can use the `tty' command to set a different device for your program. *Note Your program's input and output: Input/Output. _Warning:_ While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program; if you attempt this, GDB is likely to wind up debugging the wrong program. When you issue the `run' command, your program begins to execute immediately. *Note Stopping and continuing: Stopping, for discussion of how to arrange for your program to stop. Once your program has stopped, you may call functions in your program, using the `print' or `call' commands. *Note Examining Data: Data. If the modification time of your symbol file has changed since the last time GDB read its symbols, GDB discards its symbol table, and reads it again. When it does this, GDB tries to retain your current breakpoints. `start' The name of the main procedure can vary from language to language. With C or C++, the main procedure name is always `main', but other languages such as Ada do not require a specific name for their main procedure. The debugger provides a convenient way to start the execution of the program and to stop at the beginning of the main procedure, depending on the language used. The `start' command does the equivalent of setting a temporary breakpoint at the beginning of the main procedure and then invoking the `run' command. Some programs contain an elaboration phase where some startup code is executed before the main program is called. This depends on the languages used to write your program. In C++ for instance, constructors for static and global objects are executed before `main' is called. It is therefore possible that the debugger stops before reaching the main procedure. However, the temporary breakpoint will remain to halt execution. Specify the arguments to give to your program as arguments to the `start' command. These arguments will be given verbatim to the underlying `run' command. Note that the same arguments will be reused if no argument is provided during subsequent calls to `start' or `run'. It is sometimes necessary to debug the program during elaboration. In these cases, using the `start' command would stop the execution of your program too late, as the program would have already completed the elaboration phase. Under these circumstances, insert breakpoints in your elaboration code before running your program.  File: gdb.info, Node: Arguments, Next: Environment, Prev: Starting, Up: Running 4.3 Your program's arguments ============================ The arguments to your program can be specified by the arguments of the `run' command. They are passed to a shell, which expands wildcard characters and performs redirection of I/O, and thence to your program. Your `SHELL' environment variable (if it exists) specifies what shell GDB uses. If you do not define `SHELL', GDB uses the default shell (`/bin/sh' on Unix). On non-Unix systems, the program is usually invoked directly by GDB, which emulates I/O redirection via the appropriate system calls, and the wildcard characters are expanded by the startup code of the program, not by the shell. `run' with no arguments uses the same arguments used by the previous `run', or those set by the `set args' command. `set args' Specify the arguments to be used the next time your program is run. If `set args' has no arguments, `run' executes your program with no arguments. Once you have run your program with arguments, using `set args' before the next `run' is the only way to run it again without arguments. `show args' Show the arguments to give your program when it is started.  File: gdb.info, Node: Environment, Next: Working Directory, Prev: Arguments, Up: Running 4.4 Your program's environment ============================== The "environment" consists of a set of environment variables and their values. Environment variables conventionally record such things as your user name, your home directory, your terminal type, and your search path for programs to run. Usually you set up environment variables with the shell and they are inherited by all the other programs you run. When debugging, it can be useful to try running your program with a modified environment without having to start GDB over again. `path DIRECTORY' Add DIRECTORY to the front of the `PATH' environment variable (the search path for executables) that will be passed to your program. The value of `PATH' used by GDB does not change. You may specify several directory names, separated by whitespace or by a system-dependent separator character (`:' on Unix, `;' on MS-DOS and MS-Windows). If DIRECTORY is already in the path, it is moved to the front, so it is searched sooner. You can use the string `$cwd' to refer to whatever is the current working directory at the time GDB searches the path. If you use `.' instead, it refers to the directory where you executed the `path' command. GDB replaces `.' in the DIRECTORY argument (with the current path) before adding DIRECTORY to the search path. `show paths' Display the list of search paths for executables (the `PATH' environment variable). `show environment [VARNAME]' Print the value of environment variable VARNAME to be given to your program when it starts. If you do not supply VARNAME, print the names and values of all environment variables to be given to your program. You can abbreviate `environment' as `env'. `set environment VARNAME [=VALUE]' Set environment variable VARNAME to VALUE. The value changes for your program only, not for GDB itself. VALUE may be any string; the values of environment variables are just strings, and any interpretation is supplied by your program itself. The VALUE parameter is optional; if it is eliminated, the variable is set to a null value. For example, this command: set env USER = foo tells the debugged program, when subsequently run, that its user is named `foo'. (The spaces around `=' are used for clarity here; they are not actually required.) `unset environment VARNAME' Remove variable VARNAME from the environment to be passed to your program. This is different from `set env VARNAME ='; `unset environment' removes the variable from the environment, rather than assigning it an empty value. _Warning:_ On Unix systems, GDB runs your program using the shell indicated by your `SHELL' environment variable if it exists (or `/bin/sh' if not). If your `SHELL' variable names a shell that runs an initialization file--such as `.cshrc' for C-shell, or `.bashrc' for BASH--any variables you set in that file affect your program. You may wish to move setting of environment variables to files that are only run when you sign on, such as `.login' or `.profile'.  File: gdb.info, Node: Working Directory, Next: Input/Output, Prev: Environment, Up: Running 4.5 Your program's working directory ==================================== Each time you start your program with `run', it inherits its working directory from the current working directory of GDB. The GDB working directory is initially whatever it inherited from its parent process (typically the shell), but you can specify a new working directory in GDB with the `cd' command. The GDB working directory also serves as a default for the commands that specify files for GDB to operate on. *Note Commands to specify files: Files. `cd DIRECTORY' Set the GDB working directory to DIRECTORY. `pwd' Print the GDB working directory.  File: gdb.info, Node: Input/Output, Next: Attach, Prev: Working Directory, Up: Running 4.6 Your program's input and output =================================== By default, the program you run under GDB does input and output to the same terminal that GDB uses. GDB switches the terminal to its own terminal modes to interact with you, but it records the terminal modes your program was using and switches back to them when you continue running your program. `info terminal' Displays information recorded by GDB about the terminal modes your program is using. You can redirect your program's input and/or output using shell redirection with the `run' command. For example, run > outfile starts your program, diverting its output to the file `outfile'. Another way to specify where your program should do input and output is with the `tty' command. This command accepts a file name as argument, and causes this file to be the default for future `run' commands. It also resets the controlling terminal for the child process, for future `run' commands. For example, tty /dev/ttyb directs that processes started with subsequent `run' commands default to do input and output on the terminal `/dev/ttyb' and have that as their controlling terminal. An explicit redirection in `run' overrides the `tty' command's effect on the input/output device, but not its effect on the controlling terminal. When you use the `tty' command or redirect input in the `run' command, only the input _for your program_ is affected. The input for GDB still comes from your terminal.  File: gdb.info, Node: Attach, Next: Kill Process, Prev: Input/Output, Up: Running 4.7 Debugging an already-running process ======================================== `attach PROCESS-ID' This command attaches to a running process--one that was started outside GDB. (`info files' shows your active targets.) The command takes as argument a process ID. The usual way to find out the process-id of a Unix process is with the `ps' utility, or with the `jobs -l' shell command. `attach' does not repeat if you press a second time after executing the command. To use `attach', your program must be running in an environment which supports processes; for example, `attach' does not work for programs on bare-board targets that lack an operating system. You must also have permission to send the process a signal. When you use `attach', the debugger finds the program running in the process first by looking in the current working directory, then (if the program is not found) by using the source file search path (*note Specifying source directories: Source Path.). You can also use the `file' command to load the program. *Note Commands to Specify Files: Files. The first thing GDB does after arranging to debug the specified process is to stop it. You can examine and modify an attached process with all the GDB commands that are ordinarily available when you start processes with `run'. You can insert breakpoints; you can step and continue; you can modify storage. If you would rather the process continue running, you may use the `continue' command after attaching GDB to the process. `detach' When you have finished debugging the attached process, you can use the `detach' command to release it from GDB control. Detaching the process continues its execution. After the `detach' command, that process and GDB become completely independent once more, and you are ready to `attach' another process or start one with `run'. `detach' does not repeat if you press again after executing the command. If you exit GDB or use the `run' command while you have an attached process, you kill that process. By default, GDB asks for confirmation if you try to do either of these things; you can control whether or not you need to confirm by using the `set confirm' command (*note Optional warnings and messages: Messages/Warnings.).  File: gdb.info, Node: Kill Process, Next: Threads, Prev: Attach, Up: Running 4.8 Killing the child process ============================= `kill' Kill the child process in which your program is running under GDB. This command is useful if you wish to debug a core dump instead of a running process. GDB ignores any core dump file while your program is running. On some operating systems, a program cannot be executed outside GDB while you have breakpoints set on it inside GDB. You can use the `kill' command in this situation to permit running your program outside the debugger. The `kill' command is also useful if you wish to recompile and relink your program, since on many systems it is impossible to modify an executable file while it is running in a process. In this case, when you next type `run', GDB notices that the file has changed, and reads the symbol table again (while trying to preserve your current breakpoint settings).  File: gdb.info, Node: Threads, Next: Processes, Prev: Kill Process, Up: Running 4.9 Debugging programs with multiple threads ============================================ In some operating systems, such as HP-UX and Solaris, a single program may have more than one "thread" of execution. The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes--except that they share one address space (that is, they can all examine and modify the same variables). On the other hand, each thread has its own registers and execution stack, and perhaps private memory. GDB provides these facilities for debugging multi-thread programs: * automatic notification of new threads * `thread THREADNO', a command to switch among threads * `info threads', a command to inquire about existing threads * `thread apply [THREADNO] [ALL] ARGS', a command to apply a command to a list of threads * thread-specific breakpoints _Warning:_ These facilities are not yet available on every GDB configuration where the operating system supports threads. If your GDB does not support threads, these commands have no effect. For example, a system without thread support shows no output from `info threads', and always rejects the `thread' command, like this: (gdb) info threads (gdb) thread 1 Thread ID 1 not known. Use the "info threads" command to see the IDs of currently known threads. The GDB thread debugging facility allows you to observe all threads while your program runs--but whenever GDB takes control, one thread in particular is always the focus of debugging. This thread is called the "current thread". Debugging commands show program information from the perspective of the current thread. Whenever GDB detects a new thread in your program, it displays the target system's identification for the thread with a message in the form `[New SYSTAG]'. SYSTAG is a thread identifier whose form varies depending on the particular system. For example, on LynxOS, you might see [New process 35 thread 27] when GDB notices a new thread. In contrast, on an SGI system, the SYSTAG is simply something like `process 368', with no further qualifier. For debugging purposes, GDB associates its own thread number--always a single integer--with each thread in your program. `info threads' Display a summary of all threads currently in your program. GDB displays for each thread (in this order): 1. the thread number assigned by GDB 2. the target system's thread identifier (SYSTAG) 3. the current stack frame summary for that thread An asterisk `*' to the left of the GDB thread number indicates the current thread. For example, (gdb) info threads 3 process 35 thread 27 0x34e5 in sigpause () 2 process 35 thread 23 0x34e5 in sigpause () * 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) at threadtest.c:68 On HP-UX systems: For debugging purposes, GDB associates its own thread number--a small integer assigned in thread-creation order--with each thread in your program. Whenever GDB detects a new thread in your program, it displays both GDB's thread number and the target system's identification for the thread with a message in the form `[New SYSTAG]'. SYSTAG is a thread identifier whose form varies depending on the particular system. For example, on HP-UX, you see [New thread 2 (system thread 26594)] when GDB notices a new thread. `info threads' Display a summary of all threads currently in your program. GDB displays for each thread (in this order): 1. the thread number assigned by GDB 2. the target system's thread identifier (SYSTAG) 3. the current stack frame summary for that thread An asterisk `*' to the left of the GDB thread number indicates the current thread. For example, (gdb) info threads * 3 system thread 26607 worker (wptr=0x7b09c318 "@") \ at quicksort.c:137 2 system thread 26606 0x7b0030d8 in __ksleep () \ from /usr/lib/libc.2 1 system thread 27905 0x7b003498 in _brk () \ from /usr/lib/libc.2 `thread THREADNO' Make thread number THREADNO the current thread. The command argument THREADNO is the internal GDB thread number, as shown in the first field of the `info threads' display. GDB responds by displaying the system identifier of the thread you selected, and its current stack frame summary: (gdb) thread 2 [Switching to process 35 thread 23] 0x34e5 in sigpause () As with the `[New ...]' message, the form of the text after `Switching to' depends on your system's conventions for identifying threads. `thread apply [THREADNO] [ALL] ARGS' The `thread apply' command allows you to apply a command to one or more threads. Specify the numbers of the threads that you want affected with the command argument THREADNO. THREADNO is the internal GDB thread number, as shown in the first field of the `info threads' display. To apply a command to all threads, use `thread apply all' ARGS. Whenever GDB stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. GDB alerts you to the context switch with a message of the form `[Switching to SYSTAG]' to identify the thread. *Note Stopping and starting multi-thread programs: Thread Stops, for more information about how GDB behaves when you stop and start programs with multiple threads. *Note Setting watchpoints: Set Watchpoints, for information about watchpoints in programs with multiple threads.  File: gdb.info, Node: Processes, Prev: Threads, Up: Running 4.10 Debugging programs with multiple processes =============================================== On most systems, GDB has no special support for debugging programs which create additional processes using the `fork' function. When a program forks, GDB will continue to debug the parent process and the child process will run unimpeded. If you have set a breakpoint in any code which the child then executes, the child will get a `SIGTRAP' signal which (unless it catches the signal) will cause it to terminate. However, if you want to debug the child process there is a workaround which isn't too painful. Put a call to `sleep' in the code which the child process executes after the fork. It may be useful to sleep only if a certain environment variable is set, or a certain file exists, so that the delay need not occur when you don't want to run GDB on the child. While the child is sleeping, use the `ps' program to get its process ID. Then tell GDB (a new invocation of GDB if you are also debugging the parent process) to attach to the child process (*note Attach::). From that point on you can debug the child process just like any other process which you attached to. On some systems, GDB provides support for debugging programs that create additional processes using the `fork' or `vfork' functions. Currently, the only platforms with this feature are HP-UX (11.x and later only?) and GNU/Linux (kernel version 2.5.60 and later). By default, when a program forks, GDB will continue to debug the parent process and the child process will run unimpeded. If you want to follow the child process instead of the parent process, use the command `set follow-fork-mode'. `set follow-fork-mode MODE' Set the debugger response to a program call of `fork' or `vfork'. A call to `fork' or `vfork' creates a new process. The MODE can be: `parent' The original process is debugged after a fork. The child process runs unimpeded. This is the default. `child' The new process is debugged after a fork. The parent process runs unimpeded. `show follow-fork-mode' Display the current debugger response to a `fork' or `vfork' call. If you ask to debug a child process and a `vfork' is followed by an `exec', GDB executes the new target up to the first breakpoint in the new target. If you have a breakpoint set on `main' in your original program, the breakpoint will also be set on the child process's `main'. When a child process is spawned by `vfork', you cannot debug the child or parent until an `exec' call completes. If you issue a `run' command to GDB after an `exec' call executes, the new target restarts. To restart the parent process, use the `file' command with the parent executable name as its argument. You can use the `catch' command to make GDB stop whenever a `fork', `vfork', or `exec' call is made. *Note Setting catchpoints: Set Catchpoints.  File: gdb.info, Node: Stopping, Next: Stack, Prev: Running, Up: Top 5 Stopping and Continuing ************************* The principal purposes of using a debugger are so that you can stop your program before it terminates; or so that, if your program runs into trouble, you can investigate and find out why. Inside GDB, your program may stop for any of several reasons, such as a signal, a breakpoint, or reaching a new line after a GDB command such as `step'. You may then examine and change variables, set new breakpoints or remove old ones, and then continue execution. Usually, the messages shown by GDB provide ample explanation of the status of your program--but you can also explicitly request this information at any time. `info program' Display information about the status of your program: whether it is running or not, what process it is, and why it stopped. * Menu: * Breakpoints:: Breakpoints, watchpoints, and catchpoints * Continuing and Stepping:: Resuming execution * Signals:: Signals * Thread Stops:: Stopping and starting multi-thread programs  File: gdb.info, Node: Breakpoints, Next: Continuing and Stepping, Up: Stopping 5.1 Breakpoints, watchpoints, and catchpoints ============================================= A "breakpoint" makes your program stop whenever a certain point in the program is reached. For each breakpoint, you can add conditions to control in finer detail whether your program stops. You can set breakpoints with the `break' command and its variants (*note Setting breakpoints: Set Breaks.), to specify the place where your program should stop by line number, function name or exact address in the program. In HP-UX, SunOS 4.x, SVR4, and Alpha OSF/1 configurations, you can set breakpoints in shared libraries before the executable is run. There is a minor limitation on HP-UX systems: you must wait until the executable is run in order to set breakpoints in shared library routines that are not called directly by the program (for example, routines that are arguments in a `pthread_create' call). A "watchpoint" is a special breakpoint that stops your program when the value of an expression changes. You must use a different command to set watchpoints (*note Setting watchpoints: Set Watchpoints.), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands. You can arrange to have values from your program displayed automatically whenever GDB stops at a breakpoint. *Note Automatic display: Auto Display. A "catchpoint" is another special breakpoint that stops your program when a certain kind of event occurs, such as the throwing of a C++ exception or the loading of a library. As with watchpoints, you use a different command to set a catchpoint (*note Setting catchpoints: Set Catchpoints.), but aside from that, you can manage a catchpoint like any other breakpoint. (To stop when your program receives a signal, use the `handle' command; see *Note Signals: Signals.) GDB assigns a number to each breakpoint, watchpoint, or catchpoint when you create it; these numbers are successive integers starting with one. In many of the commands for controlling various features of breakpoints you use the breakpoint number to say which breakpoint you want to change. Each breakpoint may be "enabled" or "disabled"; if disabled, it has no effect on your program until you enable it again. Some GDB commands accept a range of breakpoints on which to operate. A breakpoint range is either a single breakpoint number, like `5', or two such numbers, in increasing order, separated by a hyphen, like `5-7'. When a breakpoint range is given to a command, all breakpoint in that range are operated on. * Menu: * Set Breaks:: Setting breakpoints * Set Watchpoints:: Setting watchpoints * Set Catchpoints:: Setting catchpoints * Delete Breaks:: Deleting breakpoints * Disabling:: Disabling breakpoints * Conditions:: Break conditions * Break Commands:: Breakpoint command lists * Breakpoint Menus:: Breakpoint menus * Error in Breakpoints:: ``Cannot insert breakpoints'' * Breakpoint related warnings:: ``Breakpoint address adjusted...''  File: gdb.info, Node: Set Breaks, Next: Set Watchpoints, Up: Breakpoints 5.1.1 Setting breakpoints ------------------------- Breakpoints are set with the `break' command (abbreviated `b'). The debugger convenience variable `$bpnum' records the number of the breakpoint you've set most recently; see *Note Convenience variables: Convenience Vars, for a discussion of what you can do with convenience variables. You have several ways to say where the breakpoint should go. `break FUNCTION' Set a breakpoint at entry to function FUNCTION. When using source languages that permit overloading of symbols, such as C++, FUNCTION may refer to more than one possible place to break. *Note Breakpoint menus: Breakpoint Menus, for a discussion of that situation. `break +OFFSET' `break -OFFSET' Set a breakpoint some number of lines forward or back from the position at which execution stopped in the currently selected "stack frame". (*Note Frames: Frames, for a description of stack frames.) `break LINENUM' Set a breakpoint at line LINENUM in the current source file. The current source file is the last file whose source text was printed. The breakpoint will stop your program just before it executes any of the code on that line. `break FILENAME:LINENUM' Set a breakpoint at line LINENUM in source file FILENAME. `break FILENAME:FUNCTION' Set a breakpoint at entry to function FUNCTION found in file FILENAME. Specifying a file name as well as a function name is superfluous except when multiple files contain similarly named functions. `break *ADDRESS' Set a breakpoint at address ADDRESS. You can use this to set breakpoints in parts of your program which do not have debugging information or source files. `break' When called without any arguments, `break' sets a breakpoint at the next instruction to be executed in the selected stack frame (*note Examining the Stack: Stack.). In any selected frame but the innermost, this makes your program stop as soon as control returns to that frame. This is similar to the effect of a `finish' command in the frame inside the selected frame--except that `finish' does not leave an active breakpoint. If you use `break' without an argument in the innermost frame, GDB stops the next time it reaches the current location; this may be useful inside loops. GDB normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies whether or not the breakpoint already existed when your program stopped. `break ... if COND' Set a breakpoint with condition COND; evaluate the expression COND each time the breakpoint is reached, and stop only if the value is nonzero--that is, if COND evaluates as true. `...' stands for one of the possible arguments described above (or no argument) specifying where to break. *Note Break conditions: Conditions, for more information on breakpoint conditions. `tbreak ARGS' Set a breakpoint enabled only for one stop. ARGS are the same as for the `break' command, and the breakpoint is set in the same way, but the breakpoint is automatically deleted after the first time your program stops there. *Note Disabling breakpoints: Disabling. `hbreak ARGS' Set a hardware-assisted breakpoint. ARGS are the same as for the `break' command and the breakpoint is set in the same way, but the breakpoint requires hardware support and some target hardware may not have this support. The main purpose of this is EPROM/ROM code debugging, so you can set a breakpoint at an instruction without changing the instruction. This can be used with the new trap-generation provided by SPARClite DSU and some x86-based targets. These targets will generate traps when a program accesses some data or instruction address that is assigned to the debug registers. However the hardware breakpoint registers can take a limited number of breakpoints. For example, on the DSU, only two data breakpoints can be set at a time, and GDB will reject this command if more than two are used. Delete or disable unused hardware breakpoints before setting new ones (*note Disabling: Disabling.). *Note Break conditions: Conditions. *Note set remote hardware-breakpoint-limit::. `thbreak ARGS' Set a hardware-assisted breakpoint enabled only for one stop. ARGS are the same as for the `hbreak' command and the breakpoint is set in the same way. However, like the `tbreak' command, the breakpoint is automatically deleted after the first time your program stops there. Also, like the `hbreak' command, the breakpoint requires hardware support and some target hardware may not have this support. *Note Disabling breakpoints: Disabling. See also *Note Break conditions: Conditions. `rbreak REGEX' Set breakpoints on all functions matching the regular expression REGEX. This command sets an unconditional breakpoint on all matches, printing a list of all breakpoints it set. Once these breakpoints are set, they are treated just like the breakpoints set with the `break' command. You can delete them, disable them, or make them conditional the same way as any other breakpoint. The syntax of the regular expression is the standard one used with tools like `grep'. Note that this is different from the syntax used by shells, so for instance `foo*' matches all functions that include an `fo' followed by zero or more `o's. There is an implicit `.*' leading and trailing the regular expression you supply, so to match only functions that begin with `foo', use `^foo'. When debugging C++ programs, `rbreak' is useful for setting breakpoints on overloaded functions that are not members of any special classes. The `rbreak' command can be used to set breakpoints in *all* the functions in a program, like this: (gdb) rbreak . `info breakpoints [N]' `info break [N]' `info watchpoints [N]' Print a table of all breakpoints, watchpoints, and catchpoints set and not deleted, with the following columns for each breakpoint: _Breakpoint Numbers_ _Type_ Breakpoint, watchpoint, or catchpoint. _Disposition_ Whether the breakpoint is marked to be disabled or deleted when hit. _Enabled or Disabled_ Enabled breakpoints are marked with `y'. `n' marks breakpoints that are not enabled. _Address_ Where the breakpoint is in your program, as a memory address. If the breakpoint is pending (see below for details) on a future load of a shared library, the address will be listed as `'. _What_ Where the breakpoint is in the source for your program, as a file and line number. For a pending breakpoint, the original string passed to the breakpoint command will be listed as it cannot be resolved until the appropriate shared library is loaded in the future. If a breakpoint is conditional, `info break' shows the condition on the line following the affected breakpoint; breakpoint commands, if any, are listed after that. A pending breakpoint is allowed to have a condition specified for it. The condition is not parsed for validity until a shared library is loaded that allows the pending breakpoint to resolve to a valid location. `info break' with a breakpoint number N as argument lists only that breakpoint. The convenience variable `$_' and the default examining-address for the `x' command are set to the address of the last breakpoint listed (*note Examining memory: Memory.). `info break' displays a count of the number of times the breakpoint has been hit. This is especially useful in conjunction with the `ignore' command. You can ignore a large number of breakpoint hits, look at the breakpoint info to see how many times the breakpoint was hit, and then run again, ignoring one less than that number. This will get you quickly to the last hit of that breakpoint. GDB allows you to set any number of breakpoints at the same place in your program. There is nothing silly or meaningless about this. When the breakpoints are conditional, this is even useful (*note Break conditions: Conditions.). If a specified breakpoint location cannot be found, it may be due to the fact that the location is in a shared library that is yet to be loaded. In such a case, you may want GDB to create a special breakpoint (known as a "pending breakpoint") that attempts to resolve itself in the future when an appropriate shared library gets loaded. Pending breakpoints are useful to set at the start of your GDB session for locations that you know will be dynamically loaded later by the program being debugged. When shared libraries are loaded, a check is made to see if the load resolves any pending breakpoint locations. If a pending breakpoint location gets resolved, a regular breakpoint is created and the original pending breakpoint is removed. GDB provides some additional commands for controlling pending breakpoint support: `set breakpoint pending auto' This is the default behavior. When GDB cannot find the breakpoint location, it queries you whether a pending breakpoint should be created. `set breakpoint pending on' This indicates that an unrecognized breakpoint location should automatically result in a pending breakpoint being created. `set breakpoint pending off' This indicates that pending breakpoints are not to be created. Any unrecognized breakpoint location results in an error. This setting does not affect any pending breakpoints previously created. `show breakpoint pending' Show the current behavior setting for creating pending breakpoints. Normal breakpoint operations apply to pending breakpoints as well. You may specify a condition for a pending breakpoint and/or commands to run when the breakpoint is reached. You can also enable or disable the pending breakpoint. When you specify a condition for a pending breakpoint, the parsing of the condition will be deferred until the point where the pending breakpoint location is resolved. Disabling a pending breakpoint tells GDB to not attempt to resolve the breakpoint on any subsequent shared library load. When a pending breakpoint is re-enabled, GDB checks to see if the location is already resolved. This is done because any number of shared library loads could have occurred since the time the breakpoint was disabled and one or more of these loads could resolve the location. GDB itself sometimes sets breakpoints in your program for special purposes, such as proper handling of `longjmp' (in C programs). These internal breakpoints are assigned negative numbers, starting with `-1'; `info breakpoints' does not display them. You can see these breakpoints with the GDB maintenance command `maint info breakpoints' (*note maint info breakpoints::).  File: gdb.info, Node: Set Watchpoints, Next: Set Catchpoints, Prev: Set Breaks, Up: Breakpoints 5.1.2 Setting watchpoints ------------------------- You can use a watchpoint to stop execution whenever the value of an expression changes, without having to predict a particular place where this may happen. Depending on your system, watchpoints may be implemented in software or hardware. GDB does software watchpointing by single-stepping your program and testing the variable's value each time, which is hundreds of times slower than normal execution. (But this may still be worth it, to catch errors where you have no clue what part of your program is the culprit.) On some systems, such as HP-UX, GNU/Linux and some other x86-based targets, GDB includes support for hardware watchpoints, which do not slow down the running of your program. `watch EXPR' Set a watchpoint for an expression. GDB will break when EXPR is written into by the program and its value changes. `rwatch EXPR' Set a watchpoint that will break when watch EXPR is read by the program. `awatch EXPR' Set a watchpoint that will break when EXPR is either read or written into by the program. `info watchpoints' This command prints a list of watchpoints, breakpoints, and catchpoints; it is the same as `info break'. GDB sets a "hardware watchpoint" if possible. Hardware watchpoints execute very quickly, and the debugger reports a change in value at the exact instruction where the change occurs. If GDB cannot set a hardware watchpoint, it sets a software watchpoint, which executes more slowly and reports the change in value at the next statement, not the instruction, after the change occurs. When you issue the `watch' command, GDB reports Hardware watchpoint NUM: EXPR if it was able to set a hardware watchpoint. Currently, the `awatch' and `rwatch' commands can only set hardware watchpoints, because accesses to data that don't change the value of the watched expression cannot be detected without examining every instruction as it is being executed, and GDB does not do that currently. If GDB finds that it is unable to set a hardware breakpoint with the `awatch' or `rwatch' command, it will print a message like this: Expression cannot be implemented with read/access watchpoint. Sometimes, GDB cannot set a hardware watchpoint because the data type of the watched expression is wider than what a hardware watchpoint on the target machine can handle. For example, some systems can only watch regions that are up to 4 bytes wide; on such systems you cannot set hardware watchpoints for an expression that yields a double-precision floating-point number (which is typically 8 bytes wide). As a work-around, it might be possible to break the large region into a series of smaller ones and watch them with separate watchpoints. If you set too many hardware watchpoints, GDB might be unable to insert all of them when you resume the execution of your program. Since the precise number of active watchpoints is unknown until such time as the program is about to be resumed, GDB might not be able to warn you about this when you set the watchpoints, and the warning will be printed only when the program is resumed: Hardware watchpoint NUM: Could not insert watchpoint If this happens, delete or disable some of the watchpoints. The SPARClite DSU will generate traps when a program accesses some data or instruction address that is assigned to the debug registers. For the data addresses, DSU facilitates the `watch' command. However the hardware breakpoint registers can only take two data watchpoints, and both watchpoints must be the same kind. For example, you can set two watchpoints with `watch' commands, two with `rwatch' commands, *or* two with `awatch' commands, but you cannot set one watchpoint with one command and the other with a different command. GDB will reject the command if you try to mix watchpoints. Delete or disable unused watchpoint commands before setting new ones. If you call a function interactively using `print' or `call', any watchpoints you have set will be inactive until GDB reaches another kind of breakpoint or the call completes. GDB automatically deletes watchpoints that watch local (automatic) variables, or expressions that involve such variables, when they go out of scope, that is, when the execution leaves the block in which these variables were defined. In particular, when the program being debugged terminates, _all_ local variables go out of scope, and so only watchpoints that watch global variables remain set. If you rerun the program, you will need to set all such watchpoints again. One way of doing that would be to set a code breakpoint at the entry to the `main' function and when it breaks, set all the watchpoints. _Warning:_ In multi-thread programs, watchpoints have only limited usefulness. With the current watchpoint implementation, GDB can only watch the value of an expression _in a single thread_. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression. _HP-UX Warning:_ In multi-thread programs, software watchpoints have only limited usefulness. If GDB creates a software watchpoint, it can only watch the value of an expression _in a single thread_. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use software watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression. (Hardware watchpoints, in contrast, watch an expression in all threads.) *Note set remote hardware-watchpoint-limit::.  File: gdb.info, Node: Set Catchpoints, Next: Delete Breaks, Prev: Set Watchpoints, Up: Breakpoints 5.1.3 Setting catchpoints ------------------------- You can use "catchpoints" to cause the debugger to stop for certain kinds of program events, such as C++ exceptions or the loading of a shared library. Use the `catch' command to set a catchpoint. `catch EVENT' Stop when EVENT occurs. EVENT can be any of the following: `throw' The throwing of a C++ exception. `catch' The catching of a C++ exception. `exec' A call to `exec'. This is currently only available for HP-UX. `fork' A call to `fork'. This is currently only available for HP-UX. `vfork' A call to `vfork'. This is currently only available for HP-UX. `load' `load LIBNAME' The dynamic loading of any shared library, or the loading of the library LIBNAME. This is currently only available for HP-UX. `unload' `unload LIBNAME' The unloading of any dynamically loaded shared library, or the unloading of the library LIBNAME. This is currently only available for HP-UX. `tcatch EVENT' Set a catchpoint that is enabled only for one stop. The catchpoint is automatically deleted after the first time the event is caught. Use the `info break' command to list the current catchpoints. There are currently some limitations to C++ exception handling (`catch throw' and `catch catch') in GDB: * If you call a function interactively, GDB normally returns control to you when the function has finished executing. If the call raises an exception, however, the call may bypass the mechanism that returns control to you and cause your program either to abort or to simply continue running until it hits a breakpoint, catches a signal that GDB is listening for, or exits. This is the case even if you set a catchpoint for the exception; catchpoints on exceptions are disabled within interactive calls. * You cannot raise an exception interactively. * You cannot install an exception handler interactively. Sometimes `catch' is not the best way to debug exception handling: if you need to know exactly where an exception is raised, it is better to stop _before_ the exception handler is called, since that way you can see the stack before any unwinding takes place. If you set a breakpoint in an exception handler instead, it may not be easy to find out where the exception was raised. To stop just before an exception handler is called, you need some knowledge of the implementation. In the case of GNU C++, exceptions are raised by calling a library function named `__raise_exception' which has the following ANSI C interface: /* ADDR is where the exception identifier is stored. ID is the exception identifier. */ void __raise_exception (void **addr, void *id); To make the debugger catch all exceptions before any stack unwinding takes place, set a breakpoint on `__raise_exception' (*note Breakpoints; watchpoints; and exceptions: Breakpoints.). With a conditional breakpoint (*note Break conditions: Conditions.) that depends on the value of ID, you can stop your program when a specific exception is raised. You can use multiple conditional breakpoints to stop your program when any of a number of exceptions are raised.  File: gdb.info, Node: Delete Breaks, Next: Disabling, Prev: Set Catchpoints, Up: Breakpoints 5.1.4 Deleting breakpoints -------------------------- It is often necessary to eliminate a breakpoint, watchpoint, or catchpoint once it has done its job and you no longer want your program to stop there. This is called "deleting" the breakpoint. A breakpoint that has been deleted no longer exists; it is forgotten. With the `clear' command you can delete breakpoints according to where they are in your program. With the `delete' command you can delete individual breakpoints, watchpoints, or catchpoints by specifying their breakpoint numbers. It is not necessary to delete a breakpoint to proceed past it. GDB automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address. `clear' Delete any breakpoints at the next instruction to be executed in the selected stack frame (*note Selecting a frame: Selection.). When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped. `clear FUNCTION' `clear FILENAME:FUNCTION' Delete any breakpoints set at entry to the function FUNCTION. `clear LINENUM' `clear FILENAME:LINENUM' Delete any breakpoints set at or within the code of the specified line. `delete [breakpoints] [RANGE...]' Delete the breakpoints, watchpoints, or catchpoints of the breakpoint ranges specified as arguments. If no argument is specified, delete all breakpoints (GDB asks confirmation, unless you have `set confirm off'). You can abbreviate this command as `d'.  File: gdb.info, Node: Disabling, Next: Conditions, Prev: Delete Breaks, Up: Breakpoints 5.1.5 Disabling breakpoints --------------------------- Rather than deleting a breakpoint, watchpoint, or catchpoint, you might prefer to "disable" it. This makes the breakpoint inoperative as if it had been deleted, but remembers the information on the breakpoint so that you can "enable" it again later. You disable and enable breakpoints, watchpoints, and catchpoints with the `enable' and `disable' commands, optionally specifying one or more breakpoint numbers as arguments. Use `info break' or `info watch' to print a list of breakpoints, watchpoints, and catchpoints if you do not know which numbers to use. A breakpoint, watchpoint, or catchpoint can have any of four different states of enablement: * Enabled. The breakpoint stops your program. A breakpoint set with the `break' command starts out in this state. * Disabled. The breakpoint has no effect on your program. * Enabled once. The breakpoint stops your program, but then becomes disabled. * Enabled for deletion. The breakpoint stops your program, but immediately after it does so it is deleted permanently. A breakpoint set with the `tbreak' command starts out in this state. You can use the following commands to enable or disable breakpoints, watchpoints, and catchpoints: `disable [breakpoints] [RANGE...]' Disable the specified breakpoints--or all breakpoints, if none are listed. A disabled breakpoint has no effect but is not forgotten. All options such as ignore-counts, conditions and commands are remembered in case the breakpoint is enabled again later. You may abbreviate `disable' as `dis'. `enable [breakpoints] [RANGE...]' Enable the specified breakpoints (or all defined breakpoints). They become effective once again in stopping your program. `enable [breakpoints] once RANGE...' Enable the specified breakpoints temporarily. GDB disables any of these breakpoints immediately after stopping your program. `enable [breakpoints] delete RANGE...' Enable the specified breakpoints to work once, then die. GDB deletes any of these breakpoints as soon as your program stops there. Except for a breakpoint set with `tbreak' (*note Setting breakpoints: Set Breaks.), breakpoints that you set are initially enabled; subsequently, they become disabled or enabled only when you use one of the commands above. (The command `until' can set and delete a breakpoint of its own, but it does not change the state of your other breakpoints; see *Note Continuing and stepping: Continuing and Stepping.)  File: gdb.info, Node: Conditions, Next: Break Commands, Prev: Disabling, Up: Breakpoints 5.1.6 Break conditions ---------------------- The simplest sort of breakpoint breaks every time your program reaches a specified place. You can also specify a "condition" for a breakpoint. A condition is just a Boolean expression in your programming language (*note Expressions: Expressions.). A breakpoint with a condition evaluates the expression each time your program reaches it, and your program stops only if the condition is _true_. This is the converse of using assertions for program validation; in that situation, you want to stop when the assertion is violated--that is, when the condition is false. In C, if you want to test an assertion expressed by the condition ASSERT, you should set the condition `! ASSERT' on the appropriate breakpoint. Conditions are also accepted for watchpoints; you may not need them, since a watchpoint is inspecting the value of an expression anyhow--but it might be simpler, say, to just set a watchpoint on a variable name, and specify a condition that tests whether the new value is an interesting one. Break conditions can have side effects, and may even call functions in your program. This can be useful, for example, to activate functions that log program progress, or to use your own print functions to format special data structures. The effects are completely predictable unless there is another enabled breakpoint at the same address. (In that case, GDB might see the other breakpoint first and stop your program without checking the condition of this one.) Note that breakpoint commands are usually more convenient and flexible than break conditions for the purpose of performing side effects when a breakpoint is reached (*note Breakpoint command lists: Break Commands.). Break conditions can be specified when a breakpoint is set, by using `if' in the arguments to the `break' command. *Note Setting breakpoints: Set Breaks. They can also be changed at any time with the `condition' command. You can also use the `if' keyword with the `watch' command. The `catch' command does not recognize the `if' keyword; `condition' is the only way to impose a further condition on a catchpoint. `condition BNUM EXPRESSION' Specify EXPRESSION as the break condition for breakpoint, watchpoint, or catchpoint number BNUM. After you set a condition, breakpoint BNUM stops your program only if the value of EXPRESSION is true (nonzero, in C). When you use `condition', GDB checks EXPRESSION immediately for syntactic correctness, and to determine whether symbols in it have referents in the context of your breakpoint. If EXPRESSION uses symbols not referenced in the context of the breakpoint, GDB prints an error message: No symbol "foo" in current context. GDB does not actually evaluate EXPRESSION at the time the `condition' command (or a command that sets a breakpoint with a condition, like `break if ...') is given, however. *Note Expressions: Expressions. `condition BNUM' Remove the condition from breakpoint number BNUM. It becomes an ordinary unconditional breakpoint. A special case of a breakpoint condition is to stop only when the breakpoint has been reached a certain number of times. This is so useful that there is a special way to do it, using the "ignore count" of the breakpoint. Every breakpoint has an ignore count, which is an integer. Most of the time, the ignore count is zero, and therefore has no effect. But if your program reaches a breakpoint whose ignore count is positive, then instead of stopping, it just decrements the ignore count by one and continues. As a result, if the ignore count value is N, the breakpoint does not stop the next N times your program reaches it. `ignore BNUM COUNT' Set the ignore count of breakpoint number BNUM to COUNT. The next COUNT times the breakpoint is reached, your program's execution does not stop; other than to decrement the ignore count, GDB takes no action. To make the breakpoint stop the next time it is reached, specify a count of zero. When you use `continue' to resume execution of your program from a breakpoint, you can specify an ignore count directly as an argument to `continue', rather than using `ignore'. *Note Continuing and stepping: Continuing and Stepping. If a breakpoint has a positive ignore count and a condition, the condition is not checked. Once the ignore count reaches zero, GDB resumes checking the condition. You could achieve the effect of the ignore count with a condition such as `$foo-- <= 0' using a debugger convenience variable that is decremented each time. *Note Convenience variables: Convenience Vars. Ignore counts apply to breakpoints, watchpoints, and catchpoints.  File: gdb.info, Node: Break Commands, Next: Breakpoint Menus, Prev: Conditions, Up: Breakpoints 5.1.7 Breakpoint command lists ------------------------------ You can give any breakpoint (or watchpoint or catchpoint) a series of commands to execute when your program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints. `commands [BNUM]' `... COMMAND-LIST ...' `end' Specify a list of commands for breakpoint number BNUM. The commands themselves appear on the following lines. Type a line containing just `end' to terminate the commands. To remove all commands from a breakpoint, type `commands' and follow it immediately with `end'; that is, give no commands. With no BNUM argument, `commands' refers to the last breakpoint, watchpoint, or catchpoint set (not to the breakpoint most recently encountered). Pressing as a means of repeating the last GDB command is disabled within a COMMAND-LIST. You can use breakpoint commands to start your program up again. Simply use the `continue' command, or `step', or any other command that resumes execution. Any other commands in the command list, after a command that resumes execution, are ignored. This is because any time you resume execution (even with a simple `next' or `step'), you may encounter another breakpoint--which could have its own command list, leading to ambiguities about which list to execute. If the first command you specify in a command list is `silent', the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue. If none of the remaining commands print anything, you see no sign that the breakpoint was reached. `silent' is meaningful only at the beginning of a breakpoint command list. The commands `echo', `output', and `printf' allow you to print precisely controlled output, and are often useful in silent breakpoints. *Note Commands for controlled output: Output. For example, here is how you could use breakpoint commands to print the value of `x' at entry to `foo' whenever `x' is positive. break foo if x>0 commands silent printf "x is %d\n",x cont end One application for breakpoint commands is to compensate for one bug so you can test for another. Put a breakpoint just after the erroneous line of code, give it a condition to detect the case in which something erroneous has been done, and give it commands to assign correct values to any variables that need them. End with the `continue' command so that your program does not stop, and start with the `silent' command so that no output is produced. Here is an example: break 403 commands silent set x = y + 4 cont end  File: gdb.info, Node: Breakpoint Menus, Next: Error in Breakpoints, Prev: Break Commands, Up: Breakpoints 5.1.8 Breakpoint menus ---------------------- Some programming languages (notably C++ and Objective-C) permit a single function name to be defined several times, for application in different contexts. This is called "overloading". When a function name is overloaded, `break FUNCTION' is not enough to tell GDB where you want a breakpoint. If you realize this is a problem, you can use something like `break FUNCTION(TYPES)' to specify which particular version of the function you want. Otherwise, GDB offers you a menu of numbered choices for different possible breakpoints, and waits for your selection with the prompt `>'. The first two options are always `[0] cancel' and `[1] all'. Typing `1' sets a breakpoint at each definition of FUNCTION, and typing `0' aborts the `break' command without setting any new breakpoints. For example, the following session excerpt shows an attempt to set a breakpoint at the overloaded symbol `String::after'. We choose three particular definitions of that function name: (gdb) b String::after [0] cancel [1] all [2] file:String.cc; line number:867 [3] file:String.cc; line number:860 [4] file:String.cc; line number:875 [5] file:String.cc; line number:853 [6] file:String.cc; line number:846 [7] file:String.cc; line number:735 > 2 4 6 Breakpoint 1 at 0xb26c: file String.cc, line 867. Breakpoint 2 at 0xb344: file String.cc, line 875. Breakpoint 3 at 0xafcc: file String.cc, line 846. Multiple breakpoints were set. Use the "delete" command to delete unwanted breakpoints. (gdb)  File: gdb.info, Node: Error in Breakpoints, Next: Breakpoint related warnings, Prev: Breakpoint Menus, Up: Breakpoints 5.1.9 "Cannot insert breakpoints" --------------------------------- Under some operating systems, breakpoints cannot be used in a program if any other process is running that program. In this situation, attempting to run or continue a program with a breakpoint causes GDB to print an error message: Cannot insert breakpoints. The same program may be running in another process. When this happens, you have three ways to proceed: 1. Remove or disable the breakpoints, then continue. 2. Suspend GDB, and copy the file containing your program to a new name. Resume GDB and use the `exec-file' command to specify that GDB should run your program under that name. Then start your program again. 3. Relink your program so that the text segment is nonsharable, using the linker option `-N'. The operating system limitation may not apply to nonsharable executables. A similar message can be printed if you request too many active hardware-assisted breakpoints and watchpoints: Stopped; cannot insert breakpoints. You may have requested too many hardware breakpoints and watchpoints. This message is printed when you attempt to resume the program, since only then GDB knows exactly how many hardware breakpoints and watchpoints it needs to insert. When this message is printed, you need to disable or remove some of the hardware-assisted breakpoints and watchpoints, and then continue.  File: gdb.info, Node: Breakpoint related warnings, Prev: Error in Breakpoints, Up: Breakpoints 5.1.10 "Breakpoint address adjusted..." --------------------------------------- Some processor architectures place constraints on the addresses at which breakpoints may be placed. For architectures thus constrained, GDB will attempt to adjust the breakpoint's address to comply with the constraints dictated by the architecture. One example of such an architecture is the Fujitsu FR-V. The FR-V is a VLIW architecture in which a number of RISC-like instructions may be bundled together for parallel execution. The FR-V architecture constrains the location of a breakpoint instruction within such a bundle to the instruction with the lowest address. GDB honors this constraint by adjusting a breakpoint's address to the first in the bundle. It is not uncommon for optimized code to have bundles which contain instructions from different source statements, thus it may happen that a breakpoint's address will be adjusted from one source statement to another. Since this adjustment may significantly alter GDB's breakpoint related behavior from what the user expects, a warning is printed when the breakpoint is first set and also when the breakpoint is hit. A warning like the one below is printed when setting a breakpoint that's been subject to address adjustment: warning: Breakpoint address adjusted from 0x00010414 to 0x00010410. Such warnings are printed both for user settable and GDB's internal breakpoints. If you see one of these warnings, you should verify that a breakpoint set at the adjusted address will have the desired affect. If not, the breakpoint in question may be removed and other breakpoints may be set which will have the desired behavior. E.g., it may be sufficient to place the breakpoint at a later instruction. A conditional breakpoint may also be useful in some cases to prevent the breakpoint from triggering too often. GDB will also issue a warning when stopping at one of these adjusted breakpoints: warning: Breakpoint 1 address previously adjusted from 0x00010414 to 0x00010410. When this warning is encountered, it may be too late to take remedial action except in cases where the breakpoint is hit earlier or more frequently than expected.  File: gdb.info, Node: Continuing and Stepping, Next: Signals, Prev: Breakpoints, Up: Stopping 5.2 Continuing and stepping =========================== "Continuing" means resuming program execution until your program completes normally. In contrast, "stepping" means executing just one more "step" of your program, where "step" may mean either one line of source code, or one machine instruction (depending on what particular command you use). Either when continuing or when stepping, your program may stop even sooner, due to a breakpoint or a signal. (If it stops due to a signal, you may want to use `handle', or use `signal 0' to resume execution. *Note Signals: Signals.) `continue [IGNORE-COUNT]' `c [IGNORE-COUNT]' `fg [IGNORE-COUNT]' Resume program execution, at the address where your program last stopped; any breakpoints set at that address are bypassed. The optional argument IGNORE-COUNT allows you to specify a further number of times to ignore a breakpoint at this location; its effect is like that of `ignore' (*note Break conditions: Conditions.). The argument IGNORE-COUNT is meaningful only when your program stopped due to a breakpoint. At other times, the argument to `continue' is ignored. The synonyms `c' and `fg' (for "foreground", as the debugged program is deemed to be the foreground program) are provided purely for convenience, and have exactly the same behavior as `continue'. To resume execution at a different place, you can use `return' (*note Returning from a function: Returning.) to go back to the calling function; or `jump' (*note Continuing at a different address: Jumping.) to go to an arbitrary location in your program. A typical technique for using stepping is to set a breakpoint (*note Breakpoints; watchpoints; and catchpoints: Breakpoints.) at the beginning of the function or the section of your program where a problem is believed to lie, run your program until it stops at that breakpoint, and then step through the suspect area, examining the variables that are interesting, until you see the problem happen. `step' Continue running your program until control reaches a different source line, then stop it and return control to GDB. This command is abbreviated `s'. _Warning:_ If you use the `step' command while control is within a function that was compiled without debugging information, execution proceeds until control reaches a function that does have debugging information. Likewise, it will not step into a function which is compiled without debugging information. To step through functions without debugging information, use the `stepi' command, described below. The `step' command only stops at the first instruction of a source line. This prevents the multiple stops that could otherwise occur in `switch' statements, `for' loops, etc. `step' continues to stop if a function that has debugging information is called within the line. In other words, `step' _steps inside_ any functions called within the line. Also, the `step' command only enters a function if there is line number information for the function. Otherwise it acts like the `next' command. This avoids problems when using `cc -gl' on MIPS machines. Previously, `step' entered subroutines if there was any debugging information about the routine. `step COUNT' Continue running as in `step', but do so COUNT times. If a breakpoint is reached, or a signal not related to stepping occurs before COUNT steps, stepping stops right away. `next [COUNT]' Continue to the next source line in the current (innermost) stack frame. This is similar to `step', but function calls that appear within the line of code are executed without stopping. Execution stops when control reaches a different line of code at the original stack level that was executing when you gave the `next' command. This command is abbreviated `n'. An argument COUNT is a repeat count, as for `step'. The `next' command only stops at the first instruction of a source line. This prevents multiple stops that could otherwise occur in `switch' statements, `for' loops, etc. `set step-mode' `set step-mode on' The `set step-mode on' command causes the `step' command to stop at the first instruction of a function which contains no debug line information rather than stepping over it. This is useful in cases where you may be interested in inspecting the machine instructions of a function which has no symbolic info and do not want GDB to automatically skip over this function. `set step-mode off' Causes the `step' command to step over any functions which contains no debug information. This is the default. `finish' Continue running until just after function in the selected stack frame returns. Print the returned value (if any). Contrast this with the `return' command (*note Returning from a function: Returning.). `until' `u' Continue running until a source line past the current line, in the current stack frame, is reached. This command is used to avoid single stepping through a loop more than once. It is like the `next' command, except that when `until' encounters a jump, it automatically continues execution until the program counter is greater than the address of the jump. This means that when you reach the end of a loop after single stepping though it, `until' makes your program continue execution until it exits the loop. In contrast, a `next' command at the end of a loop simply steps back to the beginning of the loop, which forces you to step through the next iteration. `until' always stops your program if it attempts to exit the current stack frame. `until' may produce somewhat counterintuitive results if the order of machine code does not match the order of the source lines. For example, in the following excerpt from a debugging session, the `f' (`frame') command shows that execution is stopped at line `206'; yet when we use `until', we get to line `195': (gdb) f #0 main (argc=4, argv=0xf7fffae8) at m4.c:206 206 expand_input(); (gdb) until 195 for ( ; argc > 0; NEXTARG) { This happened because, for execution efficiency, the compiler had generated code for the loop closure test at the end, rather than the start, of the loop--even though the test in a C `for'-loop is written before the body of the loop. The `until' command appeared to step back to the beginning of the loop when it advanced to this expression; however, it has not really gone to an earlier statement--not in terms of the actual machine code. `until' with no argument works by means of single instruction stepping, and hence is slower than `until' with an argument. `until LOCATION' `u LOCATION' Continue running your program until either the specified location is reached, or the current stack frame returns. LOCATION is any of the forms of argument acceptable to `break' (*note Setting breakpoints: Set Breaks.). This form of the command uses breakpoints, and hence is quicker than `until' without an argument. The specified location is actually reached only if it is in the current frame. This implies that `until' can be used to skip over recursive function invocations. For instance in the code below, if the current location is line `96', issuing `until 99' will execute the program up to line `99' in the same invocation of factorial, i.e. after the inner invocations have returned. 94 int factorial (int value) 95 { 96 if (value > 1) { 97 value *= factorial (value - 1); 98 } 99 return (value); 100 } `advance LOCATION' Continue running the program up to the given location. An argument is required, anything of the same form as arguments for the `break' command. Execution will also stop upon exit from the current stack frame. This command is similar to `until', but `advance' will not skip over recursive function calls, and the target location doesn't have to be in the same frame as the current one. `stepi' `stepi ARG' `si' Execute one machine instruction, then stop and return to the debugger. It is often useful to do `display/i $pc' when stepping by machine instructions. This makes GDB automatically display the next instruction to be executed, each time your program stops. *Note Automatic display: Auto Display. An argument is a repeat count, as in `step'. `nexti' `nexti ARG' `ni' Execute one machine instruction, but if it is a function call, proceed until the function returns. An argument is a repeat count, as in `next'.  File: gdb.info, Node: Signals, Next: Thread Stops, Prev: Continuing and Stepping, Up: Stopping 5.3 Signals =========== A signal is an asynchronous event that can happen in a program. The operating system defines the possible kinds of signals, and gives each kind a name and a number. For example, in Unix `SIGINT' is the signal a program gets when you type an interrupt character (often `C-c'); `SIGSEGV' is the signal a program gets from referencing a place in memory far away from all the areas in use; `SIGALRM' occurs when the alarm clock timer goes off (which happens only if your program has requested an alarm). Some signals, including `SIGALRM', are a normal part of the functioning of your program. Others, such as `SIGSEGV', indicate errors; these signals are "fatal" (they kill your program immediately) if the program has not specified in advance some other way to handle the signal. `SIGINT' does not indicate an error in your program, but it is normally fatal so it can carry out the purpose of the interrupt: to kill the program. GDB has the ability to detect any occurrence of a signal in your program. You can tell GDB in advance what to do for each kind of signal. Normally, GDB is set up to let the non-erroneous signals like `SIGALRM' be silently passed to your program (so as not to interfere with their role in the program's functioning) but to stop your program immediately whenever an error signal happens. You can change these settings with the `handle' command. `info signals' `info handle' Print a table of all the kinds of signals and how GDB has been told to handle each one. You can use this to see the signal numbers of all the defined types of signals. `info handle' is an alias for `info signals'. `handle SIGNAL KEYWORDS...' Change the way GDB handles signal SIGNAL. SIGNAL can be the number of a signal or its name (with or without the `SIG' at the beginning); a list of signal numbers of the form `LOW-HIGH'; or the word `all', meaning all the known signals. The KEYWORDS say what change to make. The keywords allowed by the `handle' command can be abbreviated. Their full names are: `nostop' GDB should not stop your program when this signal happens. It may still print a message telling you that the signal has come in. `stop' GDB should stop your program when this signal happens. This implies the `print' keyword as well. `print' GDB should print a message when this signal happens. `noprint' GDB should not mention the occurrence of the signal at all. This implies the `nostop' keyword as well. `pass' `noignore' GDB should allow your program to see this signal; your program can handle the signal, or else it may terminate if the signal is fatal and not handled. `pass' and `noignore' are synonyms. `nopass' `ignore' GDB should not allow your program to see this signal. `nopass' and `ignore' are synonyms. When a signal stops your program, the signal is not visible to the program until you continue. Your program sees the signal then, if `pass' is in effect for the signal in question _at that time_. In other words, after GDB reports a signal, you can use the `handle' command with `pass' or `nopass' to control whether your program sees that signal when you continue. The default is set to `nostop', `noprint', `pass' for non-erroneous signals such as `SIGALRM', `SIGWINCH' and `SIGCHLD', and to `stop', `print', `pass' for the erroneous signals. You can also use the `signal' command to prevent your program from seeing a signal, or cause it to see a signal it normally would not see, or to give it any signal at any time. For example, if your program stopped due to some sort of memory reference error, you might store correct values into the erroneous variables and continue, hoping to see more execution; but your program would probably terminate immediately as a result of the fatal signal once it saw the signal. To prevent this, you can continue with `signal 0'. *Note Giving your program a signal: Signaling.  File: gdb.info, Node: Thread Stops, Prev: Signals, Up: Stopping 5.4 Stopping and starting multi-thread programs =============================================== When your program has multiple threads (*note Debugging programs with multiple threads: Threads.), you can choose whether to set breakpoints on all threads, or on a particular thread. `break LINESPEC thread THREADNO' `break LINESPEC thread THREADNO if ...' LINESPEC specifies source lines; there are several ways of writing them, but the effect is always to specify some source line. Use the qualifier `thread THREADNO' with a breakpoint command to specify that you only want GDB to stop the program when a particular thread reaches this breakpoint. THREADNO is one of the numeric thread identifiers assigned by GDB, shown in the first column of the `info threads' display. If you do not specify `thread THREADNO' when you set a breakpoint, the breakpoint applies to _all_ threads of your program. You can use the `thread' qualifier on conditional breakpoints as well; in this case, place `thread THREADNO' before the breakpoint condition, like this: (gdb) break frik.c:13 thread 28 if bartab > lim Whenever your program stops under GDB for any reason, _all_ threads of execution stop, not just the current thread. This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot. There is an unfortunate side effect. If one thread stops for a breakpoint, or for some other reason, and another thread is blocked in a system call, then the system call may return prematurely. This is a consequence of the interaction between multiple threads and the signals that GDB uses to implement breakpoints and other events that stop execution. To handle this problem, your program should check the return value of each system call and react appropriately. This is good programming style anyways. For example, do not write code like this: sleep (10); The call to `sleep' will return early if a different thread stops at a breakpoint or for some other reason. Instead, write this: int unslept = 10; while (unslept > 0) unslept = sleep (unslept); A system call is allowed to return early, so the system is still conforming to its specification. But GDB does cause your multi-threaded program to behave differently than it would without GDB. Also, GDB uses internal breakpoints in the thread library to monitor certain events such as thread creation and thread destruction. When such an event happens, a system call in another thread may return prematurely, even though your program does not appear to stop. Conversely, whenever you restart the program, _all_ threads start executing. _This is true even when single-stepping_ with commands like `step' or `next'. In particular, GDB cannot single-step all threads in lockstep. Since thread scheduling is up to your debugging target's operating system (not controlled by GDB), other threads may execute more than one statement while the current thread completes a single step. Moreover, in general other threads stop in the middle of a statement, rather than at a clean statement boundary, when the program stops. You might even find your program stopped in another thread after continuing or even single-stepping. This happens whenever some other thread runs into a breakpoint, a signal, or an exception before the first thread completes whatever you requested. On some OSes, you can lock the OS scheduler and thus allow only a single thread to run. `set scheduler-locking MODE' Set the scheduler locking mode. If it is `off', then there is no locking and any thread may run at any time. If `on', then only the current thread may run when the inferior is resumed. The `step' mode optimizes for single-stepping. It stops other threads from "seizing the prompt" by preempting the current thread while you are stepping. Other threads will only rarely (or never) get a chance to run when you step. They are more likely to run when you `next' over a function call, and they are completely free to run when you use commands like `continue', `until', or `finish'. However, unless another thread hits a breakpoint during its timeslice, they will never steal the GDB prompt away from the thread that you are debugging. `show scheduler-locking' Display the current scheduler locking mode.  File: gdb.info, Node: Stack, Next: Source, Prev: Stopping, Up: Top 6 Examining the Stack ********************* When your program has stopped, the first thing you need to know is where it stopped and how it got there. Each time your program performs a function call, information about the call is generated. That information includes the location of the call in your program, the arguments of the call, and the local variables of the function being called. The information is saved in a block of data called a "stack frame". The stack frames are allocated in a region of memory called the "call stack". When your program stops, the GDB commands for examining the stack allow you to see all of this information. One of the stack frames is "selected" by GDB and many GDB commands refer implicitly to the selected frame. In particular, whenever you ask GDB for the value of a variable in your program, the value is found in the selected frame. There are special GDB commands to select whichever frame you are interested in. *Note Selecting a frame: Selection. When your program stops, GDB automatically selects the currently executing frame and describes it briefly, similar to the `frame' command (*note Information about a frame: Frame Info.). * Menu: * Frames:: Stack frames * Backtrace:: Backtraces * Selection:: Selecting a frame * Frame Info:: Information on a frame  File: gdb.info, Node: Frames, Next: Backtrace, Up: Stack 6.1 Stack frames ================ The call stack is divided up into contiguous pieces called "stack frames", or "frames" for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing. When your program is started, the stack has only one frame, that of the function `main'. This is called the "initial" frame or the "outermost" frame. Each time a function is called, a new frame is made. Each time a function returns, the frame for that function invocation is eliminated. If a function is recursive, there can be many frames for the same function. The frame for the function in which execution is actually occurring is called the "innermost" frame. This is the most recently created of all the stack frames that still exist. Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame. Usually this address is kept in a register called the "frame pointer register" while execution is going on in that frame. GDB assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by GDB to give you a way of designating stack frames in GDB commands. Some compilers provide a way to compile functions so that they operate without stack frames. (For example, the gcc option `-fomit-frame-pointer' generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. GDB has limited facilities for dealing with these function invocations. If the innermost function invocation has no stack frame, GDB nevertheless regards it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain. However, GDB has no provision for frameless functions elsewhere in the stack. `frame ARGS' The `frame' command allows you to move from one stack frame to another, and to print the stack frame you select. ARGS may be either the address of the frame or the stack frame number. Without an argument, `frame' prints the current stack frame. `select-frame' The `select-frame' command allows you to move from one stack frame to another without printing the frame. This is the silent version of `frame'.  File: gdb.info, Node: Backtrace, Next: Selection, Prev: Frames, Up: Stack 6.2 Backtraces ============== A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack. `backtrace' `bt' Print a backtrace of the entire stack: one line per frame for all frames in the stack. You can stop the backtrace at any time by typing the system interrupt character, normally `C-c'. `backtrace N' `bt N' Similar, but print only the innermost N frames. `backtrace -N' `bt -N' Similar, but print only the outermost N frames. The names `where' and `info stack' (abbreviated `info s') are additional aliases for `backtrace'. Each line in the backtrace shows the frame number and the function name. The program counter value is also shown--unless you use `set print address off'. The backtrace also shows the source file name and line number, as well as the arguments to the function. The program counter value is omitted if it is at the beginning of the code for that line number. Here is an example of a backtrace. It was made with the command `bt 3', so it shows the innermost three frames. #0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) at builtin.c:993 #1 0x6e38 in expand_macro (sym=0x2b600) at macro.c:242 #2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08) at macro.c:71 (More stack frames follow...) The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line `993' of `builtin.c'. Most programs have a standard user entry point--a place where system libraries and startup code transition into user code. For C this is `main'. When GDB finds the entry function in a backtrace it will terminate the backtrace, to avoid tracing into highly system-specific (and generally uninteresting) code. If you need to examine the startup code, or limit the number of levels in a backtrace, you can change this behavior: `set backtrace past-main' `set backtrace past-main on' Backtraces will continue past the user entry point. `set backtrace past-main off' Backtraces will stop when they encounter the user entry point. This is the default. `show backtrace past-main' Display the current user entry point backtrace policy. `set backtrace limit N' `set backtrace limit 0' Limit the backtrace to N levels. A value of zero means unlimited. `show backtrace limit' Display the current limit on backtrace levels.  File: gdb.info, Node: Selection, Next: Frame Info, Prev: Backtrace, Up: Stack 6.3 Selecting a frame ===================== Most commands for examining the stack and other data in your program work on whichever stack frame is selected at the moment. Here are the commands for selecting a stack frame; all of them finish by printing a brief description of the stack frame just selected. `frame N' `f N' Select frame number N. Recall that frame zero is the innermost (currently executing) frame, frame one is the frame that called the innermost one, and so on. The highest-numbered frame is the one for `main'. `frame ADDR' `f ADDR' Select the frame at address ADDR. This is useful mainly if the chaining of stack frames has been damaged by a bug, making it impossible for GDB to assign numbers properly to all frames. In addition, this can be useful when your program has multiple stacks and switches between them. On the SPARC architecture, `frame' needs two addresses to select an arbitrary frame: a frame pointer and a stack pointer. On the MIPS and Alpha architecture, it needs two addresses: a stack pointer and a program counter. On the 29k architecture, it needs three addresses: a register stack pointer, a program counter, and a memory stack pointer. `up N' Move N frames up the stack. For positive numbers N, this advances toward the outermost frame, to higher frame numbers, to frames that have existed longer. N defaults to one. `down N' Move N frames down the stack. For positive numbers N, this advances toward the innermost frame, to lower frame numbers, to frames that were created more recently. N defaults to one. You may abbreviate `down' as `do'. All of these commands end by printing two lines of output describing the frame. The first line shows the frame number, the function name, the arguments, and the source file and line number of execution in that frame. The second line shows the text of that source line. For example: (gdb) up #1 0x22f0 in main (argc=1, argv=0xf7fffbf4, env=0xf7fffbfc) at env.c:10 10 read_input_file (argv[i]); After such a printout, the `list' command with no arguments prints ten lines centered on the point of execution in the frame. You can also edit the program at the point of execution with your favorite editing program by typing `edit'. *Note Printing source lines: List, for details. `up-silently N' `down-silently N' These two commands are variants of `up' and `down', respectively; they differ in that they do their work silently, without causing display of the new frame. They are intended primarily for use in GDB command scripts, where the output might be unnecessary and distracting.  File: gdb.info, Node: Frame Info, Prev: Selection, Up: Stack 6.4 Information about a frame ============================= There are several other commands to print information about the selected stack frame. `frame' `f' When used without any argument, this command does not change which frame is selected, but prints a brief description of the currently selected stack frame. It can be abbreviated `f'. With an argument, this command is used to select a stack frame. *Note Selecting a frame: Selection. `info frame' `info f' This command prints a verbose description of the selected stack frame, including: * the address of the frame * the address of the next frame down (called by this frame) * the address of the next frame up (caller of this frame) * the language in which the source code corresponding to this frame is written * the address of the frame's arguments * the address of the frame's local variables * the program counter saved in it (the address of execution in the caller frame) * which registers were saved in the frame The verbose description is useful when something has gone wrong that has made the stack format fail to fit the usual conventions. `info frame ADDR' `info f ADDR' Print a verbose description of the frame at address ADDR, without selecting that frame. The selected frame remains unchanged by this command. This requires the same kind of address (more than one for some architectures) that you specify in the `frame' command. *Note Selecting a frame: Selection. `info args' Print the arguments of the selected frame, each on a separate line. `info locals' Print the local variables of the selected frame, each on a separate line. These are all variables (declared either static or automatic) accessible at the point of execution of the selected frame. `info catch' Print a list of all the exception handlers that are active in the current stack frame at the current point of execution. To see other exception handlers, visit the associated frame (using the `up', `down', or `frame' commands); then type `info catch'. *Note Setting catchpoints: Set Catchpoints.  File: gdb.info, Node: Source, Next: Data, Prev: Stack, Up: Top 7 Examining Source Files ************************ GDB can print parts of your program's source, since the debugging information recorded in the program tells GDB what source files were used to build it. When your program stops, GDB spontaneously prints the line where it stopped. Likewise, when you select a stack frame (*note Selecting a frame: Selection.), GDB prints the line where execution in that frame has stopped. You can print other portions of source files by explicit command. If you use GDB through its GNU Emacs interface, you may prefer to use Emacs facilities to view source; see *Note Using GDB under GNU Emacs: Emacs. * Menu: * List:: Printing source lines * Edit:: Editing source files * Search:: Searching source files * Source Path:: Specifying source directories * Machine Code:: Source and machine code  File: gdb.info, Node: List, Next: Edit, Up: Source 7.1 Printing source lines ========================= To print lines from a source file, use the `list' command (abbreviated `l'). By default, ten lines are printed. There are several ways to specify what part of the file you want to print. Here are the forms of the `list' command most commonly used: `list LINENUM' Print lines centered around line number LINENUM in the current source file. `list FUNCTION' Print lines centered around the beginning of function FUNCTION. `list' Print more lines. If the last lines printed were printed with a `list' command, this prints lines following the last lines printed; however, if the last line printed was a solitary line printed as part of displaying a stack frame (*note Examining the Stack: Stack.), this prints lines centered around that line. `list -' Print lines just before the lines last printed. By default, GDB prints ten source lines with any of these forms of the `list' command. You can change this using `set listsize': `set listsize COUNT' Make the `list' command display COUNT source lines (unless the `list' argument explicitly specifies some other number). `show listsize' Display the number of lines that `list' prints. Repeating a `list' command with discards the argument, so it is equivalent to typing just `list'. This is more useful than listing the same lines again. An exception is made for an argument of `-'; that argument is preserved in repetition so that each repetition moves up in the source file. In general, the `list' command expects you to supply zero, one or two "linespecs". Linespecs specify source lines; there are several ways of writing them, but the effect is always to specify some source line. Here is a complete description of the possible arguments for `list': `list LINESPEC' Print lines centered around the line specified by LINESPEC. `list FIRST,LAST' Print lines from FIRST to LAST. Both arguments are linespecs. `list ,LAST' Print lines ending with LAST. `list FIRST,' Print lines starting with FIRST. `list +' Print lines just after the lines last printed. `list -' Print lines just before the lines last printed. `list' As described in the preceding table. Here are the ways of specifying a single source line--all the kinds of linespec. `NUMBER' Specifies line NUMBER of the current source file. When a `list' command has two linespecs, this refers to the same source file as the first linespec. `+OFFSET' Specifies the line OFFSET lines after the last line printed. When used as the second linespec in a `list' command that has two, this specifies the line OFFSET lines down from the first linespec. `-OFFSET' Specifies the line OFFSET lines before the last line printed. `FILENAME:NUMBER' Specifies line NUMBER in the source file FILENAME. `FUNCTION' Specifies the line that begins the body of the function FUNCTION. For example: in C, this is the line with the open brace. `FILENAME:FUNCTION' Specifies the line of the open-brace that begins the body of the function FUNCTION in the file FILENAME. You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files. `*ADDRESS' Specifies the line containing the program address ADDRESS. ADDRESS may be any expression.  File: gdb.info, Node: Edit, Next: Search, Prev: List, Up: Source 7.2 Editing source files ======================== To edit the lines in a source file, use the `edit' command. The editing program of your choice is invoked with the current line set to the active line in the program. Alternatively, there are several ways to specify what part of the file you want to print if you want to see other parts of the program. Here are the forms of the `edit' command most commonly used: `edit' Edit the current source file at the active line number in the program. `edit NUMBER' Edit the current source file with NUMBER as the active line number. `edit FUNCTION' Edit the file containing FUNCTION at the beginning of its definition. `edit FILENAME:NUMBER' Specifies line NUMBER in the source file FILENAME. `edit FILENAME:FUNCTION' Specifies the line that begins the body of the function FUNCTION in the file FILENAME. You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files. `edit *ADDRESS' Specifies the line containing the program address ADDRESS. ADDRESS may be any expression. 7.2.1 Choosing your editor -------------------------- You can customize GDB to use any editor you want (1). By default, it is `/bin/ex', but you can change this by setting the environment variable `EDITOR' before using GDB. For example, to configure GDB to use the `vi' editor, you could use these commands with the `sh' shell: EDITOR=/usr/bin/vi export EDITOR gdb ... or in the `csh' shell, setenv EDITOR /usr/bin/vi gdb ... ---------- Footnotes ---------- (1) The only restriction is that your editor (say `ex'), recognizes the following command-line syntax: ex +NUMBER file The optional numeric value +NUMBER specifies the number of the line in the file where to start editing.  File: gdb.info, Node: Search, Next: Source Path, Prev: Edit, Up: Source 7.3 Searching source files ========================== There are two commands for searching through the current source file for a regular expression. `forward-search REGEXP' `search REGEXP' The command `forward-search REGEXP' checks each line, starting with the one following the last line listed, for a match for REGEXP. It lists the line that is found. You can use the synonym `search REGEXP' or abbreviate the command name as `fo'. `reverse-search REGEXP' The command `reverse-search REGEXP' checks each line, starting with the one before the last line listed and going backward, for a match for REGEXP. It lists the line that is found. You can abbreviate this command as `rev'.  File: gdb.info, Node: Source Path, Next: Machine Code, Prev: Search, Up: Source 7.4 Specifying source directories ================================= Executable programs sometimes do not record the directories of the source files from which they were compiled, just the names. Even when they do, the directories could be moved between the compilation and your debugging session. GDB has a list of directories to search for source files; this is called the "source path". Each time GDB wants a source file, it tries all the directories in the list, in the order they are present in the list, until it finds a file with the desired name. For example, suppose an executable references the file `/usr/src/foo-1.0/lib/foo.c', and our source path is `/mnt/cross'. The file is first looked up literally; if this fails, `/mnt/cross/usr/src/foo-1.0/lib/foo.c' is tried; if this fails, `/mnt/cross/foo.c' is opened; if this fails, an error message is printed. GDB does not look up the parts of the source file name, such as `/mnt/cross/src/foo-1.0/lib/foo.c'. Likewise, the subdirectories of the source path are not searched: if the source path is `/mnt/cross', and the binary refers to `foo.c', GDB would not find it under `/mnt/cross/usr/src/foo-1.0/lib'. Plain file names, relative file names with leading directories, file names containing dots, etc. are all treated as described above; for instance, if the source path is `/mnt/cross', and the source file is recorded as `../lib/foo.c', GDB would first try `../lib/foo.c', then `/mnt/cross/../lib/foo.c', and after that--`/mnt/cross/foo.c'. Note that the executable search path is _not_ used to locate the source files. Neither is the current working directory, unless it happens to be in the source path. Whenever you reset or rearrange the source path, GDB clears out any information it has cached about where source files are found and where each line is in the file. When you start GDB, its source path includes only `cdir' and `cwd', in that order. To add other directories, use the `directory' command. `directory DIRNAME ...' `dir DIRNAME ...' Add directory DIRNAME to the front of the source path. Several directory names may be given to this command, separated by `:' (`;' on MS-DOS and MS-Windows, where `:' usually appears as part of absolute file names) or whitespace. You may specify a directory that is already in the source path; this moves it forward, so GDB searches it sooner. You can use the string `$cdir' to refer to the compilation directory (if one is recorded), and `$cwd' to refer to the current working directory. `$cwd' is not the same as `.'--the former tracks the current working directory as it changes during your GDB session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path. `directory' Reset the source path to empty again. This requires confirmation. `show directories' Print the source path: show which directories it contains. If your source path is cluttered with directories that are no longer of interest, GDB may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows: 1. Use `directory' with no argument to reset the source path to empty. 2. Use `directory' with suitable arguments to reinstall the directories you want in the source path. You can add all the directories in one command.  File: gdb.info, Node: Machine Code, Prev: Source Path, Up: Source 7.5 Source and machine code =========================== You can use the command `info line' to map source lines to program addresses (and vice versa), and the command `disassemble' to display a range of addresses as machine instructions. When run under GNU Emacs mode, the `info line' command causes the arrow to point to the line specified. Also, `info line' prints addresses in symbolic form as well as hex. `info line LINESPEC' Print the starting and ending addresses of the compiled code for source line LINESPEC. You can specify source lines in any of the ways understood by the `list' command (*note Printing source lines: List.). For example, we can use `info line' to discover the location of the object code for the first line of function `m4_changequote': (gdb) info line m4_changequote Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350. We can also inquire (using `*ADDR' as the form for LINESPEC) what source line covers a particular address: (gdb) info line *0x63ff Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404. After `info line', the default address for the `x' command is changed to the starting address of the line, so that `x/i' is sufficient to begin examining the machine code (*note Examining memory: Memory.). Also, this address is saved as the value of the convenience variable `$_' (*note Convenience variables: Convenience Vars.). `disassemble' This specialized command dumps a range of memory as machine instructions. The default memory range is the function surrounding the program counter of the selected frame. A single argument to this command is a program counter value; GDB dumps the function surrounding this value. Two arguments specify a range of addresses (first inclusive, second exclusive) to dump. The following example shows the disassembly of a range of addresses of HP PA-RISC 2.0 code: (gdb) disas 0x32c4 0x32e4 Dump of assembler code from 0x32c4 to 0x32e4: 0x32c4 : addil 0,dp 0x32c8 : ldw 0x22c(sr0,r1),r26 0x32cc : ldil 0x3000,r31 0x32d0 : ble 0x3f8(sr4,r31) 0x32d4 : ldo 0(r31),rp 0x32d8 : addil -0x800,dp 0x32dc : ldo 0x588(r1),r26 0x32e0 : ldil 0x3000,r31 End of assembler dump. Some architectures have more than one commonly-used set of instruction mnemonics or other syntax. `set disassembly-flavor INSTRUCTION-SET' Select the instruction set to use when disassembling the program via the `disassemble' or `x/i' commands. Currently this command is only defined for the Intel x86 family. You can set INSTRUCTION-SET to either `intel' or `att'. The default is `att', the AT&T flavor used by default by Unix assemblers for x86-based targets.  File: gdb.info, Node: Data, Next: Macros, Prev: Source, Up: Top 8 Examining Data **************** The usual way to examine data in your program is with the `print' command (abbreviated `p'), or its synonym `inspect'. It evaluates and prints the value of an expression of the language your program is written in (*note Using GDB with Different Languages: Languages.). `print EXPR' `print /F EXPR' EXPR is an expression (in the source language). By default the value of EXPR is printed in a format appropriate to its data type; you can choose a different format by specifying `/F', where F is a letter specifying the format; see *Note Output formats: Output Formats. `print' `print /F' If you omit EXPR, GDB displays the last value again (from the "value history"; *note Value history: Value History.). This allows you to conveniently inspect the same value in an alternative format. A more low-level way of examining data is with the `x' command. It examines data in memory at a specified address and prints it in a specified format. *Note Examining memory: Memory. If you are interested in information about types, or about how the fields of a struct or a class are declared, use the `ptype EXP' command rather than `print'. *Note Examining the Symbol Table: Symbols. * Menu: * Expressions:: Expressions * Variables:: Program variables * Arrays:: Artificial arrays * Output Formats:: Output formats * Memory:: Examining memory * Auto Display:: Automatic display * Print Settings:: Print settings * Value History:: Value history * Convenience Vars:: Convenience variables * Registers:: Registers * Floating Point Hardware:: Floating point hardware * Vector Unit:: Vector Unit * Auxiliary Vector:: Auxiliary data provided by operating system * Memory Region Attributes:: Memory region attributes * Dump/Restore Files:: Copy between memory and a file * Character Sets:: Debugging programs that use a different character set than GDB does  File: gdb.info, Node: Expressions, Next: Variables, Up: Data 8.1 Expressions =============== `print' and many other GDB commands accept an expression and compute its value. Any kind of constant, variable or operator defined by the programming language you are using is valid in an expression in GDB. This includes conditional expressions, function calls, casts, and string constants. It also includes preprocessor macros, if you compiled your program to include this information; see *Note Compilation::. GDB supports array constants in expressions input by the user. The syntax is {ELEMENT, ELEMENT...}. For example, you can use the command `print {1, 2, 3}' to build up an array in memory that is `malloc'ed in the target program. Because C is so widespread, most of the expressions shown in examples in this manual are in C. *Note Using GDB with Different Languages: Languages, for information on how to use expressions in other languages. In this section, we discuss operators that you can use in GDB expressions regardless of your programming language. Casts are supported in all languages, not just in C, because it is so useful to cast a number into a pointer in order to examine a structure at that address in memory. GDB supports these operators, in addition to those common to programming languages: `@' `@' is a binary operator for treating parts of memory as arrays. *Note Artificial arrays: Arrays, for more information. `::' `::' allows you to specify a variable in terms of the file or function where it is defined. *Note Program variables: Variables. `{TYPE} ADDR' Refers to an object of type TYPE stored at address ADDR in memory. ADDR may be any expression whose value is an integer or pointer (but parentheses are required around binary operators, just as in a cast). This construct is allowed regardless of what kind of data is normally supposed to reside at ADDR.  File: gdb.info, Node: Variables, Next: Arrays, Prev: Expressions, Up: Data 8.2 Program variables ===================== The most common kind of expression to use is the name of a variable in your program. Variables in expressions are understood in the selected stack frame (*note Selecting a frame: Selection.); they must be either: * global (or file-static) or * visible according to the scope rules of the programming language from the point of execution in that frame This means that in the function foo (a) int a; { bar (a); { int b = test (); bar (b); } } you can examine and use the variable `a' whenever your program is executing within the function `foo', but you can only use or examine the variable `b' while your program is executing inside the block where `b' is declared. There is an exception: you can refer to a variable or function whose scope is a single source file even if the current execution point is not in this file. But it is possible to have more than one such variable or function with the same name (in different source files). If that happens, referring to that name has unpredictable effects. If you wish, you can specify a static variable in a particular function or file, using the colon-colon (`::') notation: FILE::VARIABLE FUNCTION::VARIABLE Here FILE or FUNCTION is the name of the context for the static VARIABLE. In the case of file names, you can use quotes to make sure GDB parses the file name as a single word--for example, to print a global value of `x' defined in `f2.c': (gdb) p 'f2.c'::x This use of `::' is very rarely in conflict with the very similar use of the same notation in C++. GDB also supports use of the C++ scope resolution operator in GDB expressions. _Warning:_ Occasionally, a local variable may appear to have the wrong value at certain points in a function--just after entry to a new scope, and just before exit. You may see this problem when you are stepping by machine instructions. This is because, on most machines, it takes more than one instruction to set up a stack frame (including local variable definitions); if you are stepping by machine instructions, variables may appear to have the wrong values until the stack frame is completely built. On exit, it usually also takes more than one machine instruction to destroy a stack frame; after you begin stepping through that group of instructions, local variable definitions may be gone. This may also happen when the compiler does significant optimizations. To be sure of always seeing accurate values, turn off all optimization when compiling. Another possible effect of compiler optimizations is to optimize unused variables out of existence, or assign variables to registers (as opposed to memory addresses). Depending on the support for such cases offered by the debug info format used by the compiler, GDB might not be able to display values for such local variables. If that happens, GDB will print a message like this: No symbol "foo" in current context. To solve such problems, either recompile without optimizations, or use a different debug info format, if the compiler supports several such formats. For example, GCC, the GNU C/C++ compiler, usually supports the `-gstabs+' option. `-gstabs+' produces debug info in a format that is superior to formats such as COFF. You may be able to use DWARF 2 (`-gdwarf-2'), which is also an effective form for debug info. *Note Options for Debugging Your Program or GNU CC: (gcc.info)Debugging Options. *Note Debugging C++: C, for more info about debug info formats that are best suited to C++ programs.  File: gdb.info, Node: Arrays, Next: Output Formats, Prev: Variables, Up: Data 8.3 Artificial arrays ===================== It is often useful to print out several successive objects of the same type in memory; a section of an array, or an array of dynamically determined size for which only a pointer exists in the program. You can do this by referring to a contiguous span of memory as an "artificial array", using the binary operator `@'. The left operand of `@' should be the first element of the desired array and be an individual object. The right operand should be the desired length of the array. The result is an array value whose elements are all of the type of the left argument. The first element is actually the left argument; the second element comes from bytes of memory immediately following those that hold the first element, and so on. Here is an example. If a program says int *array = (int *) malloc (len * sizeof (int)); you can print the contents of `array' with p *array@len The left operand of `@' must reside in memory. Array values made with `@' in this way behave just like other arrays in terms of subscripting, and are coerced to pointers when used in expressions. Artificial arrays most often appear in expressions via the value history (*note Value history: Value History.), after printing one out. Another way to create an artificial array is to use a cast. This re-interprets a value as if it were an array. The value need not be in memory: (gdb) p/x (short[2])0x12345678 $1 = {0x1234, 0x5678} As a convenience, if you leave the array length out (as in `(TYPE[])VALUE') GDB calculates the size to fill the value (as `sizeof(VALUE)/sizeof(TYPE)': (gdb) p/x (short[])0x12345678 $2 = {0x1234, 0x5678} Sometimes the artificial array mechanism is not quite enough; in moderately complex data structures, the elements of interest may not actually be adjacent--for example, if you are interested in the values of pointers in an array. One useful work-around in this situation is to use a convenience variable (*note Convenience variables: Convenience Vars.) as a counter in an expression that prints the first interesting value, and then repeat that expression via . For instance, suppose you have an array `dtab' of pointers to structures, and you are interested in the values of a field `fv' in each structure. Here is an example of what you might type: set $i = 0 p dtab[$i++]->fv ...  File: gdb.info, Node: Output Formats, Next: Memory, Prev: Arrays, Up: Data 8.4 Output formats ================== By default, GDB prints a value according to its data type. Sometimes this is not what you want. For example, you might want to print a number in hex, or a pointer in decimal. Or you might want to view data in memory at a certain address as a character string or as an instruction. To do these things, specify an "output format" when you print a value. The simplest use of output formats is to say how to print a value already computed. This is done by starting the arguments of the `print' command with a slash and a format letter. The format letters supported are: `x' Regard the bits of the value as an integer, and print the integer in hexadecimal. `d' Print as integer in signed decimal. `u' Print as integer in unsigned decimal. `o' Print as integer in octal. `t' Print as integer in binary. The letter `t' stands for "two". (1) `a' Print as an address, both absolute in hexadecimal and as an offset from the nearest preceding symbol. You can use this format used to discover where (in what function) an unknown address is located: (gdb) p/a 0x54320 $3 = 0x54320 <_initialize_vx+396> The command `info symbol 0x54320' yields similar results. *Note info symbol: Symbols. `c' Regard as an integer and print it as a character constant. `f' Regard the bits of the value as a floating point number and print using typical floating point syntax. For example, to print the program counter in hex (*note Registers::), type p/x $pc Note that no space is required before the slash; this is because command names in GDB cannot contain a slash. To reprint the last value in the value history with a different format, you can use the `print' command with just a format and no expression. For example, `p/x' reprints the last value in hex. ---------- Footnotes ---------- (1) `b' cannot be used because these format letters are also used with the `x' command, where `b' stands for "byte"; see *Note Examining memory: Memory.  File: gdb.info, Node: Memory, Next: Auto Display, Prev: Output Formats, Up: Data 8.5 Examining memory ==================== You can use the command `x' (for "examine") to examine memory in any of several formats, independently of your program's data types. `x/NFU ADDR' `x ADDR' `x' Use the `x' command to examine memory. N, F, and U are all optional parameters that specify how much memory to display and how to format it; ADDR is an expression giving the address where you want to start displaying memory. If you use defaults for NFU, you need not type the slash `/'. Several commands set convenient defaults for ADDR. N, the repeat count The repeat count is a decimal integer; the default is 1. It specifies how much memory (counting by units U) to display. F, the display format The display format is one of the formats used by `print', `s' (null-terminated string), or `i' (machine instruction). The default is `x' (hexadecimal) initially. The default changes each time you use either `x' or `print'. U, the unit size The unit size is any of `b' Bytes. `h' Halfwords (two bytes). `w' Words (four bytes). This is the initial default. `g' Giant words (eight bytes). Each time you specify a unit size with `x', that size becomes the default unit the next time you use `x'. (For the `s' and `i' formats, the unit size is ignored and is normally not written.) ADDR, starting display address ADDR is the address where you want GDB to begin displaying memory. The expression need not have a pointer value (though it may); it is always interpreted as an integer address of a byte of memory. *Note Expressions: Expressions, for more information on expressions. The default for ADDR is usually just after the last address examined--but several other commands also set the default address: `info breakpoints' (to the address of the last breakpoint listed), `info line' (to the starting address of a line), and `print' (if you use it to display a value from memory). For example, `x/3uh 0x54320' is a request to display three halfwords (`h') of memory, formatted as unsigned decimal integers (`u'), starting at address `0x54320'. `x/4xw $sp' prints the four words (`w') of memory above the stack pointer (here, `$sp'; *note Registers: Registers.) in hexadecimal (`x'). Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order works. The output specifications `4xw' and `4wx' mean exactly the same thing. (However, the count N must come first; `wx4' does not work.) Even though the unit size U is ignored for the formats `s' and `i', you might still want to use a count N; for example, `3i' specifies that you want to see three machine instructions, including any operands. The command `disassemble' gives an alternative way of inspecting machine instructions; see *Note Source and machine code: Machine Code. All the defaults for the arguments to `x' are designed to make it easy to continue scanning memory with minimal specifications each time you use `x'. For example, after you have inspected three machine instructions with `x/3i ADDR', you can inspect the next seven with just `x/7'. If you use to repeat the `x' command, the repeat count N is used again; the other arguments default as for successive uses of `x'. The addresses and contents printed by the `x' command are not saved in the value history because there is often too much of them and they would get in the way. Instead, GDB makes these values available for subsequent use in expressions as values of the convenience variables `$_' and `$__'. After an `x' command, the last address examined is available for use in expressions in the convenience variable `$_'. The contents of that address, as examined, are available in the convenience variable `$__'. If the `x' command has a repeat count, the address and contents saved are from the last memory unit printed; this is not the same as the last address printed if several units were printed on the last line of output.  File: gdb.info, Node: Auto Display, Next: Print Settings, Prev: Memory, Up: Data 8.6 Automatic display ===================== If you find that you want to print the value of an expression frequently (to see how it changes), you might want to add it to the "automatic display list" so that GDB prints its value each time your program stops. Each expression added to the list is given a number to identify it; to remove an expression from the list, you specify that number. The automatic display looks like this: 2: foo = 38 3: bar[5] = (struct hack *) 0x3804 This display shows item numbers, expressions and their current values. As with displays you request manually using `x' or `print', you can specify the output format you prefer; in fact, `display' decides whether to use `print' or `x' depending on how elaborate your format specification is--it uses `x' if you specify a unit size, or one of the two formats (`i' and `s') that are only supported by `x'; otherwise it uses `print'. `display EXPR' Add the expression EXPR to the list of expressions to display each time your program stops. *Note Expressions: Expressions. `display' does not repeat if you press again after using it. `display/FMT EXPR' For FMT specifying only a display format and not a size or count, add the expression EXPR to the auto-display list but arrange to display it each time in the specified format FMT. *Note Output formats: Output Formats. `display/FMT ADDR' For FMT `i' or `s', or including a unit-size or a number of units, add the expression ADDR as a memory address to be examined each time your program stops. Examining means in effect doing `x/FMT ADDR'. *Note Examining memory: Memory. For example, `display/i $pc' can be helpful, to see the machine instruction about to be executed each time execution stops (`$pc' is a common name for the program counter; *note Registers: Registers.). `undisplay DNUMS...' `delete display DNUMS...' Remove item numbers DNUMS from the list of expressions to display. `undisplay' does not repeat if you press after using it. (Otherwise you would just get the error `No display number ...'.) `disable display DNUMS...' Disable the display of item numbers DNUMS. A disabled display item is not printed automatically, but is not forgotten. It may be enabled again later. `enable display DNUMS...' Enable display of item numbers DNUMS. It becomes effective once again in auto display of its expression, until you specify otherwise. `display' Display the current values of the expressions on the list, just as is done when your program stops. `info display' Print the list of expressions previously set up to display automatically, each one with its item number, but without showing the values. This includes disabled expressions, which are marked as such. It also includes expressions which would not be displayed right now because they refer to automatic variables not currently available. If a display expression refers to local variables, then it does not make sense outside the lexical context for which it was set up. Such an expression is disabled when execution enters a context where one of its variables is not defined. For example, if you give the command `display last_char' while inside a function with an argument `last_char', GDB displays this argument while your program continues to stop inside that function. When it stops elsewhere--where there is no variable `last_char'--the display is disabled automatically. The next time your program stops where `last_char' is meaningful, you can enable the display expression once again.  File: gdb.info, Node: Print Settings, Next: Value History, Prev: Auto Display, Up: Data 8.7 Print settings ================== GDB provides the following ways to control how arrays, structures, and symbols are printed. These settings are useful for debugging programs in any language: `set print address' `set print address on' GDB prints memory addresses showing the location of stack traces, structure values, pointer values, breakpoints, and so forth, even when it also displays the contents of those addresses. The default is `on'. For example, this is what a stack frame display looks like with `set print address on': (gdb) f #0 set_quotes (lq=0x34c78 "<<", rq=0x34c88 ">>") at input.c:530 530 if (lquote != def_lquote) `set print address off' Do not print addresses when displaying their contents. For example, this is the same stack frame displayed with `set print address off': (gdb) set print addr off (gdb) f #0 set_quotes (lq="<<", rq=">>") at input.c:530 530 if (lquote != def_lquote) You can use `set print address off' to eliminate all machine dependent displays from the GDB interface. For example, with `print address off', you should get the same text for backtraces on all machines--whether or not they involve pointer arguments. `show print address' Show whether or not addresses are to be printed. When GDB prints a symbolic address, it normally prints the closest earlier symbol plus an offset. If that symbol does not uniquely identify the address (for example, it is a name whose scope is a single source file), you may need to clarify. One way to do this is with `info line', for example `info line *0x4537'. Alternately, you can set GDB to print the source file and line number when it prints a symbolic address: `set print symbol-filename on' Tell GDB to print the source file name and line number of a symbol in the symbolic form of an address. `set print symbol-filename off' Do not print source file name and line number of a symbol. This is the default. `show print symbol-filename' Show whether or not GDB will print the source file name and line number of a symbol in the symbolic form of an address. Another situation where it is helpful to show symbol filenames and line numbers is when disassembling code; GDB shows you the line number and source file that corresponds to each instruction. Also, you may wish to see the symbolic form only if the address being printed is reasonably close to the closest earlier symbol: `set print max-symbolic-offset MAX-OFFSET' Tell GDB to only display the symbolic form of an address if the offset between the closest earlier symbol and the address is less than MAX-OFFSET. The default is 0, which tells GDB to always print the symbolic form of an address if any symbol precedes it. `show print max-symbolic-offset' Ask how large the maximum offset is that GDB prints in a symbolic address. If you have a pointer and you are not sure where it points, try `set print symbol-filename on'. Then you can determine the name and source file location of the variable where it points, using `p/a POINTER'. This interprets the address in symbolic form. For example, here GDB shows that a variable `ptt' points at another variable `t', defined in `hi2.c': (gdb) set print symbol-filename on (gdb) p/a ptt $4 = 0xe008 _Warning:_ For pointers that point to a local variable, `p/a' does not show the symbol name and filename of the referent, even with the appropriate `set print' options turned on. Other settings control how different kinds of objects are printed: `set print array' `set print array on' Pretty print arrays. This format is more convenient to read, but uses more space. The default is off. `set print array off' Return to compressed format for arrays. `show print array' Show whether compressed or pretty format is selected for displaying arrays. `set print elements NUMBER-OF-ELEMENTS' Set a limit on how many elements of an array GDB will print. If GDB is printing a large array, it stops printing after it has printed the number of elements set by the `set print elements' command. This limit also applies to the display of strings. When GDB starts, this limit is set to 200. Setting NUMBER-OF-ELEMENTS to zero means that the printing is unlimited. `show print elements' Display the number of elements of a large array that GDB will print. If the number is 0, then the printing is unlimited. `set print null-stop' Cause GDB to stop printing the characters of an array when the first NULL is encountered. This is useful when large arrays actually contain only short strings. The default is off. `set print pretty on' Cause GDB to print structures in an indented format with one member per line, like this: $1 = { next = 0x0, flags = { sweet = 1, sour = 1 }, meat = 0x54 "Pork" } `set print pretty off' Cause GDB to print structures in a compact format, like this: $1 = {next = 0x0, flags = {sweet = 1, sour = 1}, \ meat = 0x54 "Pork"} This is the default format. `show print pretty' Show which format GDB is using to print structures. `set print sevenbit-strings on' Print using only seven-bit characters; if this option is set, GDB displays any eight-bit characters (in strings or character values) using the notation `\'NNN. This setting is best if you are working in English (ASCII) and you use the high-order bit of characters as a marker or "meta" bit. `set print sevenbit-strings off' Print full eight-bit characters. This allows the use of more international character sets, and is the default. `show print sevenbit-strings' Show whether or not GDB is printing only seven-bit characters. `set print union on' Tell GDB to print unions which are contained in structures. This is the default setting. `set print union off' Tell GDB not to print unions which are contained in structures. `show print union' Ask GDB whether or not it will print unions which are contained in structures. For example, given the declarations typedef enum {Tree, Bug} Species; typedef enum {Big_tree, Acorn, Seedling} Tree_forms; typedef enum {Caterpillar, Cocoon, Butterfly} Bug_forms; struct thing { Species it; union { Tree_forms tree; Bug_forms bug; } form; }; struct thing foo = {Tree, {Acorn}}; with `set print union on' in effect `p foo' would print $1 = {it = Tree, form = {tree = Acorn, bug = Cocoon}} and with `set print union off' in effect it would print $1 = {it = Tree, form = {...}} These settings are of interest when debugging C++ programs: `set print demangle' `set print demangle on' Print C++ names in their source form rather than in the encoded ("mangled") form passed to the assembler and linker for type-safe linkage. The default is on. `show print demangle' Show whether C++ names are printed in mangled or demangled form. `set print asm-demangle' `set print asm-demangle on' Print C++ names in their source form rather than their mangled form, even in assembler code printouts such as instruction disassemblies. The default is off. `show print asm-demangle' Show whether C++ names in assembly listings are printed in mangled or demangled form. `set demangle-style STYLE' Choose among several encoding schemes used by different compilers to represent C++ names. The choices for STYLE are currently: `auto' Allow GDB to choose a decoding style by inspecting your program. `gnu' Decode based on the GNU C++ compiler (`g++') encoding algorithm. This is the default. `hp' Decode based on the HP ANSI C++ (`aCC') encoding algorithm. `lucid' Decode based on the Lucid C++ compiler (`lcc') encoding algorithm. `arm' Decode using the algorithm in the `C++ Annotated Reference Manual'. *Warning:* this setting alone is not sufficient to allow debugging `cfront'-generated executables. GDB would require further enhancement to permit that. If you omit STYLE, you will see a list of possible formats. `show demangle-style' Display the encoding style currently in use for decoding C++ symbols. `set print object' `set print object on' When displaying a pointer to an object, identify the _actual_ (derived) type of the object rather than the _declared_ type, using the virtual function table. `set print object off' Display only the declared type of objects, without reference to the virtual function table. This is the default setting. `show print object' Show whether actual, or declared, object types are displayed. `set print static-members' `set print static-members on' Print static members when displaying a C++ object. The default is on. `set print static-members off' Do not print static members when displaying a C++ object. `show print static-members' Show whether C++ static members are printed, or not. `set print vtbl' `set print vtbl on' Pretty print C++ virtual function tables. The default is off. (The `vtbl' commands do not work on programs compiled with the HP ANSI C++ compiler (`aCC').) `set print vtbl off' Do not pretty print C++ virtual function tables. `show print vtbl' Show whether C++ virtual function tables are pretty printed, or not.  File: gdb.info, Node: Value History, Next: Convenience Vars, Prev: Print Settings, Up: Data 8.8 Value history ================= Values printed by the `print' command are saved in the GDB "value history". This allows you to refer to them in other expressions. Values are kept until the symbol table is re-read or discarded (for example with the `file' or `symbol-file' commands). When the symbol table changes, the value history is discarded, since the values may contain pointers back to the types defined in the symbol table. The values printed are given "history numbers" by which you can refer to them. These are successive integers starting with one. `print' shows you the history number assigned to a value by printing `$NUM = ' before the value; here NUM is the history number. To refer to any previous value, use `$' followed by the value's history number. The way `print' labels its output is designed to remind you of this. Just `$' refers to the most recent value in the history, and `$$' refers to the value before that. `$$N' refers to the Nth value from the end; `$$2' is the value just prior to `$$', `$$1' is equivalent to `$$', and `$$0' is equivalent to `$'. For example, suppose you have just printed a pointer to a structure and want to see the contents of the structure. It suffices to type p *$ If you have a chain of structures where the component `next' points to the next one, you can print the contents of the next one with this: p *$.next You can print successive links in the chain by repeating this command--which you can do by just typing . Note that the history records values, not expressions. If the value of `x' is 4 and you type these commands: print x set x=5 then the value recorded in the value history by the `print' command remains 4 even though the value of `x' has changed. `show values' Print the last ten values in the value history, with their item numbers. This is like `p $$9' repeated ten times, except that `show values' does not change the history. `show values N' Print ten history values centered on history item number N. `show values +' Print ten history values just after the values last printed. If no more values are available, `show values +' produces no display. Pressing to repeat `show values N' has exactly the same effect as `show values +'.  File: gdb.info, Node: Convenience Vars, Next: Registers, Prev: Value History, Up: Data 8.9 Convenience variables ========================= GDB provides "convenience variables" that you can use within GDB to hold on to a value and refer to it later. These variables exist entirely within GDB; they are not part of your program, and setting a convenience variable has no direct effect on further execution of your program. That is why you can use them freely. Convenience variables are prefixed with `$'. Any name preceded by `$' can be used for a convenience variable, unless it is one of the predefined machine-specific register names (*note Registers: Registers.). (Value history references, in contrast, are _numbers_ preceded by `$'. *Note Value history: Value History.) You can save a value in a convenience variable with an assignment expression, just as you would set a variable in your program. For example: set $foo = *object_ptr would save in `$foo' the value contained in the object pointed to by `object_ptr'. Using a convenience variable for the first time creates it, but its value is `void' until you assign a new value. You can alter the value with another assignment at any time. Convenience variables have no fixed types. You can assign a convenience variable any type of value, including structures and arrays, even if that variable already has a value of a different type. The convenience variable, when used as an expression, has the type of its current value. `show convenience' Print a list of convenience variables used so far, and their values. Abbreviated `show conv'. One of the ways to use a convenience variable is as a counter to be incremented or a pointer to be advanced. For example, to print a field from successive elements of an array of structures: set $i = 0 print bar[$i++]->contents Repeat that command by typing . Some convenience variables are created automatically by GDB and given values likely to be useful. `$_' The variable `$_' is automatically set by the `x' command to the last address examined (*note Examining memory: Memory.). Other commands which provide a default address for `x' to examine also set `$_' to that address; these commands include `info line' and `info breakpoint'. The type of `$_' is `void *' except when set by the `x' command, in which case it is a pointer to the type of `$__'. `$__' The variable `$__' is automatically set by the `x' command to the value found in the last address examined. Its type is chosen to match the format in which the data was printed. `$_exitcode' The variable `$_exitcode' is automatically set to the exit code when the program being debugged terminates. On HP-UX systems, if you refer to a function or variable name that begins with a dollar sign, GDB searches for a user or system name first, before it searches for a convenience variable.  File: gdb.info, Node: Registers, Next: Floating Point Hardware, Prev: Convenience Vars, Up: Data 8.10 Registers ============== You can refer to machine register contents, in expressions, as variables with names starting with `$'. The names of registers are different for each machine; use `info registers' to see the names used on your machine. `info registers' Print the names and values of all registers except floating-point and vector registers (in the selected stack frame). `info all-registers' Print the names and values of all registers, including floating-point and vector registers (in the selected stack frame). `info registers REGNAME ...' Print the "relativized" value of each specified register REGNAME. As discussed in detail below, register values are normally relative to the selected stack frame. REGNAME may be any register name valid on the machine you are using, with or without the initial `$'. GDB has four "standard" register names that are available (in expressions) on most machines--whenever they do not conflict with an architecture's canonical mnemonics for registers. The register names `$pc' and `$sp' are used for the program counter register and the stack pointer. `$fp' is used for a register that contains a pointer to the current stack frame, and `$ps' is used for a register that contains the processor status. For example, you could print the program counter in hex with p/x $pc or print the instruction to be executed next with x/i $pc or add four to the stack pointer(1) with set $sp += 4 Whenever possible, these four standard register names are available on your machine even though the machine has different canonical mnemonics, so long as there is no conflict. The `info registers' command shows the canonical names. For example, on the SPARC, `info registers' displays the processor status register as `$psr' but you can also refer to it as `$ps'; and on x86-based machines `$ps' is an alias for the EFLAGS register. GDB always considers the contents of an ordinary register as an integer when the register is examined in this way. Some machines have special registers which can hold nothing but floating point; these registers are considered to have floating point values. There is no way to refer to the contents of an ordinary register as floating point value (although you can _print_ it as a floating point value with `print/f $REGNAME'). Some registers have distinct "raw" and "virtual" data formats. This means that the data format in which the register contents are saved by the operating system is not the same one that your program normally sees. For example, the registers of the 68881 floating point coprocessor are always saved in "extended" (raw) format, but all C programs expect to work with "double" (virtual) format. In such cases, GDB normally works with the virtual format only (the format that makes sense for your program), but the `info registers' command prints the data in both formats. Normally, register values are relative to the selected stack frame (*note Selecting a frame: Selection.). This means that you get the value that the register would contain if all stack frames farther in were exited and their saved registers restored. In order to see the true contents of hardware registers, you must select the innermost frame (with `frame 0'). However, GDB must deduce where registers are saved, from the machine code generated by your compiler. If some registers are not saved, or if GDB is unable to locate the saved registers, the selected stack frame makes no difference. ---------- Footnotes ---------- (1) This is a way of removing one word from the stack, on machines where stacks grow downward in memory (most machines, nowadays). This assumes that the innermost stack frame is selected; setting `$sp' is not allowed when other stack frames are selected. To pop entire frames off the stack, regardless of machine architecture, use `return'; see *Note Returning from a function: Returning.  File: gdb.info, Node: Floating Point Hardware, Next: Vector Unit, Prev: Registers, Up: Data 8.11 Floating point hardware ============================ Depending on the configuration, GDB may be able to give you more information about the status of the floating point hardware. `info float' Display hardware-dependent information about the floating point unit. The exact contents and layout vary depending on the floating point chip. Currently, `info float' is supported on the ARM and x86 machines.  File: gdb.info, Node: Vector Unit, Next: Auxiliary Vector, Prev: Floating Point Hardware, Up: Data 8.12 Vector Unit ================ Depending on the configuration, GDB may be able to give you more information about the status of the vector unit. `info vector' Display information about the vector unit. The exact contents and layout vary depending on the hardware.  File: gdb.info, Node: Auxiliary Vector, Next: Memory Region Attributes, Prev: Vector Unit, Up: Data 8.13 Operating system auxiliary vector ====================================== Some operating systems supply an "auxiliary vector" to programs at startup. This is akin to the arguments and environment that you specify for a program, but contains a system-dependent variety of binary values that tell system libraries important details about the hardware, operating system, and process. Each value's purpose is identified by an integer tag; the meanings are well-known but system-specific. Depending on the configuration and operating system facilities, GDB may be able to show you this information. `info auxv' Display the auxiliary vector of the inferior, which can be either a live process or a core dump file. GDB prints each tag value numerically, and also shows names and text descriptions for recognized tags. Some values in the vector are numbers, some bit masks, and some pointers to strings or other data. GDB displays each value in the most appropriate form for a recognized tag, and in hexadecimal for an unrecognized tag.  File: gdb.info, Node: Memory Region Attributes, Next: Dump/Restore Files, Prev: Auxiliary Vector, Up: Data 8.14 Memory region attributes ============================= "Memory region attributes" allow you to describe special handling required by regions of your target's memory. GDB uses attributes to determine whether to allow certain types of memory accesses; whether to use specific width accesses; and whether to cache target memory. Defined memory regions can be individually enabled and disabled. When a memory region is disabled, GDB uses the default attributes when accessing memory in that region. Similarly, if no memory regions have been defined, GDB uses the default attributes when accessing all memory. When a memory region is defined, it is given a number to identify it; to enable, disable, or remove a memory region, you specify that number. `mem LOWER UPPER ATTRIBUTES...' Define memory region bounded by LOWER and UPPER with attributes ATTRIBUTES.... Note that UPPER == 0 is a special case: it is treated as the the target's maximum memory address. (0xffff on 16 bit targets, 0xffffffff on 32 bit targets, etc.) `delete mem NUMS...' Remove memory regions NUMS.... `disable mem NUMS...' Disable memory regions NUMS.... A disabled memory region is not forgotten. It may be enabled again later. `enable mem NUMS...' Enable memory regions NUMS.... `info mem' Print a table of all defined memory regions, with the following columns for each region. _Memory Region Number_ _Enabled or Disabled._ Enabled memory regions are marked with `y'. Disabled memory regions are marked with `n'. _Lo Address_ The address defining the inclusive lower bound of the memory region. _Hi Address_ The address defining the exclusive upper bound of the memory region. _Attributes_ The list of attributes set for this memory region. 8.14.1 Attributes ----------------- 8.14.1.1 Memory Access Mode ........................... The access mode attributes set whether GDB may make read or write accesses to a memory region. While these attributes prevent GDB from performing invalid memory accesses, they do nothing to prevent the target system, I/O DMA, etc. from accessing memory. `ro' Memory is read only. `wo' Memory is write only. `rw' Memory is read/write. This is the default. 8.14.1.2 Memory Access Size ........................... The acccess size attributes tells GDB to use specific sized accesses in the memory region. Often memory mapped device registers require specific sized accesses. If no access size attribute is specified, GDB may use accesses of any size. `8' Use 8 bit memory accesses. `16' Use 16 bit memory accesses. `32' Use 32 bit memory accesses. `64' Use 64 bit memory accesses. 8.14.1.3 Data Cache ................... The data cache attributes set whether GDB will cache target memory. While this generally improves performance by reducing debug protocol overhead, it can lead to incorrect results because GDB does not know about volatile variables or memory mapped device registers. `cache' Enable GDB to cache target memory. `nocache' Disable GDB from caching target memory. This is the default.  File: gdb.info, Node: Dump/Restore Files, Next: Character Sets, Prev: Memory Region Attributes, Up: Data 8.15 Copy between memory and a file =================================== You can use the commands `dump', `append', and `restore' to copy data between target memory and a file. The `dump' and `append' commands write data to a file, and the `restore' command reads data from a file back into the inferior's memory. Files may be in binary, Motorola S-record, Intel hex, or Tektronix Hex format; however, GDB can only append to binary files. `dump [FORMAT] memory FILENAME START_ADDR END_ADDR' `dump [FORMAT] value FILENAME EXPR' Dump the contents of memory from START_ADDR to END_ADDR, or the value of EXPR, to FILENAME in the given format. The FORMAT parameter may be any one of: `binary' Raw binary form. `ihex' Intel hex format. `srec' Motorola S-record format. `tekhex' Tektronix Hex format. GDB uses the same definitions of these formats as the GNU binary utilities, like `objdump' and `objcopy'. If FORMAT is omitted, GDB dumps the data in raw binary form. `append [binary] memory FILENAME START_ADDR END_ADDR' `append [binary] value FILENAME EXPR' Append the contents of memory from START_ADDR to END_ADDR, or the value of EXPR, to FILENAME, in raw binary form. (GDB can only append data to files in raw binary form.) `restore FILENAME [binary] BIAS START END' Restore the contents of file FILENAME into memory. The `restore' command can automatically recognize any known BFD file format, except for raw binary. To restore a raw binary file you must specify the optional keyword `binary' after the filename. If BIAS is non-zero, its value will be added to the addresses contained in the file. Binary files always start at address zero, so they will be restored at address BIAS. Other bfd files have a built-in location; they will be restored at offset BIAS from that location. If START and/or END are non-zero, then only data between file offset START and file offset END will be restored. These offsets are relative to the addresses in the file, before the BIAS argument is applied.  File: gdb.info, Node: Character Sets, Prev: Dump/Restore Files, Up: Data 8.16 Character Sets =================== If the program you are debugging uses a different character set to represent characters and strings than the one GDB uses itself, GDB can automatically translate between the character sets for you. The character set GDB uses we call the "host character set"; the one the inferior program uses we call the "target character set". For example, if you are running GDB on a GNU/Linux system, which uses the ISO Latin 1 character set, but you are using GDB's remote protocol (*note Remote Debugging: Remote.) to debug a program running on an IBM mainframe, which uses the EBCDIC character set, then the host character set is Latin-1, and the target character set is EBCDIC. If you give GDB the command `set target-charset EBCDIC-US', then GDB translates between EBCDIC and Latin 1 as you print character or string values, or use character and string literals in expressions. GDB has no way to automatically recognize which character set the inferior program uses; you must tell it, using the `set target-charset' command, described below. Here are the commands for controlling GDB's character set support: `set target-charset CHARSET' Set the current target character set to CHARSET. We list the character set names GDB recognizes below, but if you type `set target-charset' followed by , GDB will list the target character sets it supports. `set host-charset CHARSET' Set the current host character set to CHARSET. By default, GDB uses a host character set appropriate to the system it is running on; you can override that default using the `set host-charset' command. GDB can only use certain character sets as its host character set. We list the character set names GDB recognizes below, and indicate which can be host character sets, but if you type `set target-charset' followed by , GDB will list the host character sets it supports. `set charset CHARSET' Set the current host and target character sets to CHARSET. As above, if you type `set charset' followed by , GDB will list the name of the character sets that can be used for both host and target. `show charset' Show the names of the current host and target charsets. `show host-charset' Show the name of the current host charset. `show target-charset' Show the name of the current target charset. GDB currently includes support for the following character sets: `ASCII' Seven-bit U.S. ASCII. GDB can use this as its host character set. `ISO-8859-1' The ISO Latin 1 character set. This extends ASCII with accented characters needed for French, German, and Spanish. GDB can use this as its host character set. `EBCDIC-US' `IBM1047' Variants of the EBCDIC character set, used on some of IBM's mainframe operating systems. (GNU/Linux on the S/390 uses U.S. ASCII.) GDB cannot use these as its host character set. Note that these are all single-byte character sets. More work inside GDB is needed to support multi-byte or variable-width character encodings, like the UTF-8 and UCS-2 encodings of Unicode. Here is an example of GDB's character set support in action. Assume that the following source code has been placed in the file `charset-test.c': #include char ascii_hello[] = {72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33, 10, 0}; char ibm1047_hello[] = {200, 133, 147, 147, 150, 107, 64, 166, 150, 153, 147, 132, 90, 37, 0}; main () { printf ("Hello, world!\n"); } In this program, `ascii_hello' and `ibm1047_hello' are arrays containing the string `Hello, world!' followed by a newline, encoded in the ASCII and IBM1047 character sets. We compile the program, and invoke the debugger on it: $ gcc -g charset-test.c -o charset-test $ gdb -nw charset-test GNU gdb 2001-12-19-cvs Copyright 2001 Free Software Foundation, Inc. ... (gdb) We can use the `show charset' command to see what character sets GDB is currently using to interpret and display characters and strings: (gdb) show charset The current host and target character set is `ISO-8859-1'. (gdb) For the sake of printing this manual, let's use ASCII as our initial character set: (gdb) set charset ASCII (gdb) show charset The current host and target character set is `ASCII'. (gdb) Let's assume that ASCII is indeed the correct character set for our host system -- in other words, let's assume that if GDB prints characters using the ASCII character set, our terminal will display them properly. Since our current target character set is also ASCII, the contents of `ascii_hello' print legibly: (gdb) print ascii_hello $1 = 0x401698 "Hello, world!\n" (gdb) print ascii_hello[0] $2 = 72 'H' (gdb) GDB uses the target character set for character and string literals you use in expressions: (gdb) print '+' $3 = 43 '+' (gdb) The ASCII character set uses the number 43 to encode the `+' character. GDB relies on the user to tell it which character set the target program uses. If we print `ibm1047_hello' while our target character set is still ASCII, we get jibberish: (gdb) print ibm1047_hello $4 = 0x4016a8 "\310\205\223\223\226k@\246\226\231\223\204Z%" (gdb) print ibm1047_hello[0] $5 = 200 '\310' (gdb) If we invoke the `set target-charset' followed by , GDB tells us the character sets it supports: (gdb) set target-charset ASCII EBCDIC-US IBM1047 ISO-8859-1 (gdb) set target-charset We can select IBM1047 as our target character set, and examine the program's strings again. Now the ASCII string is wrong, but GDB translates the contents of `ibm1047_hello' from the target character set, IBM1047, to the host character set, ASCII, and they display correctly: (gdb) set target-charset IBM1047 (gdb) show charset The current host character set is `ASCII'. The current target character set is `IBM1047'. (gdb) print ascii_hello $6 = 0x401698 "\110\145%%?\054\040\167?\162%\144\041\012" (gdb) print ascii_hello[0] $7 = 72 '\110' (gdb) print ibm1047_hello $8 = 0x4016a8 "Hello, world!\n" (gdb) print ibm1047_hello[0] $9 = 200 'H' (gdb) As above, GDB uses the target character set for character and string literals you use in expressions: (gdb) print '+' $10 = 78 '+' (gdb) The IBM1047 character set uses the number 78 to encode the `+' character.  File: gdb.info, Node: Macros, Next: Tracepoints, Prev: Data, Up: Top 9 C Preprocessor Macros *********************** Some languages, such as C and C++, provide a way to define and invoke "preprocessor macros" which expand into strings of tokens. GDB can evaluate expressions containing macro invocations, show the result of macro expansion, and show a macro's definition, including where it was defined. You may need to compile your program specially to provide GDB with information about preprocessor macros. Most compilers do not include macros in their debugging information, even when you compile with the `-g' flag. *Note Compilation::. A program may define a macro at one point, remove that definition later, and then provide a different definition after that. Thus, at different points in the program, a macro may have different definitions, or have no definition at all. If there is a current stack frame, GDB uses the macros in scope at that frame's source code line. Otherwise, GDB uses the macros in scope at the current listing location; see *Note List::. At the moment, GDB does not support the `##' token-splicing operator, the `#' stringification operator, or variable-arity macros. Whenever GDB evaluates an expression, it always expands any macro invocations present in the expression. GDB also provides the following commands for working with macros explicitly. `macro expand EXPRESSION' `macro exp EXPRESSION' Show the results of expanding all preprocessor macro invocations in EXPRESSION. Since GDB simply expands macros, but does not parse the result, EXPRESSION need not be a valid expression; it can be any string of tokens. `macro expand-once EXPRESSION' `macro exp1 EXPRESSION' (This command is not yet implemented.) Show the results of expanding those preprocessor macro invocations that appear explicitly in EXPRESSION. Macro invocations appearing in that expansion are left unchanged. This command allows you to see the effect of a particular macro more clearly, without being confused by further expansions. Since GDB simply expands macros, but does not parse the result, EXPRESSION need not be a valid expression; it can be any string of tokens. `info macro MACRO' Show the definition of the macro named MACRO, and describe the source location where that definition was established. `macro define MACRO REPLACEMENT-LIST' `macro define MACRO(ARGLIST) REPLACEMENT-LIST' (This command is not yet implemented.) Introduce a definition for a preprocessor macro named MACRO, invocations of which are replaced by the tokens given in REPLACEMENT-LIST. The first form of this command defines an "object-like" macro, which takes no arguments; the second form defines a "function-like" macro, which takes the arguments given in ARGLIST. A definition introduced by this command is in scope in every expression evaluated in GDB, until it is removed with the `macro undef' command, described below. The definition overrides all definitions for MACRO present in the program being debugged, as well as any previous user-supplied definition. `macro undef MACRO' (This command is not yet implemented.) Remove any user-supplied definition for the macro named MACRO. This command only affects definitions provided with the `macro define' command, described above; it cannot remove definitions present in the program being debugged. Here is a transcript showing the above commands in action. First, we show our source files: $ cat sample.c #include #include "sample.h" #define M 42 #define ADD(x) (M + x) main () { #define N 28 printf ("Hello, world!\n"); #undef N printf ("We're so creative.\n"); #define N 1729 printf ("Goodbye, world!\n"); } $ cat sample.h #define Q < $ Now, we compile the program using the GNU C compiler, GCC. We pass the `-gdwarf-2' and `-g3' flags to ensure the compiler includes information about preprocessor macros in the debugging information. $ gcc -gdwarf-2 -g3 sample.c -o sample $ Now, we start GDB on our sample program: $ gdb -nw sample GNU gdb 2002-05-06-cvs Copyright 2002 Free Software Foundation, Inc. GDB is free software, ... (gdb) We can expand macros and examine their definitions, even when the program is not running. GDB uses the current listing position to decide which macro definitions are in scope: (gdb) list main 3 4 #define M 42 5 #define ADD(x) (M + x) 6 7 main () 8 { 9 #define N 28 10 printf ("Hello, world!\n"); 11 #undef N 12 printf ("We're so creative.\n"); (gdb) info macro ADD Defined at /home/jimb/gdb/macros/play/sample.c:5 #define ADD(x) (M + x) (gdb) info macro Q Defined at /home/jimb/gdb/macros/play/sample.h:1 included at /home/jimb/gdb/macros/play/sample.c:2 #define Q < (gdb) macro expand ADD(1) expands to: (42 + 1) (gdb) macro expand-once ADD(1) expands to: once (M + 1) (gdb) In the example above, note that `macro expand-once' expands only the macro invocation explicit in the original text -- the invocation of `ADD' -- but does not expand the invocation of the macro `M', which was introduced by `ADD'. Once the program is running, GDB uses the macro definitions in force at the source line of the current stack frame: (gdb) break main Breakpoint 1 at 0x8048370: file sample.c, line 10. (gdb) run Starting program: /home/jimb/gdb/macros/play/sample Breakpoint 1, main () at sample.c:10 10 printf ("Hello, world!\n"); (gdb) At line 10, the definition of the macro `N' at line 9 is in force: (gdb) info macro N Defined at /home/jimb/gdb/macros/play/sample.c:9 #define N 28 (gdb) macro expand N Q M expands to: 28 < 42 (gdb) print N Q M $1 = 1 (gdb) As we step over directives that remove `N''s definition, and then give it a new definition, GDB finds the definition (or lack thereof) in force at each point: (gdb) next Hello, world! 12 printf ("We're so creative.\n"); (gdb) info macro N The symbol `N' has no definition as a C/C++ preprocessor macro at /home/jimb/gdb/macros/play/sample.c:12 (gdb) next We're so creative. 14 printf ("Goodbye, world!\n"); (gdb) info macro N Defined at /home/jimb/gdb/macros/play/sample.c:13 #define N 1729 (gdb) macro expand N Q M expands to: 1729 < 42 (gdb) print N Q M $2 = 0 (gdb)  File: gdb.info, Node: Tracepoints, Next: Overlays, Prev: Macros, Up: Top 10 Tracepoints ************** In some applications, it is not feasible for the debugger to interrupt the program's execution long enough for the developer to learn anything helpful about its behavior. If the program's correctness depends on its real-time behavior, delays introduced by a debugger might cause the program to change its behavior drastically, or perhaps fail, even when the code itself is correct. It is useful to be able to observe the program's behavior without interrupting it. Using GDB's `trace' and `collect' commands, you can specify locations in the program, called "tracepoints", and arbitrary expressions to evaluate when those tracepoints are reached. Later, using the `tfind' command, you can examine the values those expressions had when the program hit the tracepoints. The expressions may also denote objects in memory--structures or arrays, for example--whose values GDB should record; while visiting a particular tracepoint, you may inspect those objects as if they were in memory at that moment. However, because GDB records these values without interacting with you, it can do so quickly and unobtrusively, hopefully not disturbing the program's behavior. The tracepoint facility is currently available only for remote targets. *Note Targets::. In addition, your remote target must know how to collect trace data. This functionality is implemented in the remote stub; however, none of the stubs distributed with GDB support tracepoints as of this writing. This chapter describes the tracepoint commands and features. * Menu: * Set Tracepoints:: * Analyze Collected Data:: * Tracepoint Variables::  File: gdb.info, Node: Set Tracepoints, Next: Analyze Collected Data, Up: Tracepoints 10.1 Commands to Set Tracepoints ================================ Before running such a "trace experiment", an arbitrary number of tracepoints can be set. Like a breakpoint (*note Set Breaks::), a tracepoint has a number assigned to it by GDB. Like with breakpoints, tracepoint numbers are successive integers starting from one. Many of the commands associated with tracepoints take the tracepoint number as their argument, to identify which tracepoint to work on. For each tracepoint, you can specify, in advance, some arbitrary set of data that you want the target to collect in the trace buffer when it hits that tracepoint. The collected data can include registers, local variables, or global data. Later, you can use GDB commands to examine the values these data had at the time the tracepoint was hit. This section describes commands to set tracepoints and associated conditions and actions. * Menu: * Create and Delete Tracepoints:: * Enable and Disable Tracepoints:: * Tracepoint Passcounts:: * Tracepoint Actions:: * Listing Tracepoints:: * Starting and Stopping Trace Experiment::  File: gdb.info, Node: Create and Delete Tracepoints, Next: Enable and Disable Tracepoints, Up: Set Tracepoints 10.1.1 Create and Delete Tracepoints ------------------------------------ `trace' The `trace' command is very similar to the `break' command. Its argument can be a source line, a function name, or an address in the target program. *Note Set Breaks::. The `trace' command defines a tracepoint, which is a point in the target program where the debugger will briefly stop, collect some data, and then allow the program to continue. Setting a tracepoint or changing its commands doesn't take effect until the next `tstart' command; thus, you cannot change the tracepoint attributes once a trace experiment is running. Here are some examples of using the `trace' command: (gdb) trace foo.c:121 // a source file and line number (gdb) trace +2 // 2 lines forward (gdb) trace my_function // first source line of function (gdb) trace *my_function // EXACT start address of function (gdb) trace *0x2117c4 // an address You can abbreviate `trace' as `tr'. The convenience variable `$tpnum' records the tracepoint number of the most recently set tracepoint. `delete tracepoint [NUM]' Permanently delete one or more tracepoints. With no argument, the default is to delete all tracepoints. Examples: (gdb) delete trace 1 2 3 // remove three tracepoints (gdb) delete trace // remove all tracepoints You can abbreviate this command as `del tr'.  File: gdb.info, Node: Enable and Disable Tracepoints, Next: Tracepoint Passcounts, Prev: Create and Delete Tracepoints, Up: Set Tracepoints 10.1.2 Enable and Disable Tracepoints ------------------------------------- `disable tracepoint [NUM]' Disable tracepoint NUM, or all tracepoints if no argument NUM is given. A disabled tracepoint will have no effect during the next trace experiment, but it is not forgotten. You can re-enable a disabled tracepoint using the `enable tracepoint' command. `enable tracepoint [NUM]' Enable tracepoint NUM, or all tracepoints. The enabled tracepoints will become effective the next time a trace experiment is run.  File: gdb.info, Node: Tracepoint Passcounts, Next: Tracepoint Actions, Prev: Enable and Disable Tracepoints, Up: Set Tracepoints 10.1.3 Tracepoint Passcounts ---------------------------- `passcount [N [NUM]]' Set the "passcount" of a tracepoint. The passcount is a way to automatically stop a trace experiment. If a tracepoint's passcount is N, then the trace experiment will be automatically stopped on the N'th time that tracepoint is hit. If the tracepoint number NUM is not specified, the `passcount' command sets the passcount of the most recently defined tracepoint. If no passcount is given, the trace experiment will run until stopped explicitly by the user. Examples: (gdb) passcount 5 2 // Stop on the 5th execution of `// tracepoint 2' (gdb) passcount 12 // Stop on the 12th execution of the `// most recently defined tracepoint.' (gdb) trace foo (gdb) pass 3 (gdb) trace bar (gdb) pass 2 (gdb) trace baz (gdb) pass 1 // Stop tracing when foo has been `// executed 3 times OR when bar has' `// been executed 2 times' `// OR when baz has been executed 1 time.'  File: gdb.info, Node: Tracepoint Actions, Next: Listing Tracepoints, Prev: Tracepoint Passcounts, Up: Set Tracepoints 10.1.4 Tracepoint Action Lists ------------------------------ `actions [NUM]' This command will prompt for a list of actions to be taken when the tracepoint is hit. If the tracepoint number NUM is not specified, this command sets the actions for the one that was most recently defined (so that you can define a tracepoint and then say `actions' without bothering about its number). You specify the actions themselves on the following lines, one action at a time, and terminate the actions list with a line containing just `end'. So far, the only defined actions are `collect' and `while-stepping'. To remove all actions from a tracepoint, type `actions NUM' and follow it immediately with `end'. (gdb) collect DATA // collect some data (gdb) while-stepping 5 // single-step 5 times, collect data (gdb) end // signals the end of actions. In the following example, the action list begins with `collect' commands indicating the things to be collected when the tracepoint is hit. Then, in order to single-step and collect additional data following the tracepoint, a `while-stepping' command is used, followed by the list of things to be collected while stepping. The `while-stepping' command is terminated by its own separate `end' command. Lastly, the action list is terminated by an `end' command. (gdb) trace foo (gdb) actions Enter actions for tracepoint 1, one per line: > collect bar,baz > collect $regs > while-stepping 12 > collect $fp, $sp > end end `collect EXPR1, EXPR2, ...' Collect values of the given expressions when the tracepoint is hit. This command accepts a comma-separated list of any valid expressions. In addition to global, static, or local variables, the following special arguments are supported: `$regs' collect all registers `$args' collect all function arguments `$locals' collect all local variables. You can give several consecutive `collect' commands, each one with a single argument, or one `collect' command with several arguments separated by commas: the effect is the same. The command `info scope' (*note info scope: Symbols.) is particularly useful for figuring out what data to collect. `while-stepping N' Perform N single-step traces after the tracepoint, collecting new data at each step. The `while-stepping' command is followed by the list of what to collect while stepping (followed by its own `end' command): > while-stepping 12 > collect $regs, myglobal > end > You may abbreviate `while-stepping' as `ws' or `stepping'.  File: gdb.info, Node: Listing Tracepoints, Next: Starting and Stopping Trace Experiment, Prev: Tracepoint Actions, Up: Set Tracepoints 10.1.5 Listing Tracepoints -------------------------- `info tracepoints [NUM]' Display information about the tracepoint NUM. If you don't specify a tracepoint number, displays information about all the tracepoints defined so far. For each tracepoint, the following information is shown: * its number * whether it is enabled or disabled * its address * its passcount as given by the `passcount N' command * its step count as given by the `while-stepping N' command * where in the source files is the tracepoint set * its action list as given by the `actions' command (gdb) info trace Num Enb Address PassC StepC What 1 y 0x002117c4 0 0 2 y 0x0020dc64 0 0 in g_test at g_test.c:1375 3 y 0x0020b1f4 0 0 in get_data at ../foo.c:41 (gdb) This command can be abbreviated `info tp'.  File: gdb.info, Node: Starting and Stopping Trace Experiment, Prev: Listing Tracepoints, Up: Set Tracepoints 10.1.6 Starting and Stopping Trace Experiment --------------------------------------------- `tstart' This command takes no arguments. It starts the trace experiment, and begins collecting data. This has the side effect of discarding all the data collected in the trace buffer during the previous trace experiment. `tstop' This command takes no arguments. It ends the trace experiment, and stops collecting data. *Note:* a trace experiment and data collection may stop automatically if any tracepoint's passcount is reached (*note Tracepoint Passcounts::), or if the trace buffer becomes full. `tstatus' This command displays the status of the current trace data collection. Here is an example of the commands we described so far: (gdb) trace gdb_c_test (gdb) actions Enter actions for tracepoint #1, one per line. > collect $regs,$locals,$args > while-stepping 11 > collect $regs > end > end (gdb) tstart [time passes ...] (gdb) tstop  File: gdb.info, Node: Analyze Collected Data, Next: Tracepoint Variables, Prev: Set Tracepoints, Up: Tracepoints 10.2 Using the collected data ============================= After the tracepoint experiment ends, you use GDB commands for examining the trace data. The basic idea is that each tracepoint collects a trace "snapshot" every time it is hit and another snapshot every time it single-steps. All these snapshots are consecutively numbered from zero and go into a buffer, and you can examine them later. The way you examine them is to "focus" on a specific trace snapshot. When the remote stub is focused on a trace snapshot, it will respond to all GDB requests for memory and registers by reading from the buffer which belongs to that snapshot, rather than from _real_ memory or registers of the program being debugged. This means that *all* GDB commands (`print', `info registers', `backtrace', etc.) will behave as if we were currently debugging the program state as it was when the tracepoint occurred. Any requests for data that are not in the buffer will fail. * Menu: * tfind:: How to select a trace snapshot * tdump:: How to display all data for a snapshot * save-tracepoints:: How to save tracepoints for a future run  File: gdb.info, Node: tfind, Next: tdump, Up: Analyze Collected Data 10.2.1 `tfind N' ---------------- The basic command for selecting a trace snapshot from the buffer is `tfind N', which finds trace snapshot number N, counting from zero. If no argument N is given, the next snapshot is selected. Here are the various forms of using the `tfind' command. `tfind start' Find the first snapshot in the buffer. This is a synonym for `tfind 0' (since 0 is the number of the first snapshot). `tfind none' Stop debugging trace snapshots, resume _live_ debugging. `tfind end' Same as `tfind none'. `tfind' No argument means find the next trace snapshot. `tfind -' Find the previous trace snapshot before the current one. This permits retracing earlier steps. `tfind tracepoint NUM' Find the next snapshot associated with tracepoint NUM. Search proceeds forward from the last examined trace snapshot. If no argument NUM is given, it means find the next snapshot collected for the same tracepoint as the current snapshot. `tfind pc ADDR' Find the next snapshot associated with the value ADDR of the program counter. Search proceeds forward from the last examined trace snapshot. If no argument ADDR is given, it means find the next snapshot with the same value of PC as the current snapshot. `tfind outside ADDR1, ADDR2' Find the next snapshot whose PC is outside the given range of addresses. `tfind range ADDR1, ADDR2' Find the next snapshot whose PC is between ADDR1 and ADDR2. `tfind line [FILE:]N' Find the next snapshot associated with the source line N. If the optional argument FILE is given, refer to line N in that source file. Search proceeds forward from the last examined trace snapshot. If no argument N is given, it means find the next line other than the one currently being examined; thus saying `tfind line' repeatedly can appear to have the same effect as stepping from line to line in a _live_ debugging session. The default arguments for the `tfind' commands are specifically designed to make it easy to scan through the trace buffer. For instance, `tfind' with no argument selects the next trace snapshot, and `tfind -' with no argument selects the previous trace snapshot. So, by giving one `tfind' command, and then simply hitting repeatedly you can examine all the trace snapshots in order. Or, by saying `tfind -' and then hitting repeatedly you can examine the snapshots in reverse order. The `tfind line' command with no argument selects the snapshot for the next source line executed. The `tfind pc' command with no argument selects the next snapshot with the same program counter (PC) as the current frame. The `tfind tracepoint' command with no argument selects the next trace snapshot collected by the same tracepoint as the current one. In addition to letting you scan through the trace buffer manually, these commands make it easy to construct GDB scripts that scan through the trace buffer and print out whatever collected data you are interested in. Thus, if we want to examine the PC, FP, and SP registers from each trace frame in the buffer, we can say this: (gdb) tfind start (gdb) while ($trace_frame != -1) > printf "Frame %d, PC = %08X, SP = %08X, FP = %08X\n", \ $trace_frame, $pc, $sp, $fp > tfind > end Frame 0, PC = 0020DC64, SP = 0030BF3C, FP = 0030BF44 Frame 1, PC = 0020DC6C, SP = 0030BF38, FP = 0030BF44 Frame 2, PC = 0020DC70, SP = 0030BF34, FP = 0030BF44 Frame 3, PC = 0020DC74, SP = 0030BF30, FP = 0030BF44 Frame 4, PC = 0020DC78, SP = 0030BF2C, FP = 0030BF44 Frame 5, PC = 0020DC7C, SP = 0030BF28, FP = 0030BF44 Frame 6, PC = 0020DC80, SP = 0030BF24, FP = 0030BF44 Frame 7, PC = 0020DC84, SP = 0030BF20, FP = 0030BF44 Frame 8, PC = 0020DC88, SP = 0030BF1C, FP = 0030BF44 Frame 9, PC = 0020DC8E, SP = 0030BF18, FP = 0030BF44 Frame 10, PC = 00203F6C, SP = 0030BE3C, FP = 0030BF14 Or, if we want to examine the variable `X' at each source line in the buffer: (gdb) tfind start (gdb) while ($trace_frame != -1) > printf "Frame %d, X == %d\n", $trace_frame, X > tfind line > end Frame 0, X = 1 Frame 7, X = 2 Frame 13, X = 255  File: gdb.info, Node: tdump, Next: save-tracepoints, Prev: tfind, Up: Analyze Collected Data 10.2.2 `tdump' -------------- This command takes no arguments. It prints all the data collected at the current trace snapshot. (gdb) trace 444 (gdb) actions Enter actions for tracepoint #2, one per line: > collect $regs, $locals, $args, gdb_long_test > end (gdb) tstart (gdb) tfind line 444 #0 gdb_test (p1=0x11, p2=0x22, p3=0x33, p4=0x44, p5=0x55, p6=0x66) at gdb_test.c:444 444 printp( "%s: arguments = 0x%X 0x%X 0x%X 0x%X 0x%X 0x%X\n", ) (gdb) tdump Data collected at tracepoint 2, trace frame 1: d0 0xc4aa0085 -995491707 d1 0x18 24 d2 0x80 128 d3 0x33 51 d4 0x71aea3d 119204413 d5 0x22 34 d6 0xe0 224 d7 0x380035 3670069 a0 0x19e24a 1696330 a1 0x3000668 50333288 a2 0x100 256 a3 0x322000 3284992 a4 0x3000698 50333336 a5 0x1ad3cc 1758156 fp 0x30bf3c 0x30bf3c sp 0x30bf34 0x30bf34 ps 0x0 0 pc 0x20b2c8 0x20b2c8 fpcontrol 0x0 0 fpstatus 0x0 0 fpiaddr 0x0 0 p = 0x20e5b4 "gdb-test" p1 = (void *) 0x11 p2 = (void *) 0x22 p3 = (void *) 0x33 p4 = (void *) 0x44 p5 = (void *) 0x55 p6 = (void *) 0x66 gdb_long_test = 17 '\021' (gdb)  File: gdb.info, Node: save-tracepoints, Prev: tdump, Up: Analyze Collected Data 10.2.3 `save-tracepoints FILENAME' ---------------------------------- This command saves all current tracepoint definitions together with their actions and passcounts, into a file `FILENAME' suitable for use in a later debugging session. To read the saved tracepoint definitions, use the `source' command (*note Command Files::).  File: gdb.info, Node: Tracepoint Variables, Prev: Analyze Collected Data, Up: Tracepoints 10.3 Convenience Variables for Tracepoints ========================================== `(int) $trace_frame' The current trace snapshot (a.k.a. "frame") number, or -1 if no snapshot is selected. `(int) $tracepoint' The tracepoint for the current trace snapshot. `(int) $trace_line' The line number for the current trace snapshot. `(char []) $trace_file' The source file for the current trace snapshot. `(char []) $trace_func' The name of the function containing `$tracepoint'. Note: `$trace_file' is not suitable for use in `printf', use `output' instead. Here's a simple example of using these convenience variables for stepping through all the trace snapshots and printing some of their data. (gdb) tfind start (gdb) while $trace_frame != -1 > output $trace_file > printf ", line %d (tracepoint #%d)\n", $trace_line, $tracepoint > tfind > end  File: gdb.info, Node: Overlays, Next: Languages, Prev: Tracepoints, Up: Top 11 Debugging Programs That Use Overlays *************************************** If your program is too large to fit completely in your target system's memory, you can sometimes use "overlays" to work around this problem. GDB provides some support for debugging programs that use overlays. * Menu: * How Overlays Work:: A general explanation of overlays. * Overlay Commands:: Managing overlays in GDB. * Automatic Overlay Debugging:: GDB can find out which overlays are mapped by asking the inferior. * Overlay Sample Program:: A sample program using overlays.  File: gdb.info, Node: How Overlays Work, Next: Overlay Commands, Up: Overlays 11.1 How Overlays Work ====================== Suppose you have a computer whose instruction address space is only 64 kilobytes long, but which has much more memory which can be accessed by other means: special instructions, segment registers, or memory management hardware, for example. Suppose further that you want to adapt a program which is larger than 64 kilobytes to run on this system. One solution is to identify modules of your program which are relatively independent, and need not call each other directly; call these modules "overlays". Separate the overlays from the main program, and place their machine code in the larger memory. Place your main program in instruction memory, but leave at least enough space there to hold the largest overlay as well. Now, to call a function located in an overlay, you must first copy that overlay's machine code from the large memory into the space set aside for it in the instruction memory, and then jump to its entry point there. Data Instruction Larger Address Space Address Space Address Space +-----------+ +-----------+ +-----------+ | | | | | | +-----------+ +-----------+ +-----------+<-- overlay 1 | program | | main | .----| overlay 1 | load address | variables | | program | | +-----------+ | and heap | | | | | | +-----------+ | | | +-----------+<-- overlay 2 | | +-----------+ | | | load address +-----------+ | | | .-| overlay 2 | | | | | | | mapped --->+-----------+ | | +-----------+ address | | | | | | | overlay | <-' | | | | area | <---' +-----------+<-- overlay 3 | | <---. | | load address +-----------+ `--| overlay 3 | | | | | +-----------+ | | +-----------+ | | +-----------+ A code overlay The diagram (*note A code overlay::) shows a system with separate data and instruction address spaces. To map an overlay, the program copies its code from the larger address space to the instruction address space. Since the overlays shown here all use the same mapped address, only one may be mapped at a time. For a system with a single address space for data and instructions, the diagram would be similar, except that the program variables and heap would share an address space with the main program and the overlay area. An overlay loaded into instruction memory and ready for use is called a "mapped" overlay; its "mapped address" is its address in the instruction memory. An overlay not present (or only partially present) in instruction memory is called "unmapped"; its "load address" is its address in the larger memory. The mapped address is also called the "virtual memory address", or "VMA"; the load address is also called the "load memory address", or "LMA". Unfortunately, overlays are not a completely transparent way to adapt a program to limited instruction memory. They introduce a new set of global constraints you must keep in mind as you design your program: * Before calling or returning to a function in an overlay, your program must make sure that overlay is actually mapped. Otherwise, the call or return will transfer control to the right address, but in the wrong overlay, and your program will probably crash. * If the process of mapping an overlay is expensive on your system, you will need to choose your overlays carefully to minimize their effect on your program's performance. * The executable file you load onto your system must contain each overlay's instructions, appearing at the overlay's load address, not its mapped address. However, each overlay's instructions must be relocated and its symbols defined as if the overlay were at its mapped address. You can use GNU linker scripts to specify different load and relocation addresses for pieces of your program; see *Note Overlay Description: (ld.info)Overlay Description. * The procedure for loading executable files onto your system must be able to load their contents into the larger address space as well as the instruction and data spaces. The overlay system described above is rather simple, and could be improved in many ways: * If your system has suitable bank switch registers or memory management hardware, you could use those facilities to make an overlay's load area contents simply appear at their mapped address in instruction space. This would probably be faster than copying the overlay to its mapped area in the usual way. * If your overlays are small enough, you could set aside more than one overlay area, and have more than one overlay mapped at a time. * You can use overlays to manage data, as well as instructions. In general, data overlays are even less transparent to your design than code overlays: whereas code overlays only require care when you call or return to functions, data overlays require care every time you access the data. Also, if you change the contents of a data overlay, you must copy its contents back out to its load address before you can copy a different data overlay into the same mapped area.  File: gdb.info, Node: Overlay Commands, Next: Automatic Overlay Debugging, Prev: How Overlays Work, Up: Overlays 11.2 Overlay Commands ===================== To use GDB's overlay support, each overlay in your program must correspond to a separate section of the executable file. The section's virtual memory address and load memory address must be the overlay's mapped and load addresses. Identifying overlays with sections allows GDB to determine the appropriate address of a function or variable, depending on whether the overlay is mapped or not. GDB's overlay commands all start with the word `overlay'; you can abbreviate this as `ov' or `ovly'. The commands are: `overlay off' Disable GDB's overlay support. When overlay support is disabled, GDB assumes that all functions and variables are always present at their mapped addresses. By default, GDB's overlay support is disabled. `overlay manual' Enable "manual" overlay debugging. In this mode, GDB relies on you to tell it which overlays are mapped, and which are not, using the `overlay map-overlay' and `overlay unmap-overlay' commands described below. `overlay map-overlay OVERLAY' `overlay map OVERLAY' Tell GDB that OVERLAY is now mapped; OVERLAY must be the name of the object file section containing the overlay. When an overlay is mapped, GDB assumes it can find the overlay's functions and variables at their mapped addresses. GDB assumes that any other overlays whose mapped ranges overlap that of OVERLAY are now unmapped. `overlay unmap-overlay OVERLAY' `overlay unmap OVERLAY' Tell GDB that OVERLAY is no longer mapped; OVERLAY must be the name of the object file section containing the overlay. When an overlay is unmapped, GDB assumes it can find the overlay's functions and variables at their load addresses. `overlay auto' Enable "automatic" overlay debugging. In this mode, GDB consults a data structure the overlay manager maintains in the inferior to see which overlays are mapped. For details, see *Note Automatic Overlay Debugging::. `overlay load-target' `overlay load' Re-read the overlay table from the inferior. Normally, GDB re-reads the table GDB automatically each time the inferior stops, so this command should only be necessary if you have changed the overlay mapping yourself using GDB. This command is only useful when using automatic overlay debugging. `overlay list-overlays' `overlay list' Display a list of the overlays currently mapped, along with their mapped addresses, load addresses, and sizes. Normally, when GDB prints a code address, it includes the name of the function the address falls in: (gdb) print main $3 = {int ()} 0x11a0
When overlay debugging is enabled, GDB recognizes code in unmapped overlays, and prints the names of unmapped functions with asterisks around them. For example, if `foo' is a function in an unmapped overlay, GDB prints it this way: (gdb) overlay list No sections are mapped. (gdb) print foo $5 = {int (int)} 0x100000 <*foo*> When `foo''s overlay is mapped, GDB prints the function's name normally: (gdb) overlay list Section .ov.foo.text, loaded at 0x100000 - 0x100034, mapped at 0x1016 - 0x104a (gdb) print foo $6 = {int (int)} 0x1016 When overlay debugging is enabled, GDB can find the correct address for functions and variables in an overlay, whether or not the overlay is mapped. This allows most GDB commands, like `break' and `disassemble', to work normally, even on unmapped code. However, GDB's breakpoint support has some limitations: * You can set breakpoints in functions in unmapped overlays, as long as GDB can write to the overlay at its load address. * GDB can not set hardware or simulator-based breakpoints in unmapped overlays. However, if you set a breakpoint at the end of your overlay manager (and tell GDB which overlays are now mapped, if you are using manual overlay management), GDB will re-set its breakpoints properly.  File: gdb.info, Node: Automatic Overlay Debugging, Next: Overlay Sample Program, Prev: Overlay Commands, Up: Overlays 11.3 Automatic Overlay Debugging ================================ GDB can automatically track which overlays are mapped and which are not, given some simple co-operation from the overlay manager in the inferior. If you enable automatic overlay debugging with the `overlay auto' command (*note Overlay Commands::), GDB looks in the inferior's memory for certain variables describing the current state of the overlays. Here are the variables your overlay manager must define to support GDB's automatic overlay debugging: `_ovly_table': This variable must be an array of the following structures: struct { /* The overlay's mapped address. */ unsigned long vma; /* The size of the overlay, in bytes. */ unsigned long size; /* The overlay's load address. */ unsigned long lma; /* Non-zero if the overlay is currently mapped; zero otherwise. */ unsigned long mapped; } `_novlys': This variable must be a four-byte signed integer, holding the total number of elements in `_ovly_table'. To decide whether a particular overlay is mapped or not, GDB looks for an entry in `_ovly_table' whose `vma' and `lma' members equal the VMA and LMA of the overlay's section in the executable file. When GDB finds a matching entry, it consults the entry's `mapped' member to determine whether the overlay is currently mapped. In addition, your overlay manager may define a function called `_ovly_debug_event'. If this function is defined, GDB will silently set a breakpoint there. If the overlay manager then calls this function whenever it has changed the overlay table, this will enable GDB to accurately keep track of which overlays are in program memory, and update any breakpoints that may be set in overlays. This will allow breakpoints to work even if the overlays are kept in ROM or other non-writable memory while they are not being executed.  File: gdb.info, Node: Overlay Sample Program, Prev: Automatic Overlay Debugging, Up: Overlays 11.4 Overlay Sample Program =========================== When linking a program which uses overlays, you must place the overlays at their load addresses, while relocating them to run at their mapped addresses. To do this, you must write a linker script (*note Overlay Description: (ld.info)Overlay Description.). Unfortunately, since linker scripts are specific to a particular host system, target architecture, and target memory layout, this manual cannot provide portable sample code demonstrating GDB's overlay support. However, the GDB source distribution does contain an overlaid program, with linker scripts for a few systems, as part of its test suite. The program consists of the following files from `gdb/testsuite/gdb.base': `overlays.c' The main program file. `ovlymgr.c' A simple overlay manager, used by `overlays.c'. `foo.c' `bar.c' `baz.c' `grbx.c' Overlay modules, loaded and used by `overlays.c'. `d10v.ld' `m32r.ld' Linker scripts for linking the test program on the `d10v-elf' and `m32r-elf' targets. You can build the test program using the `d10v-elf' GCC cross-compiler like this: $ d10v-elf-gcc -g -c overlays.c $ d10v-elf-gcc -g -c ovlymgr.c $ d10v-elf-gcc -g -c foo.c $ d10v-elf-gcc -g -c bar.c $ d10v-elf-gcc -g -c baz.c $ d10v-elf-gcc -g -c grbx.c $ d10v-elf-gcc -g overlays.o ovlymgr.o foo.o bar.o \ baz.o grbx.o -Wl,-Td10v.ld -o overlays The build process is identical for any other architecture, except that you must substitute the appropriate compiler and linker script for the target system for `d10v-elf-gcc' and `d10v.ld'.  File: gdb.info, Node: Languages, Next: Symbols, Prev: Overlays, Up: Top 12 Using GDB with Different Languages ************************************* Although programming languages generally have common aspects, they are rarely expressed in the same manner. For instance, in ANSI C, dereferencing a pointer `p' is accomplished by `*p', but in Modula-2, it is accomplished by `p^'. Values can also be represented (and displayed) differently. Hex numbers in C appear as `0x1ae', while in Modula-2 they appear as `1AEH'. Language-specific information is built into GDB for some languages, allowing you to express operations like the above in your program's native language, and allowing GDB to output values in a manner consistent with the syntax of your program's native language. The language you use to build expressions is called the "working language". * Menu: * Setting:: Switching between source languages * Show:: Displaying the language * Checks:: Type and range checks * Support:: Supported languages * Unsupported languages:: Unsupported languages  File: gdb.info, Node: Setting, Next: Show, Up: Languages 12.1 Switching between source languages ======================================= There are two ways to control the working language--either have GDB set it automatically, or select it manually yourself. You can use the `set language' command for either purpose. On startup, GDB defaults to setting the language automatically. The working language is used to determine how expressions you type are interpreted, how values are printed, etc. In addition to the working language, every source file that GDB knows about has its own working language. For some object file formats, the compiler might indicate which language a particular source file is in. However, most of the time GDB infers the language from the name of the file. The language of a source file controls whether C++ names are demangled--this way `backtrace' can show each frame appropriately for its own language. There is no way to set the language of a source file from within GDB, but you can set the language associated with a filename extension. *Note Displaying the language: Show. This is most commonly a problem when you use a program, such as `cfront' or `f2c', that generates C but is written in another language. In that case, make the program use `#line' directives in its C output; that way GDB will know the correct language of the source code of the original program, and will display that source code, not the generated C code. * Menu: * Filenames:: Filename extensions and languages. * Manually:: Setting the working language manually * Automatically:: Having GDB infer the source language  File: gdb.info, Node: Filenames, Next: Manually, Up: Setting 12.1.1 List of filename extensions and languages ------------------------------------------------ If a source file name ends in one of the following extensions, then GDB infers that its language is the one indicated. `.ada' `.ads' `.adb' `.a' Ada source file. `.c' C source file `.C' `.cc' `.cp' `.cpp' `.cxx' `.c++' C++ source file `.m' Objective-C source file `.f' `.F' Fortran source file `.mod' Modula-2 source file `.s' `.S' Assembler source file. This actually behaves almost like C, but GDB does not skip over function prologues when stepping. In addition, you may set the language associated with a filename extension. *Note Displaying the language: Show.  File: gdb.info, Node: Manually, Next: Automatically, Prev: Filenames, Up: Setting 12.1.2 Setting the working language ----------------------------------- If you allow GDB to set the language automatically, expressions are interpreted the same way in your debugging session and your program. If you wish, you may set the language manually. To do this, issue the command `set language LANG', where LANG is the name of a language, such as `c' or `modula-2'. For a list of the supported languages, type `set language'. Setting the language manually prevents GDB from updating the working language automatically. This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages--but means different things. For instance, if the current source file were written in C, and GDB was parsing Modula-2, a command such as: print a = b + c might not have the effect you intended. In C, this means to add `b' and `c' and place the result in `a'. The result printed would be the value of `a'. In Modula-2, this means to compare `a' to the result of `b+c', yielding a `BOOLEAN' value.  File: gdb.info, Node: Automatically, Prev: Manually, Up: Setting 12.1.3 Having GDB infer the source language ------------------------------------------- To have GDB set the working language automatically, use `set language local' or `set language auto'. GDB then infers the working language. That is, when your program stops in a frame (usually by encountering a breakpoint), GDB sets the working language to the language recorded for the function in that frame. If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and GDB issues a warning. This may not seem necessary for most programs, which are written entirely in one source language. However, program modules and libraries written in one source language can be used by a main program written in a different source language. Using `set language auto' in this case frees you from having to set the working language manually.  File: gdb.info, Node: Show, Next: Checks, Prev: Setting, Up: Languages 12.2 Displaying the language ============================ The following commands help you find out which language is the working language, and also what language source files were written in. `show language' Display the current working language. This is the language you can use with commands such as `print' to build and compute expressions that may involve variables in your program. `info frame' Display the source language for this frame. This language becomes the working language if you use an identifier from this frame. *Note Information about a frame: Frame Info, to identify the other information listed here. `info source' Display the source language of this source file. *Note Examining the Symbol Table: Symbols, to identify the other information listed here. In unusual circumstances, you may have source files with extensions not in the standard list. You can then set the extension associated with a language explicitly: `set extension-language .EXT LANGUAGE' Set source files with extension .EXT to be assumed to be in the source language LANGUAGE. `info extensions' List all the filename extensions and the associated languages.  File: gdb.info, Node: Checks, Next: Support, Prev: Show, Up: Languages 12.3 Type and range checking ============================ _Warning:_ In this release, the GDB commands for type and range checking are included, but they do not yet have any effect. This section documents the intended facilities. Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks. These include checking the type of arguments to functions and operators, and making sure mathematical overflows are caught at run time. Checks such as these help to ensure a program's correctness once it has been compiled by eliminating type mismatches, and providing active checks for range errors when your program is running. GDB can check for conditions like the above if you wish. Although GDB does not check the statements in your program, it can check expressions entered directly into GDB for evaluation via the `print' command, for example. As with the working language, GDB can also decide whether or not to check automatically based on your program's source language. *Note Supported languages: Support, for the default settings of supported languages. * Menu: * Type Checking:: An overview of type checking * Range Checking:: An overview of range checking  File: gdb.info, Node: Type Checking, Next: Range Checking, Up: Checks 12.3.1 An overview of type checking ----------------------------------- Some languages, such as Modula-2, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs. These checks prevent type mismatch errors from ever causing any run-time problems. For example, 1 + 2 => 3 but error--> 1 + 2.3 The second example fails because the `CARDINAL' 1 is not type-compatible with the `REAL' 2.3. For the expressions you use in GDB commands, you can tell the GDB type checker to skip checking; to treat any mismatches as errors and abandon the expression; or to only issue warnings when type mismatches occur, but evaluate the expression anyway. When you choose the last of these, GDB evaluates expressions like the second example above, but also issues a warning. Even if you turn type checking off, there may be other reasons related to type that prevent GDB from evaluating an expression. For instance, GDB does not know how to add an `int' and a `struct foo'. These particular type errors have nothing to do with the language in use, and usually arise from expressions, such as the one described above, which make little sense to evaluate anyway. Each language defines to what degree it is strict about type. For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers. In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators. *Note Supported languages: Support, for further details on specific languages. GDB provides some additional commands for controlling the type checker: `set check type auto' Set type checking on or off based on the current working language. *Note Supported languages: Support, for the default settings for each language. `set check type on' `set check type off' Set type checking on or off, overriding the default setting for the current working language. Issue a warning if the setting does not match the language default. If any type mismatches occur in evaluating an expression while type checking is on, GDB prints a message and aborts evaluation of the expression. `set check type warn' Cause the type checker to issue warnings, but to always attempt to evaluate the expression. Evaluating the expression may still be impossible for other reasons. For example, GDB cannot add numbers and structures. `show type' Show the current setting of the type checker, and whether or not GDB is setting it automatically.  File: gdb.info, Node: Range Checking, Prev: Type Checking, Up: Checks 12.3.2 An overview of range checking ------------------------------------ In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks. Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array. For expressions you use in GDB commands, you can tell GDB to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway. A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type. Some languages, however, do not treat overflows as an error. In many implementations of C, mathematical overflow causes the result to "wrap around" to lower values--for example, if M is the largest integer value, and S is the smallest, then M + 1 => S This, too, is specific to individual languages, and in some cases specific to individual compilers or machines. *Note Supported languages: Support, for further details on specific languages. GDB provides some additional commands for controlling the range checker: `set check range auto' Set range checking on or off based on the current working language. *Note Supported languages: Support, for the default settings for each language. `set check range on' `set check range off' Set range checking on or off, overriding the default setting for the current working language. A warning is issued if the setting does not match the language default. If a range error occurs and range checking is on, then a message is printed and evaluation of the expression is aborted. `set check range warn' Output messages when the GDB range checker detects a range error, but attempt to evaluate the expression anyway. Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many Unix systems). `show range' Show the current setting of the range checker, and whether or not it is being set automatically by GDB.  File: gdb.info, Node: Support, Next: Unsupported languages, Prev: Checks, Up: Languages 12.4 Supported languages ======================== GDB supports C, C++, Objective-C, Fortran, Java, assembly, Modula-2, and Ada. Some GDB features may be used in expressions regardless of the language you use: the GDB `@' and `::' operators, and the `{type}addr' construct (*note Expressions: Expressions.) can be used with the constructs of any supported language. The following sections detail to what degree each source language is supported by GDB. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDB expression parser accepts, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial. * Menu: * C:: C and C++ * Objective-C:: Objective-C * Modula-2:: Modula-2 * Ada:: Ada  File: gdb.info, Node: C, Next: Objective-C, Up: Support 12.4.1 C and C++ ---------------- Since C and C++ are so closely related, many features of GDB apply to both languages. Whenever this is the case, we discuss those languages together. The C++ debugging facilities are jointly implemented by the C++ compiler and GDB. Therefore, to debug your C++ code effectively, you must compile your C++ programs with a supported C++ compiler, such as GNU `g++', or the HP ANSI C++ compiler (`aCC'). For best results when using GNU C++, use the DWARF 2 debugging format; if it doesn't work on your system, try the stabs+ debugging format. You can select those formats explicitly with the `g++' command-line options `-gdwarf-2' and `-gstabs+'. *Note Options for Debugging Your Program or GNU CC: (gcc.info)Debugging Options. * Menu: * C Operators:: C and C++ operators * C Constants:: C and C++ constants * C plus plus expressions:: C++ expressions * C Defaults:: Default settings for C and C++ * C Checks:: C and C++ type and range checks * Debugging C:: GDB and C * Debugging C plus plus:: GDB features for C++  File: gdb.info, Node: C Operators, Next: C Constants, Up: C 12.4.1.1 C and C++ operators ............................ Operators must be defined on values of specific types. For instance, `+' is defined on numbers, but not on structures. Operators are often defined on groups of types. For the purposes of C and C++, the following definitions hold: * _Integral types_ include `int' with any of its storage-class specifiers; `char'; `enum'; and, for C++, `bool'. * _Floating-point types_ include `float', `double', and `long double' (if supported by the target platform). * _Pointer types_ include all types defined as `(TYPE *)'. * _Scalar types_ include all of the above. The following operators are supported. They are listed here in order of increasing precedence: `,' The comma or sequencing operator. Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated. `=' Assignment. The value of an assignment expression is the value assigned. Defined on scalar types. `OP=' Used in an expression of the form `A OP= B', and translated to `A = A OP B'. `OP=' and `=' have the same precedence. OP is any one of the operators `|', `^', `&', `<<', `>>', `+', `-', `*', `/', `%'. `?:' The ternary operator. `A ? B : C' can be thought of as: if A then B else C. A should be of an integral type. `||' Logical OR. Defined on integral types. `&&' Logical AND. Defined on integral types. `|' Bitwise OR. Defined on integral types. `^' Bitwise exclusive-OR. Defined on integral types. `&' Bitwise AND. Defined on integral types. `==, !=' Equality and inequality. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true. `<, >, <=, >=' Less than, greater than, less than or equal, greater than or equal. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true. `<<, >>' left shift, and right shift. Defined on integral types. `@' The GDB "artificial array" operator (*note Expressions: Expressions.). `+, -' Addition and subtraction. Defined on integral types, floating-point types and pointer types. `*, /, %' Multiplication, division, and modulus. Multiplication and division are defined on integral and floating-point types. Modulus is defined on integral types. `++, --' Increment and decrement. When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable's value is used before the operation takes place. `*' Pointer dereferencing. Defined on pointer types. Same precedence as `++'. `&' Address operator. Defined on variables. Same precedence as `++'. For debugging C++, GDB implements a use of `&' beyond what is allowed in the C++ language itself: you can use `&(&REF)' (or, if you prefer, simply `&&REF') to examine the address where a C++ reference variable (declared with `&REF') is stored. `-' Negative. Defined on integral and floating-point types. Same precedence as `++'. `!' Logical negation. Defined on integral types. Same precedence as `++'. `~' Bitwise complement operator. Defined on integral types. Same precedence as `++'. `., ->' Structure member, and pointer-to-structure member. For convenience, GDB regards the two as equivalent, choosing whether to dereference a pointer based on the stored type information. Defined on `struct' and `union' data. `.*, ->*' Dereferences of pointers to members. `[]' Array indexing. `A[I]' is defined as `*(A+I)'. Same precedence as `->'. `()' Function parameter list. Same precedence as `->'. `::' C++ scope resolution operator. Defined on `struct', `union', and `class' types. `::' Doubled colons also represent the GDB scope operator (*note Expressions: Expressions.). Same precedence as `::', above. If an operator is redefined in the user code, GDB usually attempts to invoke the redefined version instead of using the operator's predefined meaning. * Menu: * C Constants::  File: gdb.info, Node: C Constants, Next: C plus plus expressions, Prev: C Operators, Up: C 12.4.1.2 C and C++ constants ............................ GDB allows you to express the constants of C and C++ in the following ways: * Integer constants are a sequence of digits. Octal constants are specified by a leading `0' (i.e. zero), and hexadecimal constants by a leading `0x' or `0X'. Constants may also end with a letter `l', specifying that the constant should be treated as a `long' value. * Floating point constants are a sequence of digits, followed by a decimal point, followed by a sequence of digits, and optionally followed by an exponent. An exponent is of the form: `e[[+]|-]NNN', where NNN is another sequence of digits. The `+' is optional for positive exponents. A floating-point constant may also end with a letter `f' or `F', specifying that the constant should be treated as being of the `float' (as opposed to the default `double') type; or with a letter `l' or `L', which specifies a `long double' constant. * Enumerated constants consist of enumerated identifiers, or their integral equivalents. * Character constants are a single character surrounded by single quotes (`''), or a number--the ordinal value of the corresponding character (usually its ASCII value). Within quotes, the single character may be represented by a letter or by "escape sequences", which are of the form `\NNN', where NNN is the octal representation of the character's ordinal value; or of the form `\X', where `X' is a predefined special character--for example, `\n' for newline. * String constants are a sequence of character constants surrounded by double quotes (`"'). Any valid character constant (as described above) may appear. Double quotes within the string must be preceded by a backslash, so for instance `"a\"b'c"' is a string of five characters. * Pointer constants are an integral value. You can also write pointers to constants using the C operator `&'. * Array constants are comma-separated lists surrounded by braces `{' and `}'; for example, `{1,2,3}' is a three-element array of integers, `{{1,2}, {3,4}, {5,6}}' is a three-by-two array, and `{&"hi", &"there", &"fred"}' is a three-element array of pointers. * Menu: * C plus plus expressions:: * C Defaults:: * C Checks:: * Debugging C::  File: gdb.info, Node: C plus plus expressions, Next: C Defaults, Prev: C Constants, Up: C 12.4.1.3 C++ expressions ........................ GDB expression handling can interpret most C++ expressions. _Warning:_ GDB can only debug C++ code if you use the proper compiler and the proper debug format. Currently, GDB works best when debugging C++ code that is compiled with GCC 2.95.3 or with GCC 3.1 or newer, using the options `-gdwarf-2' or `-gstabs+'. DWARF 2 is preferred over stabs+. Most configurations of GCC emit either DWARF 2 or stabs+ as their default debug format, so you usually don't need to specify a debug format explicitly. Other compilers and/or debug formats are likely to work badly or not at all when using GDB to debug C++ code. 1. Member function calls are allowed; you can use expressions like count = aml->GetOriginal(x, y) 2. While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, GDB allows implicit references to the class instance pointer `this' following the same rules as C++. 3. You can call overloaded functions; GDB resolves the function call to the right definition, with some restrictions. GDB does not perform overload resolution involving user-defined type conversions, calls to constructors, or instantiations of templates that do not exist in the program. It also cannot handle ellipsis argument lists or default arguments. It does perform integral conversions and promotions, floating-point promotions, arithmetic conversions, pointer conversions, conversions of class objects to base classes, and standard conversions such as those of functions or arrays to pointers; it requires an exact match on the number of function arguments. Overload resolution is always performed, unless you have specified `set overload-resolution off'. *Note GDB features for C++: Debugging C plus plus. You must specify `set overload-resolution off' in order to use an explicit function signature to call an overloaded function, as in p 'foo(char,int)'('x', 13) The GDB command-completion facility can simplify this; see *Note Command completion: Completion. 4. GDB understands variables declared as C++ references; you can use them in expressions just as you do in C++ source--they are automatically dereferenced. In the parameter list shown when GDB displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The _address_ of a reference variable is always shown, unless you have specified `set print address off'. 5. GDB supports the C++ name resolution operator `::'--your expressions can use it just as expressions in your program do. Since one scope may be defined in another, you can use `::' repeatedly if necessary, for example in an expression like `SCOPE1::SCOPE2::NAME'. GDB also allows resolving name scope by reference to source files, in both C and C++ debugging (*note Program variables: Variables.). In addition, when used with HP's C++ compiler, GDB supports calling virtual functions correctly, printing out virtual bases of objects, calling functions in a base subobject, casting objects, and invoking user-defined operators.  File: gdb.info, Node: C Defaults, Next: C Checks, Prev: C plus plus expressions, Up: C 12.4.1.4 C and C++ defaults ........................... If you allow GDB to set type and range checking automatically, they both default to `off' whenever the working language changes to C or C++. This happens regardless of whether you or GDB selects the working language. If you allow GDB to set the language automatically, it recognizes source files whose names end with `.c', `.C', or `.cc', etc, and when GDB enters code compiled from one of these files, it sets the working language to C or C++. *Note Having GDB infer the source language: Automatically, for further details.  File: gdb.info, Node: C Checks, Next: Debugging C, Prev: C Defaults, Up: C 12.4.1.5 C and C++ type and range checks ........................................ By default, when GDB parses C or C++ expressions, type checking is not used. However, if you turn type checking on, GDB considers two variables type equivalent if: * The two variables are structured and have the same structure, union, or enumerated tag. * The two variables have the same type name, or types that have been declared equivalent through `typedef'. Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array.  File: gdb.info, Node: Debugging C, Next: Debugging C plus plus, Prev: C Checks, Up: C 12.4.1.6 GDB and C .................. The `set print union' and `show print union' commands apply to the `union' type. When set to `on', any `union' that is inside a `struct' or `class' is also printed. Otherwise, it appears as `{...}'. The `@' operator aids in the debugging of dynamic arrays, formed with pointers and a memory allocation function. *Note Expressions: Expressions. * Menu: * Debugging C plus plus::  File: gdb.info, Node: Debugging C plus plus, Prev: Debugging C, Up: C 12.4.1.7 GDB features for C++ ............................. Some GDB commands are particularly useful with C++, and some are designed specifically for use with C++. Here is a summary: `breakpoint menus' When you want a breakpoint in a function whose name is overloaded, GDB breakpoint menus help you specify which function definition you want. *Note Breakpoint menus: Breakpoint Menus. `rbreak REGEX' Setting breakpoints using regular expressions is helpful for setting breakpoints on overloaded functions that are not members of any special classes. *Note Setting breakpoints: Set Breaks. `catch throw' `catch catch' Debug C++ exception handling using these commands. *Note Setting catchpoints: Set Catchpoints. `ptype TYPENAME' Print inheritance relationships as well as other information for type TYPENAME. *Note Examining the Symbol Table: Symbols. `set print demangle' `show print demangle' `set print asm-demangle' `show print asm-demangle' Control whether C++ symbols display in their source form, both when displaying code as C++ source and when displaying disassemblies. *Note Print settings: Print Settings. `set print object' `show print object' Choose whether to print derived (actual) or declared types of objects. *Note Print settings: Print Settings. `set print vtbl' `show print vtbl' Control the format for printing virtual function tables. *Note Print settings: Print Settings. (The `vtbl' commands do not work on programs compiled with the HP ANSI C++ compiler (`aCC').) `set overload-resolution on' Enable overload resolution for C++ expression evaluation. The default is on. For overloaded functions, GDB evaluates the arguments and searches for a function whose signature matches the argument types, using the standard C++ conversion rules (see *Note C++ expressions: C plus plus expressions, for details). If it cannot find a match, it emits a message. `set overload-resolution off' Disable overload resolution for C++ expression evaluation. For overloaded functions that are not class member functions, GDB chooses the first function of the specified name that it finds in the symbol table, whether or not its arguments are of the correct type. For overloaded functions that are class member functions, GDB searches for a function whose signature _exactly_ matches the argument types. `Overloaded symbol names' You can specify a particular definition of an overloaded symbol, using the same notation that is used to declare such symbols in C++: type `SYMBOL(TYPES)' rather than just SYMBOL. You can also use the GDB command-line word completion facilities to list the available choices, or to finish the type list for you. *Note Command completion: Completion, for details on how to do this.  File: gdb.info, Node: Objective-C, Next: Modula-2, Prev: C, Up: Support 12.4.2 Objective-C ------------------ This section provides information about some commands and command options that are useful for debugging Objective-C code. * Menu: * Method Names in Commands:: * The Print Command with Objective-C::  File: gdb.info, Node: Method Names in Commands, Next: The Print Command with Objective-C, Prev: Objective-C, Up: Objective-C 12.4.2.1 Method Names in Commands ................................. The following commands have been extended to accept Objective-C method names as line specifications: * `clear' * `break' * `info line' * `jump' * `list' A fully qualified Objective-C method name is specified as -[CLASS METHODNAME] where the minus sign is used to indicate an instance method and a plus sign (not shown) is used to indicate a class method. The class name CLASS and method name METHODNAME are enclosed in brackets, similar to the way messages are specified in Objective-C source code. For example, to set a breakpoint at the `create' instance method of class `Fruit' in the program currently being debugged, enter: break -[Fruit create] To list ten program lines around the `initialize' class method, enter: list +[NSText initialize] In the current version of GDB, the plus or minus sign is required. In future versions of GDB, the plus or minus sign will be optional, but you can use it to narrow the search. It is also possible to specify just a method name: break create You must specify the complete method name, including any colons. If your program's source files contain more than one `create' method, you'll be presented with a numbered list of classes that implement that method. Indicate your choice by number, or type `0' to exit if none apply. As another example, to clear a breakpoint established at the `makeKeyAndOrderFront:' method of the `NSWindow' class, enter: clear -[NSWindow makeKeyAndOrderFront:]  File: gdb.info, Node: The Print Command with Objective-C, Prev: Method Names in Commands, Up: Objective-C 12.4.2.2 The Print Command With Objective-C ........................................... The print command has also been extended to accept methods. For example: print -[OBJECT hash] will tell GDB to send the `hash' message to OBJECT and print the result. Also, an additional command has been added, `print-object' or `po' for short, which is meant to print the description of an object. However, this command may only work with certain Objective-C libraries that have a particular hook function, `_NSPrintForDebugger', defined.  File: gdb.info, Node: Modula-2, Next: Ada, Prev: Objective-C, Up: Support 12.4.3 Modula-2 --------------- The extensions made to GDB to support Modula-2 only support output from the GNU Modula-2 compiler (which is currently being developed). Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them is most likely to give an error as GDB reads in the executable's symbol table. * Menu: * M2 Operators:: Built-in operators * Built-In Func/Proc:: Built-in functions and procedures * M2 Constants:: Modula-2 constants * M2 Defaults:: Default settings for Modula-2 * Deviations:: Deviations from standard Modula-2 * M2 Checks:: Modula-2 type and range checks * M2 Scope:: The scope operators `::' and `.' * GDB/M2:: GDB and Modula-2  File: gdb.info, Node: M2 Operators, Next: Built-In Func/Proc, Up: Modula-2 12.4.3.1 Operators .................. Operators must be defined on values of specific types. For instance, `+' is defined on numbers, but not on structures. Operators are often defined on groups of types. For the purposes of Modula-2, the following definitions hold: * _Integral types_ consist of `INTEGER', `CARDINAL', and their subranges. * _Character types_ consist of `CHAR' and its subranges. * _Floating-point types_ consist of `REAL'. * _Pointer types_ consist of anything declared as `POINTER TO TYPE'. * _Scalar types_ consist of all of the above. * _Set types_ consist of `SET' and `BITSET' types. * _Boolean types_ consist of `BOOLEAN'. The following operators are supported, and appear in order of increasing precedence: `,' Function argument or array index separator. `:=' Assignment. The value of VAR `:=' VALUE is VALUE. `<, >' Less than, greater than on integral, floating-point, or enumerated types. `<=, >=' Less than or equal to, greater than or equal to on integral, floating-point and enumerated types, or set inclusion on set types. Same precedence as `<'. `=, <>, #' Equality and two ways of expressing inequality, valid on scalar types. Same precedence as `<'. In GDB scripts, only `<>' is available for inequality, since `#' conflicts with the script comment character. `IN' Set membership. Defined on set types and the types of their members. Same precedence as `<'. `OR' Boolean disjunction. Defined on boolean types. `AND, &' Boolean conjunction. Defined on boolean types. `@' The GDB "artificial array" operator (*note Expressions: Expressions.). `+, -' Addition and subtraction on integral and floating-point types, or union and difference on set types. `*' Multiplication on integral and floating-point types, or set intersection on set types. `/' Division on floating-point types, or symmetric set difference on set types. Same precedence as `*'. `DIV, MOD' Integer division and remainder. Defined on integral types. Same precedence as `*'. `-' Negative. Defined on `INTEGER' and `REAL' data. `^' Pointer dereferencing. Defined on pointer types. `NOT' Boolean negation. Defined on boolean types. Same precedence as `^'. `.' `RECORD' field selector. Defined on `RECORD' data. Same precedence as `^'. `[]' Array indexing. Defined on `ARRAY' data. Same precedence as `^'. `()' Procedure argument list. Defined on `PROCEDURE' objects. Same precedence as `^'. `::, .' GDB and Modula-2 scope operators. _Warning:_ Sets and their operations are not yet supported, so GDB treats the use of the operator `IN', or the use of operators `+', `-', `*', `/', `=', , `<>', `#', `<=', and `>=' on sets as an error.