This is gdb.info, produced by makeinfo version 4.8 from ./gdb.texinfo. INFO-DIR-SECTION Software development START-INFO-DIR-ENTRY * Gdb: (gdb). The GNU debugger. END-INFO-DIR-ENTRY Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 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 1.3 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 FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom." This file documents the GNU debugger GDB. This is the Ninth Edition, of `Debugging with GDB: the GNU Source-Level Debugger' for GDB (GDB) Version 7.2. Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 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 1.3 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 FSF's Back-Cover Text is: "You are free to copy and modify this GNU Manual. Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom."  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 (GDB) Version 7.2. Copyright (C) 1988-2010 Free Software Foundation, Inc. This edition of the GDB manual is dedicated to the memory of Fred Fish. Fred was a long-standing contributor to GDB and to Free software in general. We will miss him. * 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 * Reverse Execution:: Running programs backward * Process Record and Replay:: Recording inferior's execution and replaying it * Stack:: Examining the stack * Source:: Examining source files * Data:: Examining data * Optimized Code:: Debugging optimized code * 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 * Extending GDB:: Extending GDB * Interpreters:: Command Interpreters * TUI:: GDB Text User Interface * Emacs:: Using GDB under GNU Emacs * GDB/MI:: GDB's Machine Interface. * Annotations:: GDB's annotation interface. * JIT Interface:: Using the JIT debugging interface. * GDB Bugs:: Reporting bugs in GDB * Command Line Editing:: Command Line Editing * Using History Interactively:: Using History Interactively * Formatting Documentation:: How to format and print GDB documentation * Installing GDB:: Installing GDB * Maintenance Commands:: Maintenance Commands * Remote Protocol:: GDB Remote Serial Protocol * Agent Expressions:: The GDB Agent Expression Mechanism * Target Descriptions:: How targets can describe themselves to GDB * Operating System Information:: Getting additional information from the operating system * Trace File Format:: GDB trace file format * 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: Supported Languages. For more information, see *Note C and C++: C. Support for D is partial. For information on D, see *Note D: D. 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 6.3, 6.2, 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. Andrew Cagney, Fernando Nasser, and Elena Zannoni, while working for Cygnus Solutions, implemented the original GDB/MI interface. Jim Blandy added support for preprocessor macros, while working for Red Hat. Andrew Cagney designed GDB's architecture vector. Many people including Andrew Cagney, Stephane Carrez, Randolph Chung, Nick Duffek, Richard Henderson, Mark Kettenis, Grace Sainsbury, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Andreas Schwab, Jason Thorpe, Corinna Vinschen, Ulrich Weigand, and Elena Zannoni, helped with the migration of old architectures to this new framework. Andrew Cagney completely re-designed and re-implemented GDB's unwinder framework, this consisting of a fresh new design featuring frame IDs, independent frame sniffers, and the sentinel frame. Mark Kettenis implemented the DWARF 2 unwinder, Jeff Johnston the libunwind unwinder, and Andrew Cagney the dummy, sentinel, tramp, and trad unwinders. The architecture-specific changes, each involving a complete rewrite of the architecture's frame code, were carried out by Jim Blandy, Joel Brobecker, Kevin Buettner, Andrew Cagney, Stephane Carrez, Randolph Chung, Orjan Friberg, Richard Henderson, Daniel Jacobowitz, Jeff Johnston, Mark Kettenis, Theodore A. Roth, Kei Sakamoto, Yoshinori Sato, Michael Snyder, Corinna Vinschen, and Ulrich Weigand. Christian Zankel, Ross Morley, Bob Wilson, and Maxim Grigoriev from Tensilica, Inc. contributed support for Xtensa processors. Others who have worked on the Xtensa port of GDB in the past include Steve Tjiang, John Newlin, and Scott Foehner. Michael Eager and staff of Xilinx, Inc., contributed support for the Xilinx MicroBlaze architecture.  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 Ctrl-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 7.2, 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: Ctrl-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 `Ctrl-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 * Startup:: What GDB does during startup  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 `./', e.g. `./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. `-pid NUMBER' `-p NUMBER' Connect to process ID NUMBER, as with the `attach' command. `-command FILE' `-x FILE' Execute commands from file FILE. The contents of this file is evaluated exactly as the `source' command would. *Note Command files: Command Files. `-eval-command COMMAND' `-ex COMMAND' Execute a single GDB command. This option may be used multiple times to call multiple commands. It may also be interleaved with `-command' as required. gdb -ex 'target sim' -ex 'load' \ -x setbreakpoints -ex 'run' a.out `-directory DIRECTORY' `-d DIRECTORY' Add DIRECTORY to the path to search for source and script files. `-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.  File: gdb.info, Node: Mode Options, Next: Startup, 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 also disables pagination, sets unlimited terminal width and height *note Screen Size::, and acts as if `set confirm off' were in effect (*note Messages/Warnings::). 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. `-batch-silent' Run in batch mode exactly like `-batch', but totally silently. All GDB output to `stdout' is prevented (`stderr' is unaffected). This is much quieter than `-silent' and would be useless for an interactive session. This is particularly useful when using targets that give `Loading section' messages, for example. Note that targets that give their output via GDB, as opposed to writing directly to `stdout', will also be made silent. `-return-child-result' The return code from GDB will be the return code from the child process (the process being debugged), with the following exceptions: * GDB exits abnormally. E.g., due to an incorrect argument or an internal error. In this case the exit code is the same as it would have been without `-return-child-result'. * The user quits with an explicit value. E.g., `quit 1'. * The child process never runs, or is not allowed to terminate, in which case the exit code will be -1. This option is useful in conjunction with `-batch' or `-batch-silent', when GDB is being used as a remote program loader or simulator interface. `-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 superseded 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. `-l TIMEOUT' Set the timeout (in seconds) of any communication 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 GDB 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: Startup, Prev: Mode Options, Up: Invoking GDB 2.1.3 What GDB Does During Startup ---------------------------------- Here's the description of what GDB does during session startup: 1. Sets up the command interpreter as specified by the command line (*note interpreter: Mode Options.). 2. Reads the system-wide "init file" (if `--with-system-gdbinit' was used when building GDB; *note System-wide configuration and settings: System-wide configuration.) and executes all the commands in that file. 3. Reads the init file (if any) in your home directory(1) and executes all the commands in that file. 4. Processes command line options and operands. 5. Reads and executes the commands from init file (if any) in the current working directory. This is only done if the current directory is different from your home directory. Thus, you can have more than one init file, one generic in your home directory, and another, specific to the program you are debugging, in the directory where you invoke GDB. 6. Reads command files specified by the `-x' option. *Note Command Files::, for more details about GDB command files. 7. Reads the command history recorded in the "history file". *Note Command History::, for more details about the command history and the files where GDB records it. Init files use the same syntax as "command files" (*note Command Files::) and are processed by GDB in the same way. The init file in your home directory can set options (such as `set complaints') that affect subsequent processing of command line options and operands. Init files are not executed if you use the `-nx' option (*note Choosing Modes: Mode Options.). To display the list of init files loaded by gdb at startup, you can use `gdb --help'. The GDB init files are normally called `.gdbinit'. The DJGPP port of GDB uses the name `gdb.ini', due to the limitations of file names imposed by DOS filesystems. The Windows ports of GDB use the standard name, but if they find a `gdb.ini' file, they warn you about that and suggest to rename the file to the standard name. ---------- Footnotes ---------- (1) On DOS/Windows systems, the home directory is the one pointed to by the `HOME' environment variable.  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 `Ctrl-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 `Ctrl-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. User-defined commands can disable this feature; see *Note dont-repeat: Define. 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 `Ctrl-o' binding is useful for repeating a complex sequence of commands. This command accepts the current line, like , 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. When completing in an expression which looks up a field in a structure, GDB also tries(1) to limit completions to the field names available in the type of the left-hand-side: (gdb) p gdb_stdout.M-? magic to_delete to_fputs to_put to_rewind to_data to_flush to_isatty to_read to_write This is because the `gdb_stdout' is a variable of the type `struct ui_file' that is defined in GDB sources as follows: struct ui_file { int *magic; ui_file_flush_ftype *to_flush; ui_file_write_ftype *to_write; ui_file_fputs_ftype *to_fputs; ui_file_read_ftype *to_read; ui_file_delete_ftype *to_delete; ui_file_isatty_ftype *to_isatty; ui_file_rewind_ftype *to_rewind; ui_file_put_ftype *to_put; void *to_data; } ---------- Footnotes ---------- (1) The completer can be confused by certain kinds of invalid expressions. Also, it only examines the static type of the expression, not the dynamic type.  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' 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 show the arguments passed to a function 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' `info copying' Display information about permission for copying GDB. `show warranty' `info 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 * Inferiors and Programs:: Debugging multiple inferiors and programs * Threads:: Debugging programs with multiple threads * Forks:: Debugging forks * Checkpoint/Restart:: Setting a _bookmark_ to return to later  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. Programs that are to be shipped to your customers are compiled with optimizations, using the `-O' compiler option. However, some 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/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. For more information, see *Note 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. GDB knows about preprocessor macros and can show you their expansion (*note Macros::). 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 and later 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.  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 some environments without processes, `run' jumps to the start of your program. Other targets, like `remote', are always running. If you get an error message like this one: The "remote" target does not support "run". Try "help target" or "continue". then use `continue' to run your program. You may need `load' first (*note load::). 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 procedure 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. `set exec-wrapper WRAPPER' `show exec-wrapper' `unset exec-wrapper' When `exec-wrapper' is set, the specified wrapper is used to launch programs for debugging. GDB starts your program with a shell command of the form `exec WRAPPER PROGRAM'. Quoting is added to PROGRAM and its arguments, but not to WRAPPER, so you should add quotes if appropriate for your shell. The wrapper runs until it executes your program, and then GDB takes control. You can use any program that eventually calls `execve' with its arguments as a wrapper. Several standard Unix utilities do this, e.g. `env' and `nohup'. Any Unix shell script ending with `exec "$@"' will also work. For example, you can use `env' to pass an environment variable to the debugged program, without setting the variable in your shell's environment: (gdb) set exec-wrapper env 'LD_PRELOAD=libtest.so' (gdb) run This command is available when debugging locally on most targets, excluding DJGPP, Cygwin, MS Windows, and QNX Neutrino. `set disable-randomization' `set disable-randomization on' This option (enabled by default in GDB) will turn off the native randomization of the virtual address space of the started program. This option is useful for multiple debugging sessions to make the execution better reproducible and memory addresses reusable across debugging sessions. This feature is implemented only on GNU/Linux. You can get the same behavior using (gdb) set exec-wrapper setarch `uname -m` -R `set disable-randomization off' Leave the behavior of the started executable unchanged. Some bugs rear their ugly heads only when the program is loaded at certain addresses. If your bug disappears when you run the program under GDB, that might be because GDB by default disables the address randomization on platforms, such as GNU/Linux, which do that for stand-alone programs. Use `set disable-randomization off' to try to reproduce such elusive bugs. The virtual address space randomization is implemented only on GNU/Linux. It protects the programs against some kinds of security attacks. In these cases the attacker needs to know the exact location of a concrete executable code. Randomizing its location makes it impossible to inject jumps misusing a code at its expected addresses. Prelinking shared libraries provides a startup performance advantage but it makes addresses in these libraries predictable for privileged processes by having just unprivileged access at the target system. Reading the shared library binary gives enough information for assembling the malicious code misusing it. Still even a prelinked shared library can get loaded at a new random address just requiring the regular relocation process during the startup. Shared libraries not already prelinked are always loaded at a randomly chosen address. Position independent executables (PIE) contain position independent code similar to the shared libraries and therefore such executables get loaded at a randomly chosen address upon startup. PIE executables always load even already prelinked shared libraries at a random address. You can build such executable using `gcc -fPIE -pie'. Heap (malloc storage), stack and custom mmap areas are always placed randomly (as long as the randomization is enabled). `show disable-randomization' Show the current setting of the explicit disable of the native randomization of the virtual address space of the started 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. It is generally impossible to find the current working directory of the process being debugged (since a program can change its directory during its run). If you work on a system where GDB is configured with the `/proc' support, you can use the `info proc' command (*note SVR4 Process Information::) to find out the current working directory of the debuggee.  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. `tty' is an alias for `set inferior-tty'. You can use the `show inferior-tty' command to tell GDB to display the name of the terminal that will be used for future runs of your program. `set inferior-tty /dev/ttyb' Set the tty for the program being debugged to /dev/ttyb. `show inferior-tty' Show the current tty for the program being debugged.  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 while you have an attached process, you detach that process. If you use the `run' command, 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: Inferiors and Programs, 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: Inferiors and Programs, Next: Threads, Prev: Kill Process, Up: Running 4.9 Debugging Multiple Inferiors and Programs ============================================= GDB lets you run and debug multiple programs in a single session. In addition, GDB on some systems may let you run several programs simultaneously (otherwise you have to exit from one before starting another). In the most general case, you can have multiple threads of execution in each of multiple processes, launched from multiple executables. GDB represents the state of each program execution with an object called an "inferior". An inferior typically corresponds to a process, but is more general and applies also to targets that do not have processes. Inferiors may be created before a process runs, and may be retained after a process exits. Inferiors have unique identifiers that are different from process ids. Usually each inferior will also have its own distinct address space, although some embedded targets may have several inferiors running in different parts of a single address space. Each inferior may in turn have multiple threads running in it. To find out what inferiors exist at any moment, use `info inferiors': `info inferiors' Print a list of all inferiors currently being managed by GDB. GDB displays for each inferior (in this order): 1. the inferior number assigned by GDB 2. the target system's inferior identifier 3. the name of the executable the inferior is running. An asterisk `*' preceding the GDB inferior number indicates the current inferior. For example, (gdb) info inferiors Num Description Executable 2 process 2307 hello * 1 process 3401 goodbye To switch focus between inferiors, use the `inferior' command: `inferior INFNO' Make inferior number INFNO the current inferior. The argument INFNO is the inferior number assigned by GDB, as shown in the first field of the `info inferiors' display. You can get multiple executables into a debugging session via the `add-inferior' and `clone-inferior' commands. On some systems GDB can add inferiors to the debug session automatically by following calls to `fork' and `exec'. To remove inferiors from the debugging session use the `remove-inferior' command. `add-inferior [ -copies N ] [ -exec EXECUTABLE ]' Adds N inferiors to be run using EXECUTABLE as the executable. N defaults to 1. If no executable is specified, the inferiors begins empty, with no program. You can still assign or change the program assigned to the inferior at any time by using the `file' command with the executable name as its argument. `clone-inferior [ -copies N ] [ INFNO ]' Adds N inferiors ready to execute the same program as inferior INFNO. N defaults to 1. INFNO defaults to the number of the current inferior. This is a convenient command when you want to run another instance of the inferior you are debugging. (gdb) info inferiors Num Description Executable * 1 process 29964 helloworld (gdb) clone-inferior Added inferior 2. 1 inferiors added. (gdb) info inferiors Num Description Executable 2 helloworld * 1 process 29964 helloworld You can now simply switch focus to inferior 2 and run it. `remove-inferior INFNO' Removes the inferior INFNO. It is not possible to remove an inferior that is running with this command. For those, use the `kill' or `detach' command first. To quit debugging one of the running inferiors that is not the current inferior, you can either detach from it by using the `detach inferior' command (allowing it to run independently), or kill it using the `kill inferior' command: `detach inferior INFNO' Detach from the inferior identified by GDB inferior number INFNO. Note that the inferior's entry still stays on the list of inferiors shown by `info inferiors', but its Description will show `'. `kill inferior INFNO' Kill the inferior identified by GDB inferior number INFNO. Note that the inferior's entry still stays on the list of inferiors shown by `info inferiors', but its Description will show `'. After the successful completion of a command such as `detach', `detach inferior', `kill' or `kill inferior', or after a normal process exit, the inferior is still valid and listed with `info inferiors', ready to be restarted. To be notified when inferiors are started or exit under GDB's control use `set print inferior-events': `set print inferior-events' `set print inferior-events on' `set print inferior-events off' The `set print inferior-events' command allows you to enable or disable printing of messages when GDB notices that new inferiors have started or that inferiors have exited or have been detached. By default, these messages will not be printed. `show print inferior-events' Show whether messages will be printed when GDB detects that inferiors have started, exited or have been detached. Many commands will work the same with multiple programs as with a single program: e.g., `print myglobal' will simply display the value of `myglobal' in the current inferior. Occasionaly, when debugging GDB itself, it may be useful to get more info about the relationship of inferiors, programs, address spaces in a debug session. You can do that with the `maint info program-spaces' command. `maint info program-spaces' Print a list of all program spaces currently being managed by GDB. GDB displays for each program space (in this order): 1. the program space number assigned by GDB 2. the name of the executable loaded into the program space, with e.g., the `file' command. An asterisk `*' preceding the GDB program space number indicates the current program space. In addition, below each program space line, GDB prints extra information that isn't suitable to display in tabular form. For example, the list of inferiors bound to the program space. (gdb) maint info program-spaces Id Executable 2 goodbye Bound inferiors: ID 1 (process 21561) * 1 hello Here we can see that no inferior is running the program `hello', while `process 21561' is running the program `goodbye'. On some targets, it is possible that multiple inferiors are bound to the same program space. The most common example is that of debugging both the parent and child processes of a `vfork' call. For example, (gdb) maint info program-spaces Id Executable * 1 vfork-test Bound inferiors: ID 2 (process 18050), ID 1 (process 18045) Here, both inferior 2 and inferior 1 are running in the same program space as a result of inferior 1 having executed a `vfork' call.  File: gdb.info, Node: Threads, Next: Forks, Prev: Inferiors and Programs, Up: Running 4.10 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 * `set print thread-events', which controls printing of messages on thread start and exit. * `set libthread-db-search-path PATH', which lets the user specify which `libthread_db' to use if the default choice isn't compatible with the program. _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 GNU/Linux, you might see [New Thread 46912507313328 (LWP 25582)] 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 On Solaris, you can display more information about user threads with a Solaris-specific command: `maint info sol-threads' Display info on Solaris user threads. `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. The debugger convenience variable `$_thread' contains the number of the current thread. You may find this useful in writing breakpoint conditional expressions, command scripts, and so forth. See *Note Convenience Variables: Convenience Vars, for general information on convenience variables. `thread apply [THREADNO] [ALL] COMMAND' The `thread apply' command allows you to apply the named COMMAND to one or more threads. Specify the numbers of the threads that you want affected with the command argument THREADNO. It can be a single thread number, one of the numbers shown in the first field of the `info threads' display; or it could be a range of thread numbers, as in `2-4'. To apply a command to all threads, type `thread apply all COMMAND'. `set print thread-events' `set print thread-events on' `set print thread-events off' The `set print thread-events' command allows you to enable or disable printing of messages when GDB notices that new threads have started or that threads have exited. By default, these messages will be printed if detection of these events is supported by the target. Note that these messages cannot be disabled on all targets. `show print thread-events' Show whether messages will be printed when GDB detects that threads have started and exited. *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. `set libthread-db-search-path [PATH]' If this variable is set, PATH is a colon-separated list of directories GDB will use to search for `libthread_db'. If you omit PATH, `libthread-db-search-path' will be reset to an empty list. On GNU/Linux and Solaris systems, GDB uses a "helper" `libthread_db' library to obtain information about threads in the inferior process. GDB will use `libthread-db-search-path' to find `libthread_db'. If that fails, GDB will continue with default system shared library directories, and finally the directory from which `libpthread' was loaded in the inferior process. For any `libthread_db' library GDB finds in above directories, GDB attempts to initialize it with the current inferior process. If this initialization fails (which could happen because of a version mismatch between `libthread_db' and `libpthread'), GDB will unload `libthread_db', and continue with the next directory. If none of `libthread_db' libraries initialize successfully, GDB will issue a warning and thread debugging will be disabled. Setting `libthread-db-search-path' is currently implemented only on some platforms. `show libthread-db-search-path' Display current libthread_db search path.  File: gdb.info, Node: Forks, Next: Checkpoint/Restart, Prev: Threads, Up: Running 4.11 Debugging Forks ==================== 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 argument 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. On Linux, if you want to debug both the parent and child processes, use the command `set detach-on-fork'. `set detach-on-fork MODE' Tells gdb whether to detach one of the processes after a fork, or retain debugger control over them both. `on' The child process (or parent process, depending on the value of `follow-fork-mode') will be detached and allowed to run independently. This is the default. `off' Both processes will be held under the control of GDB. One process (child or parent, depending on the value of `follow-fork-mode') is debugged as usual, while the other is held suspended. `show detach-on-fork' Show whether detach-on-fork mode is on/off. If you choose to set `detach-on-fork' mode off, then GDB will retain control of all forked processes (including nested forks). You can list the forked processes under the control of GDB by using the `info inferiors' command, and switch from one fork to another by using the `inferior' command (*note Debugging Multiple Inferiors and Programs: Inferiors and Programs.). To quit debugging one of the forked processes, you can either detach from it by using the `detach inferior' command (allowing it to run independently), or kill it using the `kill inferior' command. *Note Debugging Multiple Inferiors and Programs: Inferiors and Programs. 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'. On some systems, 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. By default, after an `exec' call executes, GDB discards the symbols of the previous executable image. You can change this behaviour with the `set follow-exec-mode' command. `set follow-exec-mode MODE' Set debugger response to a program call of `exec'. An `exec' call replaces the program image of a process. `follow-exec-mode' can be: `new' GDB creates a new inferior and rebinds the process to this new inferior. The program the process was running before the `exec' call can be restarted afterwards by restarting the original inferior. For example: (gdb) info inferiors (gdb) info inferior Id Description Executable * 1 prog1 (gdb) run process 12020 is executing new program: prog2 Program exited normally. (gdb) info inferiors Id Description Executable * 2 prog2 1 prog1 `same' GDB keeps the process bound to the same inferior. The new executable image replaces the previous executable loaded in the inferior. Restarting the inferior after the `exec' call, with e.g., the `run' command, restarts the executable the process was running after the `exec' call. This is the default mode. For example: (gdb) info inferiors Id Description Executable * 1 prog1 (gdb) run process 12020 is executing new program: prog2 Program exited normally. (gdb) info inferiors Id Description Executable * 1 prog2 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: Checkpoint/Restart, Prev: Forks, Up: Running 4.12 Setting a _Bookmark_ to Return to Later ============================================ On certain operating systems(1), GDB is able to save a "snapshot" of a program's state, called a "checkpoint", and come back to it later. Returning to a checkpoint effectively undoes everything that has happened in the program since the `checkpoint' was saved. This includes changes in memory, registers, and even (within some limits) system state. Effectively, it is like going back in time to the moment when the checkpoint was saved. Thus, if you're stepping thru a program and you think you're getting close to the point where things go wrong, you can save a checkpoint. Then, if you accidentally go too far and miss the critical statement, instead of having to restart your program from the beginning, you can just go back to the checkpoint and start again from there. This can be especially useful if it takes a lot of time or steps to reach the point where you think the bug occurs. To use the `checkpoint'/`restart' method of debugging: `checkpoint' Save a snapshot of the debugged program's current execution state. The `checkpoint' command takes no arguments, but each checkpoint is assigned a small integer id, similar to a breakpoint id. `info checkpoints' List the checkpoints that have been saved in the current debugging session. For each checkpoint, the following information will be listed: `Checkpoint ID' `Process ID' `Code Address' `Source line, or label' `restart CHECKPOINT-ID' Restore the program state that was saved as checkpoint number CHECKPOINT-ID. All program variables, registers, stack frames etc. will be returned to the values that they had when the checkpoint was saved. In essence, gdb will "wind back the clock" to the point in time when the checkpoint was saved. Note that breakpoints, GDB variables, command history etc. are not affected by restoring a checkpoint. In general, a checkpoint only restores things that reside in the program being debugged, not in the debugger. `delete checkpoint CHECKPOINT-ID' Delete the previously-saved checkpoint identified by CHECKPOINT-ID. Returning to a previously saved checkpoint will restore the user state of the program being debugged, plus a significant subset of the system (OS) state, including file pointers. It won't "un-write" data from a file, but it will rewind the file pointer to the previous location, so that the previously written data can be overwritten. For files opened in read mode, the pointer will also be restored so that the previously read data can be read again. Of course, characters that have been sent to a printer (or other external device) cannot be "snatched back", and characters received from eg. a serial device can be removed from internal program buffers, but they cannot be "pushed back" into the serial pipeline, ready to be received again. Similarly, the actual contents of files that have been changed cannot be restored (at this time). However, within those constraints, you actually can "rewind" your program to a previously saved point in time, and begin debugging it again -- and you can change the course of events so as to debug a different execution path this time. Finally, there is one bit of internal program state that will be different when you return to a checkpoint -- the program's process id. Each checkpoint will have a unique process id (or PID), and each will be different from the program's original PID. If your program has saved a local copy of its process id, this could potentially pose a problem. 4.12.1 A Non-obvious Benefit of Using Checkpoints ------------------------------------------------- On some systems such as GNU/Linux, address space randomization is performed on new processes for security reasons. This makes it difficult or impossible to set a breakpoint, or watchpoint, on an absolute address if you have to restart the program, since the absolute location of a symbol will change from one execution to the next. A checkpoint, however, is an _identical_ copy of a process. Therefore if you create a checkpoint at (eg.) the start of main, and simply return to that checkpoint instead of restarting the process, you can avoid the effects of address randomization and your symbols will all stay in the same place. ---------- Footnotes ---------- (1) Currently, only GNU/Linux.  File: gdb.info, Node: Stopping, Next: Reverse Execution, 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. On some systems, 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. The expression may be a value of a variable, or it could involve values of one or more variables combined by operators, such as `a + b'. This is sometimes called "data breakpoints". 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 breakpoints 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 * Save Breakpoints:: How to save breakpoints in a file * 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. `break LOCATION' Set a breakpoint at the given LOCATION, which can specify a function name, a line number, or an address of an instruction. (*Note Specify Location::, for a list of all the possible ways to specify a LOCATION.) The breakpoint will stop your program just before it executes any of the code in the specified LOCATION. When using source languages that permit overloading of symbols, such as C++, a function name may refer to more than one possible place to break. *Note Ambiguous Expressions: Ambiguous Expressions, for a discussion of that situation. It is also possible to insert a breakpoint that will stop the program only if a specific thread (*note Thread-Specific Breakpoints::) or a specific task (*note Ada Tasks::) hits that breakpoint. `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 most 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 Breakpoints: Disabling.). *Note Break Conditions: Conditions. For remote targets, you can restrict the number of hardware breakpoints GDB will use, see *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 . `rbreak FILE:REGEX' If `rbreak' is called with a filename qualification, it limits the search for functions matching the given regular expression to the specified FILE. This can be used, for example, to set breakpoints on every function in a given file: (gdb) rbreak file.c:. The colon separating the filename qualifier from the regex may optionally be surrounded by spaces. `info breakpoints [N]' `info break [N]' Print a table of all breakpoints, watchpoints, and catchpoints set and not deleted. Optional argument N means print information only about the specified breakpoint (or watchpoint or catchpoint). For each breakpoint, following columns are printed: _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. For a pending breakpoint whose address is not yet known, this field will contain `'. Such breakpoint won't fire until a shared library that has the symbol or line referred by breakpoint is loaded. See below for details. A breakpoint with several locations will have `' in this field--see below for details. _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.). It is possible that a breakpoint corresponds to several locations in your program. Examples of this situation are: * For a C++ constructor, the GCC compiler generates several instances of the function body, used in different cases. * For a C++ template function, a given line in the function can correspond to any number of instantiations. * For an inlined function, a given source line can correspond to several places where that function is inlined. In all those cases, GDB will insert a breakpoint at all the relevant locations(1). A breakpoint with multiple locations is displayed in the breakpoint table using several rows--one header row, followed by one row for each breakpoint location. The header row has `' in the address column. The rows for individual locations contain the actual addresses for locations, and show the functions to which those locations belong. The number column for a location is of the form BREAKPOINT-NUMBER.LOCATION-NUMBER. For example: Num Type Disp Enb Address What 1 breakpoint keep y stop only if i==1 breakpoint already hit 1 time 1.1 y 0x080486a2 in void foo() at t.cc:8 1.2 y 0x080486ca in void foo() at t.cc:8 Each location can be individually enabled or disabled by passing BREAKPOINT-NUMBER.LOCATION-NUMBER as argument to the `enable' and `disable' commands. Note that you cannot delete the individual locations from the list, you can only delete the entire list of locations that belong to their parent breakpoint (with the `delete NUM' command, where NUM is the number of the parent breakpoint, 1 in the above example). Disabling or enabling the parent breakpoint (*note Disabling::) affects all of the locations that belong to that breakpoint. It's quite common to have a breakpoint inside a shared library. Shared libraries can be loaded and unloaded explicitly, and possibly repeatedly, as the program is executed. To support this use case, GDB updates breakpoint locations whenever any shared library is loaded or unloaded. Typically, you would set a breakpoint in a shared library at the beginning of your debugging session, when the library is not loaded, and when the symbols from the library are not available. When you try to set breakpoint, GDB will ask you if you want to set a so called "pending breakpoint"--breakpoint whose address is not yet resolved. After the program is run, whenever a new shared library is loaded, GDB reevaluates all the breakpoints. When a newly loaded shared library contains the symbol or line referred to by some pending breakpoint, that breakpoint is resolved and becomes an ordinary breakpoint. When a library is unloaded, all breakpoints that refer to its symbols or source lines become pending again. This logic works for breakpoints with multiple locations, too. For example, if you have a breakpoint in a C++ template function, and a newly loaded shared library has an instantiation of that template, a new location is added to the list of locations for the breakpoint. Except for having unresolved address, pending breakpoints do not differ from regular breakpoints. You can set conditions or commands, enable and disable them and perform other breakpoint operations. GDB provides some additional commands for controlling what happens when the `break' command cannot resolve breakpoint address specification to an address: `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. The settings above only affect the `break' command and its variants. Once breakpoint is set, it will be automatically updated as shared libraries are loaded and unloaded. For some targets, GDB can automatically decide if hardware or software breakpoints should be used, depending on whether the breakpoint address is read-only or read-write. This applies to breakpoints set with the `break' command as well as to internal breakpoints set by commands like `next' and `finish'. For breakpoints set with `hbreak', GDB will always use hardware breakpoints. You can control this automatic behaviour with the following commands:: `set breakpoint auto-hw on' This is the default behavior. When GDB sets a breakpoint, it will try to use the target memory map to decide if software or hardware breakpoint must be used. `set breakpoint auto-hw off' This indicates GDB should not automatically select breakpoint type. If the target provides a memory map, GDB will warn when trying to set software breakpoint at a read-only address. GDB normally implements breakpoints by replacing the program code at the breakpoint address with a special instruction, which, when executed, given control to the debugger. By default, the program code is so modified only when the program is resumed. As soon as the program stops, GDB restores the original instructions. This behaviour guards against leaving breakpoints inserted in the target should gdb abrubptly disconnect. However, with slow remote targets, inserting and removing breakpoint can reduce the performance. This behavior can be controlled with the following commands:: `set breakpoint always-inserted off' All breakpoints, including newly added by the user, are inserted in the target only when the target is resumed. All breakpoints are removed from the target when it stops. `set breakpoint always-inserted on' Causes all breakpoints to be inserted in the target at all times. If the user adds a new breakpoint, or changes an existing breakpoint, the breakpoints in the target are updated immediately. A breakpoint is removed from the target only when breakpoint itself is removed. `set breakpoint always-inserted auto' This is the default mode. If GDB is controlling the inferior in non-stop mode (*note Non-Stop Mode::), gdb behaves as if `breakpoint always-inserted' mode is on. If GDB is controlling the inferior in all-stop mode, GDB behaves as if `breakpoint always-inserted' mode is off. 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::). ---------- Footnotes ---------- (1) As of this writing, multiple-location breakpoints work only if there's line number information for all the locations. This means that they will generally not work in system libraries, unless you have debug info with line numbers for them.  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. (This is sometimes called a "data breakpoint".) The expression may be as simple as the value of a single variable, or as complex as many variables combined by operators. Examples include: * A reference to the value of a single variable. * An address cast to an appropriate data type. For example, `*(int *)0x12345678' will watch a 4-byte region at the specified address (assuming an `int' occupies 4 bytes). * An arbitrarily complex expression, such as `a*b + c/d'. The expression can use any operators valid in the program's native language (*note Languages::). You can set a watchpoint on an expression even if the expression can not be evaluated yet. For instance, you can set a watchpoint on `*global_ptr' before `global_ptr' is initialized. GDB will stop when your program sets `global_ptr' and the expression produces a valid value. If the expression becomes valid in some other way than changing a variable (e.g. if the memory pointed to by `*global_ptr' becomes readable as the result of a `malloc' call), GDB may not stop until the next time the expression changes. 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, PowerPC, GNU/Linux and most other x86-based targets, GDB includes support for hardware watchpoints, which do not slow down the running of your program. `watch EXPR [thread THREADNUM]' Set a watchpoint for an expression. GDB will break when the expression EXPR is written into by the program and its value changes. The simplest (and the most popular) use of this command is to watch the value of a single variable: (gdb) watch foo If the command includes a `[thread THREADNUM]' clause, GDB breaks only when the thread identified by THREADNUM changes the value of EXPR. If any other threads change the value of EXPR, GDB will not break. Note that watchpoints restricted to a single thread in this way only work with Hardware Watchpoints. `rwatch EXPR [thread THREADNUM]' Set a watchpoint that will break when the value of EXPR is read by the program. `awatch EXPR [thread THREADNUM]' Set a watchpoint that will break when EXPR is either read from or written into by the program. `info watchpoints' This command prints a list of watchpoints, using the same format as `info break' (*note Set Breaks::). If you watch for a change in a numerically entered address you need to dereference it, as the address itself is just a constant number which will never change. GDB refuses to create a watchpoint that watches a never-changing value: (gdb) watch 0x600850 Cannot watch constant value 0x600850. (gdb) watch *(int *) 0x600850 Watchpoint 1: *(int *) 6293584 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. You can force GDB to use only software watchpoints with the `set can-use-hw-watchpoints 0' command. With this variable set to zero, GDB will never try to use hardware watchpoints, even if the underlying system supports them. (Note that hardware-assisted watchpoints that were set _before_ setting `can-use-hw-watchpoints' to zero will still use the hardware mechanism of watching expression values.) `set can-use-hw-watchpoints' Set whether or not to use hardware watchpoints. `show can-use-hw-watchpoints' Show the current mode of using hardware watchpoints. For remote targets, you can restrict the number of hardware watchpoints GDB will use, see *Note set remote hardware-breakpoint-limit::. 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. Watching complex expressions that reference many variables can also exhaust the resources available for hardware-assisted watchpoints. That's because GDB needs to watch every variable in the expression with separately allocated resources. 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. In multi-threaded programs, watchpoints will detect changes to the watched expression from every thread. _Warning:_ In multi-threaded 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. `exception' An Ada exception being raised. If an exception name is specified at the end of the command (eg `catch exception Program_Error'), the debugger will stop only when this specific exception is raised. Otherwise, the debugger stops execution when any Ada exception is raised. When inserting an exception catchpoint on a user-defined exception whose name is identical to one of the exceptions defined by the language, the fully qualified name must be used as the exception name. Otherwise, GDB will assume that it should stop on the pre-defined exception rather than the user-defined one. For instance, assuming an exception called `Constraint_Error' is defined in package `Pck', then the command to use to catch such exceptions is `catch exception Pck.Constraint_Error'. `exception unhandled' An exception that was raised but is not handled by the program. `assert' A failed Ada assertion. `exec' A call to `exec'. This is currently only available for HP-UX and GNU/Linux. `syscall' `syscall [NAME | NUMBER] ...' A call to or return from a system call, a.k.a. "syscall". A syscall is a mechanism for application programs to request a service from the operating system (OS) or one of the OS system services. GDB can catch some or all of the syscalls issued by the debuggee, and show the related information for each syscall. If no argument is specified, calls to and returns from all system calls will be caught. NAME can be any system call name that is valid for the underlying OS. Just what syscalls are valid depends on the OS. On GNU and Unix systems, you can find the full list of valid syscall names on `/usr/include/asm/unistd.h'. Normally, GDB knows in advance which syscalls are valid for each OS, so you can use the GDB command-line completion facilities (*note command completion: Completion.) to list the available choices. You may also specify the system call numerically. A syscall's number is the value passed to the OS's syscall dispatcher to identify the requested service. When you specify the syscall by its name, GDB uses its database of syscalls to convert the name into the corresponding numeric code, but using the number directly may be useful if GDB's database does not have the complete list of syscalls on your system (e.g., because GDB lags behind the OS upgrades). The example below illustrates how this command works if you don't provide arguments to it: (gdb) catch syscall Catchpoint 1 (syscall) (gdb) r Starting program: /tmp/catch-syscall Catchpoint 1 (call to syscall 'close'), \ 0xffffe424 in __kernel_vsyscall () (gdb) c Continuing. Catchpoint 1 (returned from syscall 'close'), \ 0xffffe424 in __kernel_vsyscall () (gdb) Here is an example of catching a system call by name: (gdb) catch syscall chroot Catchpoint 1 (syscall 'chroot' [61]) (gdb) r Starting program: /tmp/catch-syscall Catchpoint 1 (call to syscall 'chroot'), \ 0xffffe424 in __kernel_vsyscall () (gdb) c Continuing. Catchpoint 1 (returned from syscall 'chroot'), \ 0xffffe424 in __kernel_vsyscall () (gdb) An example of specifying a system call numerically. In the case below, the syscall number has a corresponding entry in the XML file, so GDB finds its name and prints it: (gdb) catch syscall 252 Catchpoint 1 (syscall(s) 'exit_group') (gdb) r Starting program: /tmp/catch-syscall Catchpoint 1 (call to syscall 'exit_group'), \ 0xffffe424 in __kernel_vsyscall () (gdb) c Continuing. Program exited normally. (gdb) However, there can be situations when there is no corresponding name in XML file for that syscall number. In this case, GDB prints a warning message saying that it was not able to find the syscall name, but the catchpoint will be set anyway. See the example below: (gdb) catch syscall 764 warning: The number '764' does not represent a known syscall. Catchpoint 2 (syscall 764) (gdb) If you configure GDB using the `--without-expat' option, it will not be able to display syscall names. Also, if your architecture does not have an XML file describing its system calls, you will not be able to see the syscall names. It is important to notice that these two features are used for accessing the syscall name database. In either case, you will see a warning like this: (gdb) catch syscall warning: Could not open "syscalls/i386-linux.xml" warning: Could not load the syscall XML file 'syscalls/i386-linux.xml'. GDB will not be able to display syscall names. Catchpoint 1 (syscall) (gdb) Of course, the file name will change depending on your architecture and system. Still using the example above, you can also try to catch a syscall by its number. In this case, you would see something like: (gdb) catch syscall 252 Catchpoint 1 (syscall(s) 252) Again, in this case GDB would not be able to display syscall's names. `fork' A call to `fork'. This is currently only available for HP-UX and GNU/Linux. `vfork' A call to `vfork'. This is currently only available for HP-UX and GNU/Linux. `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 LOCATION' Delete any breakpoints set at the specified LOCATION. *Note Specify Location::, for the various forms of LOCATION; the most useful ones are listed below: `clear FUNCTION' `clear FILENAME:FUNCTION' Delete any breakpoints set at entry to the named FUNCTION. `clear LINENUM' `clear FILENAME:LINENUM' Delete any breakpoints set at or within the code of the specified LINENUM of the specified FILENAME. `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' to print a list of all breakpoints, watchpoints, and catchpoints if you do not know which numbers to use. Disabling and enabling a breakpoint that has multiple locations affects all of its locations. 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. Breakpoints set by the `tbreak' command start out in this state. 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: Save Breakpoints, 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 [RANGE...]' `... COMMAND-LIST ...' `end' Specify a list of commands for the given breakpoints. 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 argument, `commands' refers to the last breakpoint, watchpoint, or catchpoint set (not to the breakpoint most recently encountered). If the most recent breakpoints were set with a single command, then the `commands' will apply to all the breakpoints set by that command. This applies to breakpoints set by `rbreak', and also applies when a single `break' command creates multiple breakpoints (*note Ambiguous Expressions: Ambiguous Expressions.). 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: Save Breakpoints, Next: Error in Breakpoints, Prev: Break Commands, Up: Breakpoints 5.1.8 How to save breakpoints to a file --------------------------------------- To save breakpoint definitions to a file use the `save breakpoints' command. `save breakpoints [FILENAME]' This command saves all current breakpoint definitions together with their commands and ignore counts, into a file `FILENAME' suitable for use in a later debugging session. This includes all types of breakpoints (breakpoints, watchpoints, catchpoints, tracepoints). To read the saved breakpoint definitions, use the `source' command (*note Command Files::). Note that watchpoints with expressions involving local variables may fail to be recreated because it may not be possible to access the context where the watchpoint is valid anymore. Because the saved breakpoint definitions are simply a sequence of GDB commands that recreate the breakpoints, you can edit the file in your favorite editing program, and remove the breakpoint definitions you're not interested in, or that can no longer be recreated.  File: gdb.info, Node: Error in Breakpoints, Next: Breakpoint-related Warnings, Prev: Save Breakpoints, Up: Breakpoints 5.1.9 "Cannot insert breakpoints" --------------------------------- If you request too many active hardware-assisted breakpoints and watchpoints, you will see this error message: 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. `show step-mode' Show whether GDB will stop in or step over functions without source line debug information. `finish' Continue running until just after function in the selected stack frame returns. Print the returned value (if any). This command can be abbreviated as `fin'. 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 described in *Note Specify Location::. This form of the command uses temporary 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, which should be of one of the forms described in *Note Specify Location::. 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 `Ctrl-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 signals SIG' Similar, but print information only about the specified signal number. `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. Optional arguments KEYWORDS, described below, 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. On some targets, GDB can inspect extra signal information associated with the intercepted signal, before it is actually delivered to the program being debugged. This information is exported by the convenience variable `$_siginfo', and consists of data that is passed by the kernel to the signal handler at the time of the receipt of a signal. The data type of the information itself is target dependent. You can see the data type using the `ptype $_siginfo' command. On Unix systems, it typically corresponds to the standard `siginfo_t' type, as defined in the `signal.h' system header. Here's an example, on a GNU/Linux system, printing the stray referenced address that raised a segmentation fault. (gdb) continue Program received signal SIGSEGV, Segmentation fault. 0x0000000000400766 in main () 69 *(int *)p = 0; (gdb) ptype $_siginfo type = struct { int si_signo; int si_errno; int si_code; union { int _pad[28]; struct {...} _kill; struct {...} _timer; struct {...} _rt; struct {...} _sigchld; struct {...} _sigfault; struct {...} _sigpoll; } _sifields; } (gdb) ptype $_siginfo._sifields._sigfault type = struct { void *si_addr; } (gdb) p $_siginfo._sifields._sigfault.si_addr $1 = (void *) 0x7ffff7ff7000 Depending on target support, `$_siginfo' may also be writable.  File: gdb.info, Node: Thread Stops, Prev: Signals, Up: Stopping 5.4 Stopping and Starting Multi-thread Programs =============================================== GDB supports debugging programs with multiple threads (*note Debugging Programs with Multiple Threads: Threads.). There are two modes of controlling execution of your program within the debugger. In the default mode, referred to as "all-stop mode", when any thread in your program stops (for example, at a breakpoint or while being stepped), all other threads in the program are also stopped by GDB. On some targets, GDB also supports "non-stop mode", in which other threads can continue to run freely while you examine the stopped thread in the debugger. * Menu: * All-Stop Mode:: All threads stop when GDB takes control * Non-Stop Mode:: Other threads continue to execute * Background Execution:: Running your program asynchronously * Thread-Specific Breakpoints:: Controlling breakpoints * Interrupted System Calls:: GDB may interfere with system calls * Observer Mode:: GDB does not alter program behavior  File: gdb.info, Node: All-Stop Mode, Next: Non-Stop Mode, Up: Thread Stops 5.4.1 All-Stop Mode ------------------- In all-stop mode, 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. 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. 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 such as `[Switching to Thread N]' to identify the thread. On some OSes, you can modify GDB's default behavior by locking the OS scheduler to 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 prevents other threads from preempting the current thread while you are stepping, so that the focus of debugging does not change unexpectedly. Other threads 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, GDB does not change the current thread away from the thread that you are debugging. `show scheduler-locking' Display the current scheduler locking mode. By default, when you issue one of the execution commands such as `continue', `next' or `step', GDB allows only threads of the current inferior to run. For example, if GDB is attached to two inferiors, each with two threads, the `continue' command resumes only the two threads of the current inferior. This is useful, for example, when you debug a program that forks and you want to hold the parent stopped (so that, for instance, it doesn't run to exit), while you debug the child. In other situations, you may not be interested in inspecting the current state of any of the processes GDB is attached to, and you may want to resume them all until some breakpoint is hit. In the latter case, you can instruct GDB to allow all threads of all the inferiors to run with the `set schedule-multiple' command. `set schedule-multiple' Set the mode for allowing threads of multiple processes to be resumed when an execution command is issued. When `on', all threads of all processes are allowed to run. When `off', only the threads of the current process are resumed. The default is `off'. The `scheduler-locking' mode takes precedence when set to `on', or while you are stepping and set to `step'. `show schedule-multiple' Display the current mode for resuming the execution of threads of multiple processes.  File: gdb.info, Node: Non-Stop Mode, Next: Background Execution, Prev: All-Stop Mode, Up: Thread Stops 5.4.2 Non-Stop Mode ------------------- For some multi-threaded targets, GDB supports an optional mode of operation in which you can examine stopped program threads in the debugger while other threads continue to execute freely. This minimizes intrusion when debugging live systems, such as programs where some threads have real-time constraints or must continue to respond to external events. This is referred to as "non-stop" mode. In non-stop mode, when a thread stops to report a debugging event, _only_ that thread is stopped; GDB does not stop other threads as well, in contrast to the all-stop mode behavior. Additionally, execution commands such as `continue' and `step' apply by default only to the current thread in non-stop mode, rather than all threads as in all-stop mode. This allows you to control threads explicitly in ways that are not possible in all-stop mode -- for example, stepping one thread while allowing others to run freely, stepping one thread while holding all others stopped, or stepping several threads independently and simultaneously. To enter non-stop mode, use this sequence of commands before you run or attach to your program: # Enable the async interface. set target-async 1 # If using the CLI, pagination breaks non-stop. set pagination off # Finally, turn it on! set non-stop on You can use these commands to manipulate the non-stop mode setting: `set non-stop on' Enable selection of non-stop mode. `set non-stop off' Disable selection of non-stop mode. `show non-stop' Show the current non-stop enablement setting. Note these commands only reflect whether non-stop mode is enabled, not whether the currently-executing program is being run in non-stop mode. In particular, the `set non-stop' preference is only consulted when GDB starts or connects to the target program, and it is generally not possible to switch modes once debugging has started. Furthermore, since not all targets support non-stop mode, even when you have enabled non-stop mode, GDB may still fall back to all-stop operation by default. In non-stop mode, all execution commands apply only to the current thread by default. That is, `continue' only continues one thread. To continue all threads, issue `continue -a' or `c -a'. You can use GDB's background execution commands (*note Background Execution::) to run some threads in the background while you continue to examine or step others from GDB. The MI execution commands (*note GDB/MI Program Execution::) are always executed asynchronously in non-stop mode. Suspending execution is done with the `interrupt' command when running in the background, or `Ctrl-c' during foreground execution. In all-stop mode, this stops the whole process; but in non-stop mode the interrupt applies only to the current thread. To stop the whole program, use `interrupt -a'. Other execution commands do not currently support the `-a' option. In non-stop mode, when a thread stops, GDB doesn't automatically make that thread current, as it does in all-stop mode. This is because the thread stop notifications are asynchronous with respect to GDB's command interpreter, and it would be confusing if GDB unexpectedly changed to a different thread just as you entered a command to operate on the previously current thread.  File: gdb.info, Node: Background Execution, Next: Thread-Specific Breakpoints, Prev: Non-Stop Mode, Up: Thread Stops 5.4.3 Background Execution -------------------------- GDB's execution commands have two variants: the normal foreground (synchronous) behavior, and a background (asynchronous) behavior. In foreground execution, GDB waits for the program to report that some thread has stopped before prompting for another command. In background execution, GDB immediately gives a command prompt so that you can issue other commands while your program runs. You need to explicitly enable asynchronous mode before you can use background execution commands. You can use these commands to manipulate the asynchronous mode setting: `set target-async on' Enable asynchronous mode. `set target-async off' Disable asynchronous mode. `show target-async' Show the current target-async setting. If the target doesn't support async mode, GDB issues an error message if you attempt to use the background execution commands. To specify background execution, add a `&' to the command. For example, the background form of the `continue' command is `continue&', or just `c&'. The execution commands that accept background execution are: `run' *Note Starting your Program: Starting. `attach' *Note Debugging an Already-running Process: Attach. `step' *Note step: Continuing and Stepping. `stepi' *Note stepi: Continuing and Stepping. `next' *Note next: Continuing and Stepping. `nexti' *Note nexti: Continuing and Stepping. `continue' *Note continue: Continuing and Stepping. `finish' *Note finish: Continuing and Stepping. `until' *Note until: Continuing and Stepping. Background execution is especially useful in conjunction with non-stop mode for debugging programs with multiple threads; see *Note Non-Stop Mode::. However, you can also use these commands in the normal all-stop mode with the restriction that you cannot issue another execution command until the previous one finishes. Examples of commands that are valid in all-stop mode while the program is running include `help' and `info break'. You can interrupt your program while it is running in the background by using the `interrupt' command. `interrupt' `interrupt -a' Suspend execution of the running program. In all-stop mode, `interrupt' stops the whole process, but in non-stop mode, it stops only the current thread. To stop the whole program in non-stop mode, use `interrupt -a'.  File: gdb.info, Node: Thread-Specific Breakpoints, Next: Interrupted System Calls, Prev: Background Execution, Up: Thread Stops 5.4.4 Thread-Specific Breakpoints --------------------------------- 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 (*note Specify Location::), 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 or after the breakpoint condition, like this: (gdb) break frik.c:13 thread 28 if bartab > lim  File: gdb.info, Node: Interrupted System Calls, Next: Observer Mode, Prev: Thread-Specific Breakpoints, Up: Thread Stops 5.4.5 Interrupted System Calls ------------------------------ There is an unfortunate side effect when using GDB to debug multi-threaded programs. 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.  File: gdb.info, Node: Observer Mode, Prev: Interrupted System Calls, Up: Thread Stops 5.4.6 Observer Mode ------------------- If you want to build on non-stop mode and observe program behavior without any chance of disruption by GDB, you can set variables to disable all of the debugger's attempts to modify state, whether by writing memory, inserting breakpoints, etc. These operate at a low level, intercepting operations from all commands. When all of these are set to `off', then GDB is said to be "observer mode". As a convenience, the variable `observer' can be set to disable these, plus enable non-stop mode. Note that GDB will not prevent you from making nonsensical combinations of these settings. For instance, if you have enabled `may-insert-breakpoints' but disabled `may-write-memory', then breakpoints that work by writing trap instructions into the code stream will still not be able to be placed. `set observer on' `set observer off' When set to `on', this disables all the permission variables below (except for `insert-fast-tracepoints'), plus enables non-stop debugging. Setting this to `off' switches back to normal debugging, though remaining in non-stop mode. `show observer' Show whether observer mode is on or off. `set may-write-registers on' `set may-write-registers off' This controls whether GDB will attempt to alter the values of registers, such as with assignment expressions in `print', or the `jump' command. It defaults to `on'. `show may-write-registers' Show the current permission to write registers. `set may-write-memory on' `set may-write-memory off' This controls whether GDB will attempt to alter the contents of memory, such as with assignment expressions in `print'. It defaults to `on'. `show may-write-memory' Show the current permission to write memory. `set may-insert-breakpoints on' `set may-insert-breakpoints off' This controls whether GDB will attempt to insert breakpoints. This affects all breakpoints, including internal breakpoints defined by GDB. It defaults to `on'. `show may-insert-breakpoints' Show the current permission to insert breakpoints. `set may-insert-tracepoints on' `set may-insert-tracepoints off' This controls whether GDB will attempt to insert (regular) tracepoints at the beginning of a tracing experiment. It affects only non-fast tracepoints, fast tracepoints being under the control of `may-insert-fast-tracepoints'. It defaults to `on'. `show may-insert-tracepoints' Show the current permission to insert tracepoints. `set may-insert-fast-tracepoints on' `set may-insert-fast-tracepoints off' This controls whether GDB will attempt to insert fast tracepoints at the beginning of a tracing experiment. It affects only fast tracepoints, regular (non-fast) tracepoints being under the control of `may-insert-tracepoints'. It defaults to `on'. `show may-insert-fast-tracepoints' Show the current permission to insert fast tracepoints. `set may-interrupt on' `set may-interrupt off' This controls whether GDB will attempt to interrupt or stop program execution. When this variable is `off', the `interrupt' command will have no effect, nor will `Ctrl-c'. It defaults to `on'. `show may-interrupt' Show the current permission to interrupt or stop the program.  File: gdb.info, Node: Reverse Execution, Next: Process Record and Replay, Prev: Stopping, Up: Top 6 Running programs backward *************************** When you are debugging a program, it is not unusual to realize that you have gone too far, and some event of interest has already happened. If the target environment supports it, GDB can allow you to "rewind" the program by running it backward. A target environment that supports reverse execution should be able to "undo" the changes in machine state that have taken place as the program was executing normally. Variables, registers etc. should revert to their previous values. Obviously this requires a great deal of sophistication on the part of the target environment; not all target environments can support reverse execution. When a program is executed in reverse, the instructions that have most recently been executed are "un-executed", in reverse order. The program counter runs backward, following the previous thread of execution in reverse. As each instruction is "un-executed", the values of memory and/or registers that were changed by that instruction are reverted to their previous states. After executing a piece of source code in reverse, all side effects of that code should be "undone", and all variables should be returned to their prior values(1). If you are debugging in a target environment that supports reverse execution, GDB provides the following commands. `reverse-continue [IGNORE-COUNT]' `rc [IGNORE-COUNT]' Beginning at the point where your program last stopped, start executing in reverse. Reverse execution will stop for breakpoints and synchronous exceptions (signals), just like normal execution. Behavior of asynchronous signals depends on the target environment. `reverse-step [COUNT]' Run the program backward until control reaches the start of a different source line; then stop it, and return control to GDB. Like the `step' command, `reverse-step' will only stop at the beginning of a source line. It "un-executes" the previously executed source line. If the previous source line included calls to debuggable functions, `reverse-step' will step (backward) into the called function, stopping at the beginning of the _last_ statement in the called function (typically a return statement). Also, as with the `step' command, if non-debuggable functions are called, `reverse-step' will run thru them backward without stopping. `reverse-stepi [COUNT]' Reverse-execute one machine instruction. Note that the instruction to be reverse-executed is _not_ the one pointed to by the program counter, but the instruction executed prior to that one. For instance, if the last instruction was a jump, `reverse-stepi' will take you back from the destination of the jump to the jump instruction itself. `reverse-next [COUNT]' Run backward to the beginning of the previous line executed in the current (innermost) stack frame. If the line contains function calls, they will be "un-executed" without stopping. Starting from the first line of a function, `reverse-next' will take you back to the caller of that function, _before_ the function was called, just as the normal `next' command would take you from the last line of a function back to its return to its caller (2). `reverse-nexti [COUNT]' Like `nexti', `reverse-nexti' executes a single instruction in reverse, except that called functions are "un-executed" atomically. That is, if the previously executed instruction was a return from another function, `reverse-nexti' will continue to execute in reverse until the call to that function (from the current stack frame) is reached. `reverse-finish' Just as the `finish' command takes you to the point where the current function returns, `reverse-finish' takes you to the point where it was called. Instead of ending up at the end of the current function invocation, you end up at the beginning. `set exec-direction' Set the direction of target execution. `set exec-direction reverse' GDB will perform all execution commands in reverse, until the exec-direction mode is changed to "forward". Affected commands include `step, stepi, next, nexti, continue, and finish'. The `return' command cannot be used in reverse mode. `set exec-direction forward' GDB will perform all execution commands in the normal fashion. This is the default. ---------- Footnotes ---------- (1) Note that some side effects are easier to undo than others. For instance, memory and registers are relatively easy, but device I/O is hard. Some targets may be able undo things like device I/O, and some may not. The contract between GDB and the reverse executing target requires only that the target do something reasonable when GDB tells it to execute backwards, and then report the results back to GDB. Whatever the target reports back to GDB, GDB will report back to the user. GDB assumes that the memory and registers that the target reports are in a consistant state, but GDB accepts whatever it is given. (2) Unless the code is too heavily optimized.  File: gdb.info, Node: Process Record and Replay, Next: Stack, Prev: Reverse Execution, Up: Top 7 Recording Inferior's Execution and Replaying It ************************************************* On some platforms, GDB provides a special "process record and replay" target that can record a log of the process execution, and replay it later with both forward and reverse execution commands. When this target is in use, if the execution log includes the record for the next instruction, GDB will debug in "replay mode". In the replay mode, the inferior does not really execute code instructions. Instead, all the events that normally happen during code execution are taken from the execution log. While code is not really executed in replay mode, the values of registers (including the program counter register) and the memory of the inferior are still changed as they normally would. Their contents are taken from the execution log. If the record for the next instruction is not in the execution log, GDB will debug in "record mode". In this mode, the inferior executes normally, and GDB records the execution log for future replay. The process record and replay target supports reverse execution (*note Reverse Execution::), even if the platform on which the inferior runs does not. However, the reverse execution is limited in this case by the range of the instructions recorded in the execution log. In other words, reverse execution on platforms that don't support it directly can only be done in the replay mode. When debugging in the reverse direction, GDB will work in replay mode as long as the execution log includes the record for the previous instruction; otherwise, it will work in record mode, if the platform supports reverse execution, or stop if not. For architecture environments that support process record and replay, GDB provides the following commands: `target record' This command starts the process record and replay target. The process record and replay target can only debug a process that is already running. Therefore, you need first to start the process with the `run' or `start' commands, and then start the recording with the `target record' command. Both `record' and `rec' are aliases of `target record'. Displaced stepping (*note displaced stepping: Maintenance Commands.) will be automatically disabled when process record and replay target is started. That's because the process record and replay target doesn't support displaced stepping. If the inferior is in the non-stop mode (*note Non-Stop Mode::) or in the asynchronous execution mode (*note Background Execution::), the process record and replay target cannot be started because it doesn't support these two modes. `record stop' Stop the process record and replay target. When process record and replay target stops, the entire execution log will be deleted and the inferior will either be terminated, or will remain in its final state. When you stop the process record and replay target in record mode (at the end of the execution log), the inferior will be stopped at the next instruction that would have been recorded. In other words, if you record for a while and then stop recording, the inferior process will be left in the same state as if the recording never happened. On the other hand, if the process record and replay target is stopped while in replay mode (that is, not at the end of the execution log, but at some earlier point), the inferior process will become "live" at that earlier state, and it will then be possible to continue the usual "live" debugging of the process from that state. When the inferior process exits, or GDB detaches from it, process record and replay target will automatically stop itself. `record save FILENAME' Save the execution log to a file `FILENAME'. Default filename is `gdb_record.PROCESS_ID', where PROCESS_ID is the process ID of the inferior. `record restore FILENAME' Restore the execution log from a file `FILENAME'. File must have been created with `record save'. `set record insn-number-max LIMIT' Set the limit of instructions to be recorded. Default value is 200000. If LIMIT is a positive number, then GDB will start deleting instructions from the log once the number of the record instructions becomes greater than LIMIT. For every new recorded instruction, GDB will delete the earliest recorded instruction to keep the number of recorded instructions at the limit. (Since deleting recorded instructions loses information, GDB lets you control what happens when the limit is reached, by means of the `stop-at-limit' option, described below.) If LIMIT is zero, GDB will never delete recorded instructions from the execution log. The number of recorded instructions is unlimited in this case. `show record insn-number-max' Show the limit of instructions to be recorded. `set record stop-at-limit' Control the behavior when the number of recorded instructions reaches the limit. If ON (the default), GDB will stop when the limit is reached for the first time and ask you whether you want to stop the inferior or continue running it and recording the execution log. If you decide to continue recording, each new recorded instruction will cause the oldest one to be deleted. If this option is OFF, GDB will automatically delete the oldest record to make room for each new one, without asking. `show record stop-at-limit' Show the current setting of `stop-at-limit'. `set record memory-query' Control the behavior when GDB is unable to record memory changes caused by an instruction. If ON, GDB will query whether to stop the inferior in that case. If this option is OFF (the default), GDB will automatically ignore the effect of such instructions on memory. Later, when GDB replays this execution log, it will mark the log of this instruction as not accessible, and it will not affect the replay results. `show record memory-query' Show the current setting of `memory-query'. `info record' Show various statistics about the state of process record and its in-memory execution log buffer, including: * Whether in record mode or replay mode. * Lowest recorded instruction number (counting from when the current execution log started recording instructions). * Highest recorded instruction number. * Current instruction about to be replayed (if in replay mode). * Number of instructions contained in the execution log. * Maximum number of instructions that may be contained in the execution log. `record delete' When record target runs in replay mode ("in the past"), delete the subsequent execution log and begin to record a new execution log starting from the current address. This means you will abandon the previously recorded "future" and begin recording a new "future".  File: gdb.info, Node: Stack, Next: Source, Prev: Process Record and Replay, Up: Top 8 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 8.1 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" (*note $fp: Registers.) 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 8.2 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 `Ctrl-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. `backtrace full' `bt full' `bt full N' `bt full -N' Print the values of the local variables also. N specifies the number of frames to print, as described above. The names `where' and `info stack' (abbreviated `info s') are additional aliases for `backtrace'. In a multi-threaded program, GDB by default shows the backtrace only for the current thread. To display the backtrace for several or all of the threads, use the command `thread apply' (*note thread apply: Threads.). For example, if you type `thread apply all backtrace', GDB will display the backtrace for all the threads; this is handy when you debug a core dump of a multi-threaded program. 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, data=...) 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'. The value of parameter `data' in frame 1 has been replaced by `...'. By default, GDB prints the value of a parameter only if it is a scalar (integer, pointer, enumeration, etc). See command `set print frame-arguments' in *Note Print Settings:: for more details on how to configure the way function parameter values are printed. If your program was compiled with optimizations, some compilers will optimize away arguments passed to functions if those arguments are never used after the call. Such optimizations generate code that passes arguments through registers, but doesn't store those arguments in the stack frame. GDB has no way of displaying such arguments in stack frames other than the innermost one. Here's what such a backtrace might look like: #0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) at builtin.c:993 #1 0x6e38 in expand_macro (sym=) at macro.c:242 #2 0x6840 in expand_token (obs=0x0, t=, td=0xf7fffb08) at macro.c:71 (More stack frames follow...) The values of arguments that were not saved in their stack frames are shown as `'. If you need to display the values of such optimized-out arguments, either deduce that from other variables whose values depend on the one you are interested in, or recompile without optimizations. 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'(1). 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 past-entry' `set backtrace past-entry on' Backtraces will continue past the internal entry point of an application. This entry point is encoded by the linker when the application is built, and is likely before the user entry point `main' (or equivalent) is called. `set backtrace past-entry off' Backtraces will stop when they encounter the internal entry point of an application. This is the default. `show backtrace past-entry' Display the current internal 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. ---------- Footnotes ---------- (1) Note that embedded programs (the so-called "free-standing" environment) are not required to have a `main' function as the entry point. They could even have multiple entry points.  File: gdb.info, Node: Selection, Next: Frame Info, Prev: Backtrace, Up: Stack 8.3 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 8.4 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 9 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 * Specify Location:: How to specify code locations * 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: Specify Location, Up: Source 9.1 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; see *Note Specify Location::, for the full list. 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 (*note Specify Location::), 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. When a `list' command has two linespecs, and the source file of the second linespec is omitted, this refers to the same source file as the first linespec. `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.  File: gdb.info, Node: Specify Location, Next: Edit, Prev: List, Up: Source 9.2 Specifying a Location ========================= Several GDB commands accept arguments that specify a location of your program's code. Since GDB is a source-level debugger, a location usually specifies some line in the source code; for that reason, locations are also known as "linespecs". Here are all the different ways of specifying a code location that GDB understands: `LINENUM' Specifies the line number LINENUM of the current source file. `-OFFSET' `+OFFSET' Specifies the line OFFSET lines before or after the "current line". For the `list' command, the current line is the last one printed; for the breakpoint commands, this is the line at which execution stopped in the currently selected "stack frame" (*note Frames: Frames, for a description of stack frames.) When used as the second of the two linespecs in a `list' command, this specifies the line OFFSET lines up or down from the first linespec. `FILENAME:LINENUM' Specifies the line LINENUM 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 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 program address ADDRESS. For line-oriented commands, such as `list' and `edit', this specifies a source line that contains ADDRESS. For `break' and other breakpoint oriented commands, this can be used to set breakpoints in parts of your program which do not have debugging information or source files. Here ADDRESS may be any expression valid in the current working language (*note working language: Languages.) that specifies a code address. In addition, as a convenience, GDB extends the semantics of expressions used in locations to cover the situations that frequently happen during debugging. Here are the various forms of ADDRESS: `EXPRESSION' Any expression valid in the current working language. `FUNCADDR' An address of a function or procedure derived from its name. In C, C++, Java, Objective-C, Fortran, minimal, and assembly, this is simply the function's name FUNCTION (and actually a special case of a valid expression). In Pascal and Modula-2, this is `&FUNCTION'. In Ada, this is `FUNCTION'Address' (although the Pascal form also works). This form specifies the address of the function's first instruction, before the stack frame and arguments have been set up. `'FILENAME'::FUNCADDR' Like FUNCADDR above, but also specifies the name of the source file explicitly. This is useful if the name of the function does not specify the function unambiguously, e.g., if there are several functions with identical names in different source files.  File: gdb.info, Node: Edit, Next: Search, Prev: Specify Location, Up: Source 9.3 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: `edit LOCATION' Edit the source file specified by `location'. Editing starts at that LOCATION, e.g., at the specified source line of the specified file. *Note Specify Location::, for all the possible forms of the LOCATION argument; here are the forms of the `edit' command most commonly used: `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. 9.3.1 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 9.4 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 9.5 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. 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. The search path is used to find both program source files and GDB script files (read using the `-command' option and `source' command). In addition to the source path, GDB provides a set of commands that manage a list of source path substitution rules. A "substitution rule" specifies how to rewrite source directories stored in the program's debug information in case the sources were moved to a different directory between compilation and debugging. A rule is made of two strings, the first specifying what needs to be rewritten in the path, and the second specifying how it should be rewritten. In *Note set substitute-path::, we name these two parts FROM and TO respectively. GDB does a simple string replacement of FROM with TO at the start of the directory part of the source file name, and uses that result instead of the original file name to look up the sources. Using the previous example, suppose the `foo-1.0' tree has been moved from `/usr/src' to `/mnt/cross', then you can tell GDB to replace `/usr/src' in all source path names with `/mnt/cross'. The first lookup will then be `/mnt/cross/foo-1.0/lib/foo.c' in place of the original location of `/usr/src/foo-1.0/lib/foo.c'. To define a source path substitution rule, use the `set substitute-path' command (*note set substitute-path::). To avoid unexpected substitution results, a rule is applied only if the FROM part of the directory name ends at a directory separator. For instance, a rule substituting `/usr/source' into `/mnt/cross' will be applied to `/usr/source/foo-1.0' but not to `/usr/sourceware/foo-2.0'. And because the substitution is applied only at the beginning of the directory name, this rule will not be applied to `/root/usr/source/baz.c' either. In many cases, you can achieve the same result using the `directory' command. However, `set substitute-path' can be more efficient in the case where the sources are organized in a complex tree with multiple subdirectories. With the `directory' command, you need to add each subdirectory of your project. If you moved the entire tree while preserving its internal organization, then `set substitute-path' allows you to direct the debugger to all the sources with one single command. `set substitute-path' is also more than just a shortcut command. The source path is only used if the file at the original location no longer exists. On the other hand, `set substitute-path' modifies the debugger behavior to look at the rewritten location instead. So, if for any reason a source file that is not relevant to your executable is located at the original location, a substitution rule is the only method available to point GDB at the new location. You can configure a default source path substitution rule by configuring GDB with the `--with-relocated-sources=DIR' option. The DIR should be the name of a directory under GDB's configured prefix (set with `--prefix' or `--exec-prefix'), and directory names in debug information under DIR will be adjusted automatically if the installed GDB is moved to a new location. This is useful if GDB, libraries or executables with debug information and corresponding source code are being moved together. `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 its default value (`$cdir:$cwd' on Unix systems). This requires confirmation. `show directories' Print the source path: show which directories it contains. `set substitute-path FROM TO' Define a source path substitution rule, and add it at the end of the current list of existing substitution rules. If a rule with the same FROM was already defined, then the old rule is also deleted. For example, if the file `/foo/bar/baz.c' was moved to `/mnt/cross/baz.c', then the command (gdb) set substitute-path /usr/src /mnt/cross will tell GDB to replace `/usr/src' with `/mnt/cross', which will allow GDB to find the file `baz.c' even though it was moved. In the case when more than one substitution rule have been defined, the rules are evaluated one by one in the order where they have been defined. The first one matching, if any, is selected to perform the substitution. For instance, if we had entered the following commands: (gdb) set substitute-path /usr/src/include /mnt/include (gdb) set substitute-path /usr/src /mnt/src GDB would then rewrite `/usr/src/include/defs.h' into `/mnt/include/defs.h' by using the first rule. However, it would use the second rule to rewrite `/usr/src/lib/foo.c' into `/mnt/src/lib/foo.c'. `unset substitute-path [path]' If a path is specified, search the current list of substitution rules for a rule that would rewrite that path. Delete that rule if found. A warning is emitted by the debugger if no rule could be found. If no path is specified, then all substitution rules are deleted. `show substitute-path [path]' If a path is specified, then print the source path substitution rule which would rewrite that path, if any. If no path is specified, then print all existing source path substitution rules. 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 its default value. 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 9.6 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. You can use the command `set disassemble-next-line' to set whether to disassemble next source line when execution stops. 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 documented in *Note Specify Location::. 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 `$_' (*note Convenience Variables: Convenience Vars.). `disassemble' `disassemble /m' `disassemble /r' This specialized command dumps a range of memory as machine instructions. It can also print mixed source+disassembly by specifying the `/m' modifier and print the raw instructions in hex as well as in symbolic form by specifying the `/r'. The default memory range is the function surrounding the program counter of the selected frame. A single argument to this command is a program counter value; GDB dumps the function surrounding this value. When two arguments are given, they should be separated by a comma, possibly surrounded by whitespace. The arguments specify a range of addresses to dump, in one of two forms: `START,END' the addresses from START (inclusive) to END (exclusive) `START,+LENGTH' the addresses from START (inclusive) to `START+LENGTH' (exclusive). When 2 arguments are specified, the name of the function is also printed (since there could be several functions in the given range). The argument(s) can be any expression yielding a numeric value, such as `0x32c4', `&main+10' or `$pc - 8'. If the range of memory being disassembled contains current program counter, the instruction at that location is shown with a `=>' marker. 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. Here is an example showing mixed source+assembly for Intel x86, when the program is stopped just after function prologue: (gdb) disas /m main Dump of assembler code for function main: 5 { 0x08048330 <+0>: push %ebp 0x08048331 <+1>: mov %esp,%ebp 0x08048333 <+3>: sub $0x8,%esp 0x08048336 <+6>: and $0xfffffff0,%esp 0x08048339 <+9>: sub $0x10,%esp 6 printf ("Hello.\n"); => 0x0804833c <+12>: movl $0x8048440,(%esp) 0x08048343 <+19>: call 0x8048284 7 return 0; 8 } 0x08048348 <+24>: mov $0x0,%eax 0x0804834d <+29>: leave 0x0804834e <+30>: ret End of assembler dump. Here is another example showing raw instructions in hex for AMD x86-64, (gdb) disas /r 0x400281,+10 Dump of assembler code from 0x400281 to 0x40028b: 0x0000000000400281: 38 36 cmp %dh,(%rsi) 0x0000000000400283: 2d 36 34 2e 73 sub $0x732e3436,%eax 0x0000000000400288: 6f outsl %ds:(%rsi),(%dx) 0x0000000000400289: 2e 32 00 xor %cs:(%rax),%al End of assembler dump. Some architectures have more than one commonly-used set of instruction mnemonics or other syntax. For programs that were dynamically linked and use shared libraries, instructions that call functions or branch to locations in the shared libraries might show a seemingly bogus location--it's actually a location of the relocation table. On some architectures, GDB might be able to resolve these to actual function names. `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. `show disassembly-flavor' Show the current setting of the disassembly flavor. `set disassemble-next-line' `show disassemble-next-line' Control whether or not GDB will disassemble the next source line or instruction when execution stops. If ON, GDB will display disassembly of the next source line when execution of the program being debugged stops. This is _in addition_ to displaying the source line itself, which GDB always does if possible. If the next source line cannot be displayed for some reason (e.g., if GDB cannot find the source file, or there's no line info in the debug info), GDB will display disassembly of the next _instruction_ instead of showing the next source line. If AUTO, GDB will display disassembly of next instruction only if the source line cannot be displayed. This setting causes GDB to display some feedback when you step through a function with no line info or whose source file is unavailable. The default is OFF, which means never display the disassembly of the next line or instruction.  File: gdb.info, Node: Data, Next: Optimized Code, Prev: Source, Up: Top 10 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.). It may also print the expression using a Python-based pretty-printer (*note Pretty Printing::). `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 * Ambiguous Expressions:: Ambiguous Expressions * Variables:: Program variables * Arrays:: Artificial arrays * Output Formats:: Output formats * Memory:: Examining memory * Auto Display:: Automatic display * Print Settings:: Print settings * Pretty Printing:: Python pretty printing * Value History:: Value history * Convenience Vars:: Convenience variables * Registers:: Registers * Floating Point Hardware:: Floating point hardware * Vector Unit:: Vector Unit * OS Information:: Auxiliary data provided by operating system * Memory Region Attributes:: Memory region attributes * Dump/Restore Files:: Copy between memory and a file * Core File Generation:: Cause a program dump its core * Character Sets:: Debugging programs that use a different character set than GDB does * Caching Remote Data:: Data caching for remote targets * Searching Memory:: Searching memory for a sequence of bytes  File: gdb.info, Node: Expressions, Next: Ambiguous Expressions, Up: Data 10.1 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 create an array of three integers. If you pass an array to a function or assign it to a program variable, GDB copies the array to 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: Ambiguous Expressions, Next: Variables, Prev: Expressions, Up: Data 10.2 Ambiguous Expressions ========================== Expressions can sometimes contain some ambiguous elements. For instance, some programming languages (notably Ada, C++ and Objective-C) permit a single function name to be defined several times, for application in different contexts. This is called "overloading". Another example involving Ada is generics. A "generic package" is similar to C++ templates and is typically instantiated several times, resulting in the same function name being defined in different contexts. In some cases and depending on the language, it is possible to adjust the expression to remove the ambiguity. For instance in C++, you can specify the signature of the function you want to break on, as in `break FUNCTION(TYPES)'. In Ada, using the fully qualified name of your function often makes the expression unambiguous as well. When an ambiguity that needs to be resolved is detected, the debugger has the capability to display a menu of numbered choices for each possibility, and then waits for the selection with the prompt `>'. The first option is always `[0] cancel', and typing `0 ' aborts the current command. If the command in which the expression was used allows more than one choice to be selected, the next option in the menu is `[1] all', and typing `1 ' selects all possible choices. 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) `set multiple-symbols MODE' This option allows you to adjust the debugger behavior when an expression is ambiguous. By default, MODE is set to `all'. If the command with which the expression is used allows more than one choice, then GDB automatically selects all possible choices. For instance, inserting a breakpoint on a function using an ambiguous name results in a breakpoint inserted on each possible match. However, if a unique choice must be made, then GDB uses the menu to help you disambiguate the expression. For instance, printing the address of an overloaded function will result in the use of the menu. When MODE is set to `ask', the debugger always uses the menu when an ambiguity is detected. Finally, when MODE is set to `cancel', the debugger reports an error due to the ambiguity and the command is aborted. `show multiple-symbols' Show the current value of the `multiple-symbols' setting.  File: gdb.info, Node: Variables, Next: Arrays, Prev: Ambiguous Expressions, Up: Data 10.3 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 GCC: (gcc.info)Debugging Options. *Note C and C++: C, for more information about debug info formats that are best suited to C++ programs. If you ask to print an object whose contents are unknown to GDB, e.g., because its data type is not completely specified by the debug information, GDB will say `'. *Note incomplete type: Symbols, for more about this. Strings are identified as arrays of `char' values without specified signedness. Arrays of either `signed char' or `unsigned char' get printed as arrays of 1 byte sized integers. `-fsigned-char' or `-funsigned-char' GCC options have no effect as GDB defines literal string type `"char"' as `char' without a sign. For program code char var0[] = "A"; signed char var1[] = "A"; You get during debugging (gdb) print var0 $1 = "A" (gdb) print var1 $2 = {65 'A', 0 '\0'}  File: gdb.info, Node: Arrays, Next: Output Formats, Prev: Variables, Up: Data 10.4 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 10.5 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. This prints both the numerical value and its character representation. The character representation is replaced with the octal escape `\nnn' for characters outside the 7-bit ASCII range. Without this format, GDB displays `char', `unsigned char', and `signed char' data as character constants. Single-byte members of vectors are displayed as integer data. `f' Regard the bits of the value as a floating point number and print using typical floating point syntax. `s' Regard as a string, if possible. With this format, pointers to single-byte data are displayed as null-terminated strings and arrays of single-byte data are displayed as fixed-length strings. Other values are displayed in their natural types. Without this format, GDB displays pointers to and arrays of `char', `unsigned char', and `signed char' as strings. Single-byte members of a vector are displayed as an integer array. `r' Print using the `raw' formatting. By default, GDB will use a Python-based pretty-printer, if one is available (*note Pretty Printing::). This typically results in a higher-level display of the value's contents. The `r' format bypasses any Python pretty-printer which might exist. 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 10.6 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' (`x', `d', `u', `o', `t', `a', `c', `f', `s'), and in addition `i' (for machine instructions). 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 `i' format, the unit size is ignored and is normally not written. For the `s' format, the unit size defaults to `b', unless it is explicitly given. Use `x /hs' to display 16-bit char strings and `x /ws' to display 32-bit strings. The next use of `x /s' will again display 8-bit strings. Note that the results depend on the programming language of the current compilation unit. If the language is C, the `s' modifier will use the UTF-16 encoding while `w' will use UTF-32. The encoding is set by the programming language and cannot be altered. 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. For convenience, especially when used with the `display' command, the `i' format also prints branch delay slot instructions, if any, beyond the count specified, which immediately follow the last instruction that is within the count. 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'. When examining machine instructions, the instruction at current program counter is shown with a `=>' marker. For example: (gdb) x/5i $pc-6 0x804837f : mov %esp,%ebp 0x8048381 : push %ecx 0x8048382 : sub $0x4,%esp => 0x8048385 : movl $0x8048460,(%esp) 0x804838c : call 0x80482d4 The addresses and contents printed by the `x' command are not saved in the value history because there is often too much of them and they would get in the way. Instead, GDB makes these values available for subsequent use in expressions as values of the convenience variables `$_' 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. When you are debugging a program running on a remote target machine (*note Remote Debugging::), you may wish to verify the program's image in the remote machine's memory against the executable file you downloaded to the target. The `compare-sections' command is provided for such situations. `compare-sections [SECTION-NAME]' Compare the data of a loadable section SECTION-NAME in the executable file of the program being debugged with the same section in the remote machine's memory, and report any mismatches. With no arguments, compares all loadable sections. This command's availability depends on the target's support for the `"qCRC"' remote request.  File: gdb.info, Node: Auto Display, Next: Print Settings, Prev: Memory, Up: Data 10.7 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 your format specification--it uses `x' if you specify either the `i' or `s' format, or a unit size; 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: Pretty Printing, Prev: Auto Display, Up: Data 10.8 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 array-indexes' `set print array-indexes on' Print the index of each element when displaying arrays. May be more convenient to locate a given element in the array or quickly find the index of a given element in that printed array. The default is off. `set print array-indexes off' Stop printing element indexes when displaying arrays. `show print array-indexes' Show whether the index of each element is printed when 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 frame-arguments VALUE' This command allows to control how the values of arguments are printed when the debugger prints a frame (*note Frames::). The possible values are: `all' The values of all arguments are printed. `scalars' Print the value of an argument only if it is a scalar. The value of more complex arguments such as arrays, structures, unions, etc, is replaced by `...'. This is the default. Here is an example where only scalar arguments are shown: #1 0x08048361 in call_me (i=3, s=..., ss=0xbf8d508c, u=..., e=green) at frame-args.c:23 `none' None of the argument values are printed. Instead, the value of each argument is replaced by `...'. In this case, the example above now becomes: #1 0x08048361 in call_me (i=..., s=..., ss=..., u=..., e=...) at frame-args.c:23 By default, only scalar arguments are printed. This command can be used to configure the debugger to print the value of all arguments, regardless of their type. However, it is often advantageous to not print the value of more complex parameters. For instance, it reduces the amount of information printed in each frame, making the backtrace more readable. Also, it improves performance when displaying Ada frames, because the computation of large arguments can sometimes be CPU-intensive, especially in large applications. Setting `print frame-arguments' to `scalars' (the default) or `none' avoids this computation, thus speeding up the display of each Ada frame. `show print frame-arguments' Show how the value of arguments should be displayed when printing a frame. `set print repeats' Set the threshold for suppressing display of repeated array elements. When the number of consecutive identical elements of an array exceeds the threshold, GDB prints the string `""', where N is the number of identical repetitions, instead of displaying the identical elements themselves. Setting the threshold to zero will cause all elements to be individually printed. The default threshold is 10. `show print repeats' Display the current threshold for printing repeated identical elements. `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. `show print null-stop' Show whether GDB stops printing an array on the first NULL character. `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 and other unions. This is the default setting. `set print union off' Tell GDB not to print unions which are contained in structures and other unions. GDB will print `"{...}"' instead. `show print union' Ask GDB whether or not it will print unions which are contained in structures and other unions. 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 = {...}} `set print union' affects programs written in C-like languages and in Pascal. 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 pascal_static-members' `set print pascal_static-members on' Print static members when displaying a Pascal object. The default is on. `set print pascal_static-members off' Do not print static members when displaying a Pascal object. `show print pascal_static-members' Show whether Pascal 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: Pretty Printing, Next: Value History, Prev: Print Settings, Up: Data 10.9 Pretty Printing ==================== GDB provides a mechanism to allow pretty-printing of values using Python code. It greatly simplifies the display of complex objects. This mechanism works for both MI and the CLI. For example, here is how a C++ `std::string' looks without a pretty-printer: (gdb) print s $1 = { static npos = 4294967295, _M_dataplus = { > = { <__gnu_cxx::new_allocator> = { }, }, members of std::basic_string, std::allocator >::_Alloc_hider: _M_p = 0x804a014 "abcd" } } With a pretty-printer for `std::string' only the contents are printed: (gdb) print s $2 = "abcd" For implementing pretty printers for new types you should read the Python API details (*note Pretty Printing API::).  File: gdb.info, Node: Value History, Next: Convenience Vars, Prev: Pretty Printing, Up: Data 10.10 Value History =================== Values printed by the `print' command are saved in the GDB "value history". This allows you to refer to them in other expressions. Values are kept until the symbol table is re-read or discarded (for example with the `file' or `symbol-file' commands). When the symbol table changes, the value history is discarded, since the values may contain pointers back to the types defined in the symbol table. The values printed are given "history numbers" by which you can refer to them. These are successive integers starting with one. `print' shows you the history number assigned to a value by printing `$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 10.11 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'. `init-if-undefined $VARIABLE = EXPRESSION' Set a convenience variable if it has not already been set. This is useful for user-defined commands that keep some state. It is similar, in concept, to using local static variables with initializers in C (except that convenience variables are global). It can also be used to allow users to override default values used in a command script. If the variable is already defined then the expression is not evaluated so any side-effects do not occur. 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 `$_exitcode' is automatically set to the exit code when the program being debugged terminates. `$_sdata' The variable `$_sdata' contains extra collected static tracepoint data. *Note Tracepoint Action Lists: Tracepoint Actions. Note that `$_sdata' could be empty, if not inspecting a trace buffer, or if extra static tracepoint data has not been collected. `$_siginfo' The variable `$_siginfo' contains extra signal information (*note extra signal information::). Note that `$_siginfo' could be empty, if the application has not yet received any signals. For example, it will be empty before you execute the `run' command. `$_tlb' The variable `$_tlb' is automatically set when debugging applications running on MS-Windows in native mode or connected to gdbserver that supports the `qGetTIBAddr' request. *Note General Query Packets::. This variable contains the address of the thread information block. 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. GDB also supplies some "convenience functions". These have a syntax similar to convenience variables. A convenience function can be used in an expression just like an ordinary function; however, a convenience function is implemented internally to GDB. `help function' Print a list of all convenience functions.