START-INFO-DIR-ENTRY * Gdb: (gdb). The GNU debugger. END-INFO-DIR-ENTRY This file documents the GNU debugger GDB. This is the Fifth Edition, April 1998, of `Debugging with GDB: the GNU Source-Level Debugger' for GDB Version . Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions. Debugging with GDB ****************** This file describes GDB, the GNU symbolic debugger. This is the Fifth Edition, April 1998, for GDB Version . 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 or C++. For more information, see *Note C and C++: C. Support for Modula-2 and Chill is partial. For information on Modula-2, see *Note Modula-2: Modula-2. There is no further documentation on Chill yet. 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 does not yet support entering expressions, printing values, or similar features using Fortran syntax. It may be necessary to refer to some variables with a trailing underscore. 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. 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 long labor as thankless, we particularly thank those who shepherded GDB through major releases: Stan Shebs (release 4.14), Fred Fish (releases 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). As major maintainer of GDB for some period, each contributed significantly to the structure, stability, and capabilities of the entire debugger. 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. 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 4 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. 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, Ltd. sponsored the support for Hitachi microprocessors. 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. Cygnus Solutions has sponsored GDB maintenance and much of its development since 1991. A Sample GDB Session ******************** You can use this manual at your leisure to read all about GDB. However, a handful of commands are enough to get started using the debugger. This chapter illustrates those commands. One of the preliminary versions of GNU `m4' (a generic macro processor) exhibits the following bug: sometimes, when we change its quote strings from the default, the commands used to capture one macro definition within another stop working. In the following short `m4' session, we define a macro `foo' which expands to `0000'; we then use the `m4' built-in `defn' to define `bar' as the same thing. However, when we change the open quote string to `' and the close quote string to `', the same procedure fails to define a new synonym `baz': $ cd gnu/m4 $ ./m4 define(foo,0000) foo 0000 define(bar,defn(`foo')) bar 0000 changequote(,) define(baz,defn(foo)) baz C-d m4: End of input: 0: fatal error: EOF in string Let us use GDB to try to see what is going on. $ gdb m4 GDB is free software and you are welcome to distribute copies of it under certain conditions; type "show copying" to see the conditions. There is absolutely no warranty for GDB; type "show warranty" for details. GDB , Copyright 1995 Free Software Foundation, Inc... (gdb) GDB reads only enough symbol data to know where to find the rest when needed; as a result, the first prompt comes up very quickly. We now tell GDB to use a narrower display width than usual, so that examples fit in this manual. (gdb) set width 70 We need to see how the `m4' built-in `changequote' works. Having looked at the source, we know the relevant subroutine is `m4_changequote', so we set a breakpoint there with the GDB `break' command. (gdb) break m4_changequote Breakpoint 1 at 0x62f4: file builtin.c, line 879. Using the `run' command, we start `m4' running under GDB control; as long as control does not reach the `m4_changequote' subroutine, the program runs as usual: (gdb) run Starting program: /work/Editorial/gdb/gnu/m4/m4 define(foo,0000) foo 0000 To trigger the breakpoint, we call `changequote'. GDB suspends execution of `m4', displaying information about the context where it stops. changequote(,) Breakpoint 1, m4_changequote (argc=3, argv=0x33c70) at builtin.c:879 879 if (bad_argc(TOKEN_DATA_TEXT(argv[0]),argc,1,3)) Now we use the command `n' (`next') to advance execution to the next line of the current function. (gdb) n 882 set_quotes((argc >= 2) ? TOKEN_DATA_TEXT(argv[1])\ : nil, `set_quotes' looks like a promising subroutine. We can go into it by using the command `s' (`step') instead of `next'. `step' goes to the next line to be executed in *any* subroutine, so it steps into `set_quotes'. (gdb) s set_quotes (lq=0x34c78 "", rq=0x34c88 "") at input.c:530 530 if (lquote != def_lquote) The display that shows the subroutine where `m4' is now suspended (and its arguments) is called a stack frame display. It shows a summary of the stack. We can use the `backtrace' command (which can also be spelled `bt'), to see where we are in the stack as a whole: the `backtrace' command displays a stack frame for each active subroutine. (gdb) bt #0 set_quotes (lq=0x34c78 "", rq=0x34c88 "") at input.c:530 #1 0x6344 in m4_changequote (argc=3, argv=0x33c70) at builtin.c:882 #2 0x8174 in expand_macro (sym=0x33320) at macro.c:242 #3 0x7a88 in expand_token (obs=0x0, t=209696, td=0xf7fffa30) at macro.c:71 #4 0x79dc in expand_input () at macro.c:40 #5 0x2930 in main (argc=0, argv=0xf7fffb20) at m4.c:195 We step through a few more lines to see what happens. The first two times, we can use `s'; the next two times we use `n' to avoid falling into the `xstrdup' subroutine. (gdb) s 0x3b5c 532 if (rquote != def_rquote) (gdb) s 0x3b80 535 lquote = (lq == nil || *lq == '\0') ? \ def_lquote : xstrdup(lq); (gdb) n 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup(rq); (gdb) n 538 len_lquote = strlen(rquote); The last line displayed looks a little odd; we can examine the variables `lquote' and `rquote' to see if they are in fact the new left and right quotes we specified. We use the command `p' (`print') to see their values. (gdb) p lquote $1 = 0x35d40 "" (gdb) p rquote $2 = 0x35d50 "" `lquote' and `rquote' are indeed the new left and right quotes. To look at some context, we can display ten lines of source surrounding the current line with the `l' (`list') command. (gdb) l 533 xfree(rquote); 534 535 lquote = (lq == nil || *lq == '\0') ? def_lquote\ : xstrdup (lq); 536 rquote = (rq == nil || *rq == '\0') ? def_rquote\ : xstrdup (rq); 537 538 len_lquote = strlen(rquote); 539 len_rquote = strlen(lquote); 540 } 541 542 void Let us step past the two lines that set `len_lquote' and `len_rquote', and then examine the values of those variables. (gdb) n 539 len_rquote = strlen(lquote); (gdb) n 540 } (gdb) p len_lquote $3 = 9 (gdb) p len_rquote $4 = 7 That certainly looks wrong, assuming `len_lquote' and `len_rquote' are meant to be the lengths of `lquote' and `rquote' respectively. We can set them to better values using the `p' command, since it can print the value of any expression--and that expression can include subroutine calls and assignments. (gdb) p len_lquote=strlen(lquote) $5 = 7 (gdb) p len_rquote=strlen(rquote) $6 = 9 Is that enough to fix the problem of using the new quotes with the `m4' built-in `defn'? We can allow `m4' to continue executing with the `c' (`continue') command, and then try the example that caused trouble initially: (gdb) c Continuing. define(baz,defn(foo)) baz 0000 Success! The new quotes now work just as well as the default ones. The problem seems to have been just the two typos defining the wrong lengths. We allow `m4' exit by giving it an EOF as input: C-d Program exited normally. The message `Program exited normally.' is from GDB; it indicates `m4' has finished executing. We can end our GDB session with the GDB `quit' command. (gdb) quit Getting In and Out of GDB ************************* This chapter discusses how to start GDB, and how to get out of it. The essentials are: * type `gdb' to start GDB. * type `quit' or `C-d' to exit. 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. 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. 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' 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' option followed by that argument.) Many options have both long and short forms; both are shown in the following list. GDB also recognizes the long forms if you truncate them, so long as enough of the option is present to be unambiguous. (If you prefer, you can flag option arguments with `--' rather than `-', though we illustrate the more usual convention.) `-symbols FILE' `-s FILE' Read symbol table from file FILE. `-exec FILE' `-e FILE' Use file FILE as the executable file to execute when appropriate, and for examining pure data in conjunction with a core dump. `-se FILE' Read symbol table from file FILE and use it as the executable file. `-core FILE' `-c FILE' Use file FILE as a core dump to examine. `-c NUMBER' Connect to process ID NUMBER, as with the `attach' command (unless there is a file in core-dump format named NUMBER, in which case `-c' specifies that file as a core dump to read). `-command FILE' `-x FILE' Execute GDB commands from file FILE. *Note Command files: Command Files. `-directory DIRECTORY' `-d DIRECTORY' Add DIRECTORY to the path to search for source files. `-m' `-mapped' *Warning: this option depends on operating system facilities that are not supported on all systems.* If memory-mapped files are available on your system through the `mmap' system call, you can use this option to have GDB write the symbols from your program into a reusable file in the current directory. If the program you are debugging is called `/tmp/fred', the mapped symbol file is `./fred.syms'. Future GDB debugging sessions notice the presence of this file, and can quickly map in symbol information from it, rather than reading the symbol table from the executable program. The `.syms' file is specific to the host machine where GDB is run. It holds an exact image of the internal GDB symbol table. It cannot be shared across multiple host platforms. `-r' `-readnow' Read each symbol file's entire symbol table immediately, rather than the default, which is to read it incrementally as it is needed. This makes startup slower, but makes future operations faster. The `-mapped' and `-readnow' options are typically combined in order to build a `.syms' file that contains complete symbol information. (*Note Commands to specify files: Files, for information a `.syms' file for future use is: gdb -batch -nx -mapped -readnow programname Choosing modes -------------- You can run GDB in various alternative modes--for example, in batch mode or quiet mode. `-nx' `-n' Do not execute commands from any initialization files (normally called `.gdbinit'). Normally, the commands in these files are executed after all the command options and arguments have been processed. *Note Command files: Command Files. `-quiet' `-q' "Quiet". Do not print the introductory and copyright messages. These messages are also suppressed in batch mode. `-batch' Run in batch mode. Exit with status `0' after processing all the command files specified with `-x' (and all commands from initialization files, if not inhibited with `-n'). Exit with nonzero status if an error occurs in executing the GDB commands in the command files. Batch mode may be useful for running GDB as a filter, for example to download and run a program on another computer; in order to make this more useful, the message Program exited normally. (which is ordinarily issued whenever a program running under GDB control terminates) is not issued when running in batch mode. `-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. `-b BPS' Set the line speed (baud rate or bits per second) of any serial interface used by GDB for remote debugging. `-tty DEVICE' Run using DEVICE for your program's standard input and output. Quitting GDB ============ `quit' To exit GDB, use the `quit' command (abbreviated `q'), or type an end-of-file character (usually `C-d'). If you do not supply EXPRESSION, GDB will terminate normally; otherwise it will terminate using the result of EXPRESSION as the error code. An interrupt (often `C-c') does not exit from GDB, but rather terminates the action of any GDB command that is in progress and returns to GDB command level. It is safe to type the interrupt character at any time because GDB does not allow it to take effect until a time when it is safe. If you have been using GDB to control an attached process or device, you can release it with the `detach' command (*note Debugging an already-running process: Attach.). 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 the standard shell to execute COMMAND STRING. If it exists, the environment variable `SHELL' determines which shell to run. Otherwise GDB uses `/bin/sh'. 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'. 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). 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 command names do not allow any arguments. GDB command names may always be truncated if that abbreviation is unambiguous. Other possible command abbreviations are listed in the documentation for individual commands. In some cases, even ambiguous abbreviations are allowed; for example, `s' is specially defined as equivalent to `step' even though there are other commands whose names start with `s'. You can test abbreviations by using them as arguments to the `help' command. A blank line as input to GDB (typing just ) means to repeat the previous command. Certain commands (for example, `run') will not repeat this way; these are commands whose unintentional repetition might cause trouble and which you are unlikely to want to repeat. The `list' and `x' commands, when you repeat them with , construct new arguments rather than repeating exactly as typed. This permits easy scanning of source or memory. GDB can also use in another way: to partition lengthy output, in a way similar to the common utility `more' (*note Screen size: Screen Size.). Since it is easy to press one too many in this situation, GDB disables command repetition after any command that generates this sort of display. Any text from a `#' to the end of the line is a comment; it does nothing. This is useful mainly in command files (*note Command files: Command Files.). 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( 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. 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: running -- Running the program stack -- Examining the stack data -- Examining data breakpoints -- Making program stop at certain points files -- Specifying and examining files status -- Status inquiries support -- Support facilities user-defined -- User-defined commands aliases -- Aliases of other commands obscure -- Obscure features 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: show -- Generic command for showing things set with "set" info -- Generic command for printing status 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. `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: info inspect ignore This is intended for use by GNU Emacs. In addition to `help', you can use the GDB commands `info' and `show' to inquire about the state of your program, or the state of GDB itself. Each command supports many topics of inquiry; this manual introduces each of them in the appropriate context. The listings under `info' and under `show' in the Index point to all the sub-commands. *Note Index::. `info' This command (abbreviated `i') is for describing the state of your program. For example, you can list the arguments given to your program with `info args', list the registers currently in use with `info registers', or list the breakpoints you have set with `info breakpoints'. You can get a complete list of the `info' sub-commands with `help info'. `set' You can assign the result of an expresson 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 occasionally want to determine which version of GDB you are running; as GDB evolves, new commands are introduced, and old ones may wither away. The version number is also announced when you start GDB. `show copying' Display information about permission for copying GDB. `show warranty' Display the GNU "NO WARRANTY" statement. 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. You may redirect your program's input and output, debug an already running process, or kill a child process. 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. Many C compilers are unable to handle the `-g' and `-O' options together. Using those compilers, you cannot generate optimized executables containing debugging information. GCC, the GNU C compiler, supports `-g' with or without `-O', making it possible to debug optimized code. We recommend that you *always* use `-g' whenever you compile a program. You may think your program is correct, but there is no sense in pushing your luck. When you debug a program compiled with `-g -O', remember that the optimizer is rearranging your code; the debugger shows you what is really there. Do not be too surprised when the execution path does not exactly match your source file! An extreme example: if you define a variable, but never use it, GDB never sees that variable--because the compiler optimizes it out of existence. Some things do not work as well with `-g -O' as with just `-g', particularly on machines with instruction scheduling. If in doubt, recompile with `-g' alone, and if this fixes the problem, please report it to us as a bug (including a test case!). 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. Starting your program ===================== `run' `r' Use the `run' command to start your program under GDB. You must first specify the program name (except on VxWorks) with an argument to GDB (*note Getting In and Out of GDB: Invocation.), or by using the `file' or `exec-file' command (*note Commands to specify files: Files.). If you are running your program in an execution environment that supports processes, `run' creates an inferior process and makes that process run your program. (In environments without processes, `run' jumps to the start of your program.) The execution of a program is affected by certain information it receives from its superior. GDB provides ways to specify this information, which you must do *before* starting your program. (You can change it after starting your program, but such changes only affect your program the next time you start it.) This information may be divided into four categories: The *arguments.* Specify the arguments to give your program as the arguments of the `run' command. If a shell is available on your target, the shell is used to pass the arguments, so that you may use normal conventions (such as wildcard expansion or variable substitution) in describing the arguments. In Unix systems, you can control which shell is used with the `SHELL' environment variable. *Note Your program's arguments: Arguments. The *environment.* Your program normally inherits its environment from GDB, but you can use the GDB commands `set environment' and `unset environment' to change parts of the environment that affect your program. *Note Your program's environment: Environment. The *working directory.* Your program inherits its working directory from GDB. You can set the GDB working directory with the `cd' command in GDB. *Note Your program's working directory: Working Directory. The *standard input and output.* Your program normally uses the same device for standard input and standard output as GDB is using. You can redirect input and output in the `run' command line, or you can use the `tty' command to set a different device for your program. *Note Your program's input and output: Input/Output. *Warning:* While input and output redirection work, you cannot use pipes to pass the output of the program you are debugging to another program; if you attempt this, GDB is likely to wind up debugging the wrong program. When you issue the `run' command, your program begins to execute immediately. *Note Stopping and continuing: Stopping, for discussion of how to arrange for your program to stop. Once your program has stopped, you may call functions in your program, using the `print' or `call' commands. *Note Examining Data: Data. If the modification time of your symbol file has changed since the last time GDB read its symbols, GDB discards its symbol table, and reads it again. When it does this, GDB tries to retain your current breakpoints. 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 `/bin/sh'. `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. 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), for both GDB and your program. You may specify several directory names, separated by `:' or whitespace. 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 a Unix 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:* 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'. 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. 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. 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 using `attach', you should first use the `file' command to specify the program running in the process and load its symbol table. *Note Commands to Specify Files: Files. The first thing GDB does after arranging to debug the specified process is to stop it. You can examine and modify an attached process with all the GDB commands that are ordinarily available when you start processes with `run'. You can insert breakpoints; you can step and continue; you can modify storage. If you would rather the process continue running, you may use the `continue' command after attaching GDB to the process. `detach' When you have finished debugging the attached process, you can use the `detach' command to release it from GDB control. Detaching the process continues its execution. After the `detach' command, that process and GDB become completely independent once more, and you are ready to `attach' another process or start one with `run'. `detach' does not repeat if you press again after executing the command. If you exit GDB or use the `run' command while you have an attached process, you kill that process. By default, GDB asks for confirmation if you try to do either of these things; you can control whether or not you need to confirm by using the `set confirm' command (*note Optional warnings and messages: Messages/Warnings.). 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). Additional process information ============================== Some operating systems provide a facility called `/proc' that can be used to examine the image of a running process using file-system subroutines. If GDB is configured for an operating system with this facility, the command `info proc' is available to report on several kinds of information about the process running your program. `info proc' works only on SVR4 systems that support `procfs'. `info proc' Summarize available information about the process. `info proc mappings' Report on the address ranges accessible in the program, with information on whether your program may read, write, or execute each range. `info proc times' Starting time, user CPU time, and system CPU time for your program and its children. `info proc id' Report on the process IDs related to your program: its own process ID, the ID of its parent, the process group ID, and the session ID. `info proc status' General information on the state of the process. If the process is stopped, this report includes the reason for stopping, and any signal received. `info proc all' Show all the above information about the process. Debugging programs with multiple threads ======================================== In some operating systems, a single program may have more than one "thread" of execution. The precise semantics of threads differ from one operating system to another, but in general the threads of a single program are akin to multiple processes--except that they share one address space (that is, they can all examine and modify the same variables). On the other hand, each thread has its own registers and execution stack, and perhaps private memory. GDB provides these facilities for debugging multi-thread programs: * automatic notification of new threads * `thread THREADNO', a command to switch among threads * `info threads', a command to inquire about existing threads * `thread apply [THREADNO] [ALL] ARGS', a command to apply a command to a list of threads * thread-specific breakpoints *Warning:* These facilities are not yet available on every GDB configuration where the operating system supports threads. If your GDB does not support threads, these commands have no effect. For example, a system without thread support shows no output from `info threads', and always rejects the `thread' command, like this: (gdb) info threads (gdb) thread 1 Thread ID 1 not known. Use the "info threads" command to see the IDs of currently known threads. The GDB thread debugging facility allows you to observe all threads while your program runs--but whenever GDB takes control, one thread in particular is always the focus of debugging. This thread is called the "current thread". Debugging commands show program information from the perspective of the current thread. Whenever GDB detects a new thread in your program, it displays the target system's identification for the thread with a message in the form `[New SYSTAG]'. SYSTAG is a thread identifier whose form varies depending on the particular system. For example, on LynxOS, you might see [New process 35 thread 27] when GDB notices a new thread. In contrast, on an SGI system, the SYSTAG is simply something like `process 368', with no further qualifier. For debugging purposes, GDB associates its own thread number--always a single integer--with each thread in your program. `info threads' Display a summary of all threads currently in your program. GDB displays for each thread (in this order): 1. the thread number assigned by GDB 2. the target system's thread identifier (SYSTAG) 3. the current stack frame summary for that thread An asterisk `*' to the left of the GDB thread number indicates the current thread. For example, (gdb) info threads 3 process 35 thread 27 0x34e5 in sigpause () 2 process 35 thread 23 0x34e5 in sigpause () * 1 process 35 thread 13 main (argc=1, argv=0x7ffffff8) at threadtest.c:68 `thread THREADNO' Make thread number THREADNO the current thread. The command argument THREADNO is the internal GDB thread number, as shown in the first field of the `info threads' display. GDB responds by displaying the system identifier of the thread you selected, and its current stack frame summary: (gdb) thread 2 [Switching to process 35 thread 23] 0x34e5 in sigpause () As with the `[New ...]' message, the form of the text after `Switching to' depends on your system's conventions for identifying threads. `thread apply [THREADNO] [ALL] ARGS' The `thread apply' command allows you to apply a command to one or more threads. Specify the numbers of the threads that you want affected with the command argument THREADNO. THREADNO is the internal GDB thread number, as shown in the first field of the `info threads' display. To apply a command to all threads, use `thread apply all' ARGS. Whenever GDB stops your program, due to a breakpoint or a signal, it automatically selects the thread where that breakpoint or signal happened. GDB alerts you to the context switch with a message of the form `[Switching to SYSTAG]' to identify the thread. *Note Stopping and starting multi-thread programs: Thread Stops, for more information about how GDB behaves when you stop and start programs with multiple threads. *Note Setting watchpoints: Set Watchpoints, for information about watchpoints in programs with multiple threads. Debugging programs with multiple processes ========================================== 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 (see *Note Attach::). From that point on you can debug the child process just like any other process which you attached to. 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. Breakpoints, watchpoints, and exceptions ======================================== A "breakpoint" makes your program stop whenever a certain point in the program is reached. For each breakpoint, you can add conditions to control in finer detail whether your program stops. You can set breakpoints with the `break' command and its variants (*note Setting breakpoints: Set Breaks.), to specify the place where your program should stop by line number, function name or exact address in the program. In languages with exception handling (such as GNU C++), you can also set breakpoints where an exception is raised (*note Breakpoints and exceptions: Exception Handling.). In SunOS 4.x, SVR4, and Alpha OSF/1 configurations, you can now set breakpoints in shared libraries before the executable is run. A "watchpoint" is a special breakpoint that stops your program when the value of an expression changes. You must use a different command to set watchpoints (*note Setting watchpoints: Set Watchpoints.), but aside from that, you can manage a watchpoint like any other breakpoint: you enable, disable, and delete both breakpoints and watchpoints using the same commands. You can arrange to have values from your program displayed automatically whenever GDB stops at a breakpoint. *Note Automatic display: Auto Display. GDB assigns a number to each breakpoint or watchpoint 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. Setting breakpoints ------------------- Breakpoints are set with the `break' command (abbreviated `b'). The debugger convenience variable `$bpnum' records the number of the breakpoints you've set most recently; see *Note Convenience variables: Convenience Vars, for a discussion of what you can do with convenience variables. You have several ways to say where the breakpoint should go. `break FUNCTION' Set a breakpoint at entry to function FUNCTION. When using source languages that permit overloading of symbols, such as C++, FUNCTION may refer to more than one possible place to break. *Note Breakpoint menus: Breakpoint Menus, for a discussion of that situation. `break +OFFSET' `break -OFFSET' Set a breakpoint some number of lines forward or back from the position at which execution stopped in the currently selected frame. `break LINENUM' Set a breakpoint at line LINENUM in the current source file. That file is the last file whose source text was printed. This breakpoint stops your program just before it executes any of the code on that line. `break FILENAME:LINENUM' Set a breakpoint at line LINENUM in source file FILENAME. `break FILENAME:FUNCTION' Set a breakpoint at entry to function FUNCTION found in file FILENAME. Specifying a file name as well as a function name is superfluous except when multiple files contain similarly named functions. `break *ADDRESS' Set a breakpoint at address ADDRESS. You can use this to set breakpoints in parts of your program which do not have debugging information or source files. `break' When called without any arguments, `break' sets a breakpoint at the next instruction to be executed in the selected stack frame (*note Examining the Stack: Stack.). In any selected frame but the innermost, this makes your program stop as soon as control returns to that frame. This is similar to the effect of a `finish' command in the frame inside the selected frame--except that `finish' does not leave an active breakpoint. If you use `break' without an argument in the innermost frame, GDB stops the next time it reaches the current location; this may be useful inside loops. GDB normally ignores breakpoints when it resumes execution, until at least one instruction has been executed. If it did not do this, you would be unable to proceed past a breakpoint without first disabling the breakpoint. This rule applies whether or not the breakpoint already existed when your program stopped. `break ... if COND' Set a breakpoint with condition COND; evaluate the expression COND each time the breakpoint is reached, and stop only if the value is nonzero--that is, if COND evaluates as true. `...' stands for one of the possible arguments described above (or no argument) specifying where to break. *Note Break conditions: Conditions, for more information on breakpoint conditions. `tbreak ARGS' Set a breakpoint enabled only for one stop. ARGS are the same as for the `break' command, and the breakpoint is set in the same way, but the breakpoint is automatically deleted after the first time your program stops there. *Note Disabling breakpoints: Disabling. `hbreak ARGS' Set a hardware-assisted breakpoint. ARGS are the same as for the `break' command and the breakpoint is set in the same way, but the breakpoint requires hardware support and some target hardware may not have this support. The main purpose of this is EPROM/ROM code debugging, so you can set a breakpoint at an instruction without changing the instruction. This can be used with the new trap-generation provided by SPARClite DSU. DSU will generate traps when a program accesses some date or instruction address that is assigned to the debug registers. However the hardware breakpoint registers can only take two data breakpoints, and GDB will reject this command if more than two are used. Delete or disable usused hardware breakpoints before setting new ones. *Note Break conditions: Conditions. `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. 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. When debugging C++ programs, `rbreak' is useful for setting breakpoints on overloaded functions that are not members of any special classes. `info breakpoints [N]' `info break [N]' `info watchpoints [N]' Print a table of all breakpoints and watchpoints set and not deleted, with the following columns for each breakpoint: *Breakpoint Numbers* *Type* Breakpoint or watchpoint. *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 *What* Where the breakpoint is in the source for your program, as a file and line number. 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. `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' now 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.). 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'. `maint info breakpoints' Using the same format as `info breakpoints', display both the breakpoints you've set explicitly, and those GDB is using for internal purposes. Internal breakpoints are shown with negative breakpoint numbers. The type column identifies what kind of breakpoint is shown: `breakpoint' Normal, explicitly set breakpoint. `watchpoint' Normal, explicitly set watchpoint. `longjmp' Internal breakpoint, used to handle correctly stepping through `longjmp' calls. `longjmp resume' Internal breakpoint at the target of a `longjmp'. `until' Temporary internal breakpoint used by the GDB `until' command. `finish' Temporary internal breakpoint used by the GDB `finish' command. 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. Watchpoints currently execute two orders of magnitude more slowly than other breakpoints, but this can be well worth it to catch errors where you have no clue what part of your program is the culprit. `watch EXPR' Set a watchpoint for an expression. GDB will break when EXPR is written into by the program and its value changes. This can be used with the new trap-generation provided by SPARClite DSU. DSU will generate traps when a program accesses some date or instruction address that is assigned to the debug registers. For the data addresses, DSU facilitates the `watch' command. However the hardware breakpoint registers can only take two data watchpoints, and both watchpoints must be the same kind. For example, you can set two watchpoints with `watch' commands, two with `rwatch' commands, *or* two with `awatch' commands, but you cannot set one watchpoint with one command and the other with a different command. {No Value For "GBDN"} will reject the command if you try to mix watchpoints. Delete or disable unused watchpoint commands before setting new ones. `rwatch EXPR' Set a watchpoint that will break when watch ARGS is read by the program. If you use both watchpoints, both must be set with the `rwatch' command. `awatch EXPR' Set a watchpoint that will break when ARGS is read and written into by the program. If you use both watchpoints, both must be set with the `awatch' command. `info watchpoints' This command prints a list of watchpoints and breakpoints; it is the same as `info break'. *Warning:* in multi-thread programs, watchpoints have only limited usefulness. With the current watchpoint implementation, GDB can only watch the value of an expression *in a single thread*. If you are confident that the expression can only change due to the current thread's activity (and if you are also confident that no other thread can become current), then you can use watchpoints as usual. However, GDB may not notice when a non-current thread's activity changes the expression. Breakpoints and exceptions -------------------------- Some languages, such as GNU C++, implement exception handling. You can use GDB to examine what caused your program to raise an exception, and to list the exceptions your program is prepared to handle at a given point in time. `catch EXCEPTIONS' You can set breakpoints at active exception handlers by using the `catch' command. EXCEPTIONS is a list of names of exceptions to catch. You can use `info catch' to list active exception handlers. *Note Information about a frame: Frame Info. There are currently some limitations to exception handling 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 to simply continue running until it hits a breakpoint, catches a signal that GDB is listening for, or exits. * 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. Deleting breakpoints -------------------- It is often necessary to eliminate a breakpoint or watchpoint 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 or watchpoints by specifying their breakpoint numbers. It is not necessary to delete a breakpoint to proceed past it. GDB automatically ignores breakpoints on the first instruction to be executed when you continue execution without changing the execution address. `clear' Delete any breakpoints at the next instruction to be executed in the selected stack frame (*note Selecting a frame: Selection.). When the innermost frame is selected, this is a good way to delete a breakpoint where your program just stopped. `clear FUNCTION' `clear FILENAME:FUNCTION' Delete any breakpoints set at entry to the function FUNCTION. `clear LINENUM' `clear FILENAME:LINENUM' Delete any breakpoints set at or within the code of the specified line. `delete [breakpoints] [BNUMS...]' Delete the breakpoints or watchpoints of the numbers 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'. Disabling breakpoints --------------------- Rather than deleting a breakpoint or watchpoint, 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 and watchpoints with the `enable' and `disable' commands, optionally specifying one or more breakpoint numbers as arguments. Use `info break' or `info watch' to print a list of breakpoints or watchpoints if you do not know which numbers to use. A breakpoint or watchpoint 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. A breakpoint set with the `tbreak' command starts out in this state. * Enabled for deletion. The breakpoint stops your program, but immediately after it does so it is deleted permanently. You can use the following commands to enable or disable breakpoints and watchpoints: `disable [breakpoints] [BNUMS...]' 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] [BNUMS...]' Enable the specified breakpoints (or all defined breakpoints). They become effective once again in stopping your program. `enable [breakpoints] once BNUMS...' Enable the specified breakpoints temporarily. GDB disables any of these breakpoints immediately after stopping your program. `enable [breakpoints] delete BNUMS...' Enable the specified breakpoints to work once, then die. GDB deletes any of these breakpoints as soon as your program stops there. Except for a breakpoint set with `tbreak' (*note Setting breakpoints: Set Breaks.), breakpoints that you set are initially enabled; subsequently, they become disabled or enabled only when you use one of the commands above. (The command `until' can set and delete a breakpoint of its own, but it does not change the state of your other breakpoints; see *Note Continuing and stepping: Continuing and Stepping.) 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 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. The `watch' command does not recognize the `if' keyword; `condition' is the only way to impose a further condition on a watchpoint. `condition BNUM EXPRESSION' Specify EXPRESSION as the break condition for breakpoint or watchpoint 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. GDB does not actually evaluate EXPRESSION at the time the `condition' command 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. Breakpoint command lists ------------------------ You can give any breakpoint (or watchpoint) a series of commands to execute when your program stops due to that breakpoint. For example, you might want to print the values of certain expressions, or enable other breakpoints. `commands [BNUM]' `... COMMAND-LIST ...' `end' Specify a list of commands for breakpoint number BNUM. The commands themselves appear on the following lines. Type a line containing just `end' to terminate the commands. To remove all commands from a breakpoint, type `commands' and follow it immediately with `end'; that is, give no commands. With no BNUM argument, `commands' refers to the last breakpoint or watchpoint set (not to the breakpoint most recently encountered). Pressing as a means of repeating the last GDB command is disabled within a COMMAND-LIST. You can use breakpoint commands to start your program up again. Simply use the `continue' command, or `step', or any other command that resumes execution. Any other commands in the command list, after a command that resumes execution, are ignored. This is because any time you resume execution (even with a simple `next' or `step'), you may encounter another breakpoint--which could have its own command list, leading to ambiguities about which list to execute. If the first command you specify in a command list is `silent', the usual message about stopping at a breakpoint is not printed. This may be desirable for breakpoints that are to print a specific message and then continue. If none of the remaining commands print anything, you see no sign that the breakpoint was reached. `silent' is meaningful only at the beginning of a breakpoint command list. The commands `echo', `output', and `printf' allow you to print precisely controlled output, and are often useful in silent breakpoints. *Note Commands for controlled output: Output. For example, here is how you could use breakpoint commands to print the value of `x' at entry to `foo' whenever `x' is positive. break foo if x>0 commands silent printf "x is %d\n",x cont end One application for breakpoint commands is to compensate for one bug so you can test for another. Put a breakpoint just after the erroneous line of code, give it a condition to detect the case in which something erroneous has been done, and give it commands to assign correct values to any variables that need them. End with the `continue' command so that your program does not stop, and start with the `silent' command so that no output is produced. Here is an example: break 403 commands silent set x = y + 4 cont end Breakpoint menus ---------------- Some programming languages (notably C++) permit a single function name to be defined several times, for application in different contexts. This is called "overloading". When a function name is overloaded, `break FUNCTION' is not enough to tell GDB where you want a breakpoint. If you realize this is a problem, you can use something like `break FUNCTION(TYPES)' to specify which particular version of the function you want. Otherwise, GDB offers you a menu of numbered choices for different possible breakpoints, and waits for your selection with the prompt `>'. The first two options are always `[0] cancel' and `[1] all'. Typing `1' sets a breakpoint at each definition of FUNCTION, and typing `0' aborts the `break' command without setting any new breakpoints. For example, the following session excerpt shows an attempt to set a breakpoint at the overloaded symbol `String::after'. We choose three particular definitions of that function name: (gdb) b String::after [0] cancel [1] all [2] file:String.cc; line number:867 [3] file:String.cc; line number:860 [4] file:String.cc; line number:875 [5] file:String.cc; line number:853 [6] file:String.cc; line number:846 [7] file:String.cc; line number:735 > 2 4 6 Breakpoint 1 at 0xb26c: file String.cc, line 867. Breakpoint 2 at 0xb344: file String.cc, line 875. Breakpoint 3 at 0xafcc: file String.cc, line 846. Multiple breakpoints were set. Use the "delete" command to delete unwanted breakpoints. (gdb) 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 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' 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 exceptions: 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 now only stops at the first instruction of a source line. This prevents the multiple stops that used to occur in switch statements, for loops, etc. `step' continues to stop if a function that has debugging information is called within the line. Also, the `step' command now only enters a subroutine if there is line number information for the subroutine. 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 now only stops at the first instruction of a source line. This prevents the multiple stops that used to occur in swtch statements, for loops, etc. `finish' Continue running until just after function in the selected stack frame returns. Print the returned value (if any). Contrast this with the `return' command (*note Returning from a function: Returning.). `u' `until' Continue running until a source line past the current line, in the current stack frame, is reached. This command is used to avoid single stepping through a loop more than once. It is like the `next' command, except that when `until' encounters a jump, it automatically continues execution until the program counter is greater than the address of the jump. This means that when you reach the end of a loop after single stepping though it, `until' makes your program continue execution until it exits the loop. In contrast, a `next' command at the end of a loop simply steps back to the beginning of the loop, which forces you to step through the next iteration. `until' always stops your program if it attempts to exit the current stack frame. `until' may produce somewhat counterintuitive results if the order of machine code does not match the order of the source lines. For example, in the following excerpt from a debugging session, the `f' (`frame') command shows that execution is stopped at line `206'; yet when we use `until', we get to line `195': (gdb) f #0 main (argc=4, argv=0xf7fffae8) at m4.c:206 206 expand_input(); (gdb) until 195 for ( ; argc > 0; NEXTARG) { This happened because, for execution efficiency, the compiler had generated code for the loop closure test at the end, rather than the start, of the loop--even though the test in a C `for'-loop is written before the body of the loop. The `until' command appeared to step back to the beginning of the loop when it advanced to this expression; however, it has not really gone to an earlier statement--not in terms of the actual machine code. `until' with no argument works by means of single instruction stepping, and hence is slower than `until' with an argument. `until LOCATION' `u LOCATION' Continue running your program until either the specified location is reached, or the current stack frame returns. LOCATION is any of the forms of argument acceptable to `break' (*note Setting breakpoints: Set Breaks.). This form of the command uses breakpoints, and hence is quicker than `until' without an argument. `stepi' `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' `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'. 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 (often `C-c'); `SIGSEGV' is the signal a program gets from referencing a place in memory far away from all the areas in use; `SIGALRM' occurs when the alarm clock timer goes off (which happens only if your program has requested an alarm). Some signals, including `SIGALRM', are a normal part of the functioning of your program. Others, such as `SIGSEGV', indicate errors; these signals are "fatal" (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 ignore non-erroneous signals like `SIGALRM' (so as not to interfere with their role in the functioning of your program) but to stop your program immediately whenever an error signal happens. You can change these settings with the `handle' command. `info signals' Print a table of all the kinds of signals and how GDB has been told to handle each one. You can use this to see the signal numbers of all the defined types of signals. `info handle' is the new 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). The KEYWORDS say what change to make. The keywords allowed by the `handle' command can be abbreviated. Their full names are: `nostop' GDB should not stop your program when this signal happens. It may still print a message telling you that the signal has come in. `stop' GDB should stop your program when this signal happens. This implies the `print' keyword as well. `print' GDB should print a message when this signal happens. `noprint' GDB should not mention the occurrence of the signal at all. This implies the `nostop' keyword as well. `pass' 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. `nopass' GDB should not allow your program to see this signal. When a signal stops your program, the signal is not visible 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. 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. Stopping and starting multi-thread programs =========================================== When your program has multiple threads (*note Debugging programs with multiple threads: Threads.), you can choose whether to set breakpoints on all threads, or on a particular thread. `break LINESPEC thread THREADNO' `break LINESPEC thread THREADNO if ...' LINESPEC specifies source lines; there are several ways of writing them, but the effect is always to specify some source line. Use the qualifier `thread THREADNO' with a breakpoint command to specify that you only want GDB to stop the program when a particular thread reaches this breakpoint. THREADNO is one of the numeric thread identifiers assigned by GDB, shown in the first column of the `info threads' display. If you do not specify `thread THREADNO' when you set a breakpoint, the breakpoint applies to *all* threads of your program. You can use the `thread' qualifier on conditional breakpoints as well; in this case, place `thread THREADNO' before the breakpoint condition, like this: (gdb) break frik.c:13 thread 28 if bartab > lim Whenever your program stops under GDB for any reason, *all* threads of execution stop, not just the current thread. This allows you to examine the overall state of the program, including switching between threads, without worrying that things may change underfoot. 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. 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.). Stack frames ============ The call stack is divided up into contiguous pieces called "stack frames", or "frames" for short; each frame is the data associated with one call to one function. The frame contains the arguments given to the function, the function's local variables, and the address at which the function is executing. When your program is started, the stack has only one frame, that of the function `main'. This is called the "initial" frame or the "outermost" frame. Each time a function is called, a new frame is made. Each time a function returns, the frame for that function invocation is eliminated. If a function is recursive, there can be many frames for the same function. The frame for the function in which execution is actually occurring is called the "innermost" frame. This is the most recently created of all the stack frames that still exist. Inside your program, stack frames are identified by their addresses. A stack frame consists of many bytes, each of which has its own address; each kind of computer has a convention for choosing one byte whose address serves as the address of the frame. Usually this address is kept in a register called the "frame pointer register" while execution is going on in that frame. GDB assigns numbers to all existing stack frames, starting with zero for the innermost frame, one for the frame that called it, and so on upward. These numbers do not really exist in your program; they are assigned by GDB to give you a way of designating stack frames in GDB commands. Some compilers provide a way to compile functions so that they operate without stack frames. (For example, the `gcc' option `-fomit-frame-pointer' generates functions without a frame.) This is occasionally done with heavily used library functions to save the frame setup time. GDB has limited facilities for dealing with these function invocations. If the innermost function invocation has no stack frame, GDB nevertheless regards it as though it had a separate frame, which is numbered zero as usual, allowing correct tracing of the function call chain. However, GDB has no provision for frameless functions elsewhere in the stack. `frame ARGS' The `frame' command allows you to move from one stack frame to another, and to print the stack frame you select. ARGS may be either the address of the frame or the stack frame number. Without an argument, `frame' prints the current stack frame. `select-frame' The `select-frame' command allows you to move from one stack frame to another without printing the frame. This is the silent version of `frame'. Backtraces ========== A backtrace is a summary of how your program got where it is. It shows one line per frame, for many frames, starting with the currently executing frame (frame zero), followed by its caller (frame one), and on up the stack. `backtrace' `bt' Print a backtrace of the entire stack: one line per frame for all frames in the stack. You can stop the backtrace at any time by typing the system interrupt character, normally `C-c'. `backtrace N' `bt N' Similar, but print only the innermost N frames. `backtrace -N' `bt -N' Similar, but print only the outermost N frames. The names `where' and `info stack' (abbreviated `info s') are additional aliases for `backtrace'. Each line in the backtrace shows the frame number and the function name. The program counter value is also shown--unless you use `set print address off'. The backtrace also shows the source file name and line number, as well as the arguments to the function. The program counter value is omitted if it is at the beginning of the code for that line number. Here is an example of a backtrace. It was made with the command `bt 3', so it shows the innermost three frames. #0 m4_traceon (obs=0x24eb0, argc=1, argv=0x2b8c8) at builtin.c:993 #1 0x6e38 in expand_macro (sym=0x2b600) at macro.c:242 #2 0x6840 in expand_token (obs=0x0, t=177664, td=0xf7fffb08) at macro.c:71 (More stack frames follow...) The display for frame zero does not begin with a program counter value, indicating that your program has stopped at the beginning of the code for line `993' of `builtin.c'. 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. *Note Printing source lines: List. `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. 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 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 Breakpoints and exceptions: Exception Handling. MIPS machines and the function stack ==================================== MIPS based computers use an unusual stack frame, which sometimes requires GDB to search backward in the object code to find the beginning of a function. To improve response time (especially for embedded applications, where GDB may be restricted to a slow serial line for this search) you may want to limit the size of this search, using one of these commands: `set heuristic-fence-post LIMIT' Restrict GDB to examining at most LIMIT bytes in its search for the beginning of a function. A value of 0 (the default) means there is no limit. However, except for 0, the larger the limit the more bytes `heuristic-fence-post' must search and therefore the longer it takes to run. `show heuristic-fence-post' Display the current limit. These commands are available *only* when GDB is configured for debugging programs on MIPS processors. 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; *note Using GDB under GNU Emacs: Emacs.. Printing source lines ===================== To print lines from a source file, use the `list' command (abbreviated `l'). By default, ten lines are printed. There are several ways to specify what part of the file you want to print. Here are the forms of the `list' command most commonly used: `list LINENUM' Print lines centered around line number LINENUM in the current source file. `list FUNCTION' Print lines centered around the beginning of function FUNCTION. `list' Print more lines. If the last lines printed were printed with a `list' command, this prints lines following the last lines printed; however, if the last line printed was a solitary line printed as part of displaying a stack frame (*note Examining the Stack: Stack.), this prints lines centered around that line. `list -' Print lines just before the lines last printed. By default, GDB prints ten source lines with any of these forms of the `list' command. You can change this using `set listsize': `set listsize COUNT' Make the `list' command display COUNT source lines (unless the `list' argument explicitly specifies some other number). `show listsize' Display the number of lines that `list' prints. Repeating a `list' command with discards the argument, so it is equivalent to typing just `list'. This is more useful than listing the same lines again. An exception is made for an argument of `-'; that argument is preserved in repetition so that each repetition moves up in the source file. In general, the `list' command expects you to supply zero, one or two "linespecs". Linespecs specify source lines; there are several ways of writing them but the effect is always to specify some source line. Here is a complete description of the possible arguments for `list': `list LINESPEC' Print lines centered around the line specified by LINESPEC. `list FIRST,LAST' Print lines from FIRST to LAST. Both arguments are linespecs. `list ,LAST' Print lines ending with LAST. `list FIRST,' Print lines starting with FIRST. `list +' Print lines just after the lines last printed. `list -' Print lines just before the lines last printed. `list' As described in the preceding table. Here are the ways of specifying a single source line--all the kinds of linespec. `NUMBER' Specifies line NUMBER of the current source file. When a `list' command has two linespecs, this refers to the same source file as the first linespec. `+OFFSET' Specifies the line OFFSET lines after the last line printed. When used as the second linespec in a `list' command that has two, this specifies the line OFFSET lines down from the first linespec. `-OFFSET' Specifies the line OFFSET lines before the last line printed. `FILENAME:NUMBER' Specifies line NUMBER in the source file FILENAME. `FUNCTION' Specifies the line that begins the body of the function FUNCTION. For example: in C, this is the line with the open brace. `FILENAME:FUNCTION' Specifies the line of the open-brace that begins the body of the function FUNCTION in the file FILENAME. You only need the file name with a function name to avoid ambiguity when there are identically named functions in different source files. `*ADDRESS' Specifies the line containing the program address ADDRESS. ADDRESS may be any expression. 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'. 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. Note that the executable search path is *not* used for this purpose. Neither is the current working directory, unless it happens to be in the source path. If GDB cannot find a source file in the source path, and the object program records a directory, GDB tries that directory too. If the source path is empty, and there is no record of the compilation directory, GDB looks in the current directory as a last resort. 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 is empty. To add other directories, use the `directory' command. `directory DIRNAME ...' `dir DIRNAME ...' Add directory DIRNAME to the front of the source path. Several directory names may be given to this command, separated by `:' or whitespace. You may specify a directory that is already in the source path; this moves it forward, so GDB searches it sooner. You can use the string `$cdir' to refer to the compilation directory (if one is recorded), and `$cwd' to refer to the current working directory. `$cwd' is not the same as `.'--the former tracks the current working directory as it changes during your GDB session, while the latter is immediately expanded to the current directory at the time you add an entry to the source path. `directory' Reset the source path to empty again. This requires confirmation. `show directories' Print the source path: show which directories it contains. If your source path is cluttered with directories that are no longer of interest, GDB may sometimes cause confusion by finding the wrong versions of source. You can correct the situation as follows: 1. Use `directory' with no argument to reset the source path to empty. 2. Use `directory' with suitable arguments to reinstall the directories you want in the source path. You can add all the directories in one command. Source and machine code ======================= You can use the command `info line' to map source lines to program addresses (and vice versa), and the command `disassemble' to display a range of addresses as machine instructions. When run under GNU Emacs mode, the `info line' command now causes the arrow to point to the line specified. Also, `info line' prints addresses in symbolic form as well as hex. `info line LINESPEC' Print the starting and ending addresses of the compiled code for source line LINESPEC. You can specify source lines in any of the ways understood by the `list' command (*note Printing source lines: List.). For example, we can use `info line' to discover the location of the object code for the first line of function `m4_changequote': (gdb) info line m4_changecom Line 895 of "builtin.c" starts at pc 0x634c and ends at 0x6350. We can also inquire (using `*ADDR' as the form for LINESPEC) what source line covers a particular address: (gdb) info line *0x63ff Line 926 of "builtin.c" starts at pc 0x63e4 and ends at 0x6404. After `info line', the default address for the `x' command is changed to the starting address of the line, so that `x/i' is sufficient to begin examining the machine code (*note Examining memory: Memory.). Also, this address is saved as the value of the convenience variable `$_' (*note Convenience variables: Convenience Vars.). `disassemble' This specialized command dumps a range of memory as machine instructions. The default memory range is the function surrounding the program counter of the selected frame. A single argument to this command is a program counter value; GDB dumps the function surrounding this value. Two arguments specify a range of addresses (first inclusive, second exclusive) to dump. We can use `disassemble' to inspect the object code range shown in the last `info line' example (the example shows SPARC machine instructions): (gdb) disas 0x63e4 0x6404 Dump of assembler code from 0x63e4 to 0x6404: 0x63e4 : ble 0x63f8 0x63e8 : sethi %hi(0x4c00), %o0 0x63ec : ld [%i1+4], %o0 0x63f0 : b 0x63fc 0x63f4 : ld [%o0+4], %o0 0x63f8 : or %o0, 0x1a4, %o0 0x63fc : call 0x9288 0x6400 : nop End of assembler dump. `set assembly-language INSTRUCTION-SET' This command selects the instruction set to use when disassembling the program via the `disassemble' or `x/i' commands. It is useful for architectures that have more than one native instruction set. Currently it is only defined for the Intel x86 family. You can set INSTRUCTION-SET to either `i386' or `i8086'. The default is `i386'. Examining Data ************** The usual way to examine data in your program is with the `print' command (abbreviated `p'), or its synonym `inspect'. It evaluates and prints the value of an expression of the language your program is written in (*note Using GDB with Different Languages: Languages.). `print EXP' `print /F EXP' EXP is an expression (in the source language). By default the value of EXP 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; *note Output formats: Output Formats.. `print' `print /F' If you omit EXP, 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 class are declared, use the `ptype EXP' command rather than `print'. *Note Examining the Symbol Table: Symbols. 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 unfortunately does not include symbols defined by preprocessor `#define' commands. GDB now supports array constants in expressions input by the user. The syntax is {ELEMENT, ELEMENT...}. For example, you can now use the command `print {1, 2, 3}' to build up an array in memory that is malloc'd 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. 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 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. 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 ... 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> `c' Regard as an integer and print it as a character constant. `f' Regard the bits of the value as a floating point number and print using typical floating point syntax. For example, to print the program counter in hex (*note Registers::.), type p/x $pc Note that no space is required before the slash; this is because command names in GDB cannot contain a slash. To reprint the last value in the value history with a different format, you can use the `print' command with just a format and no expression. For example, `p/x' reprints the last value in hex. ---------- Footnotes ---------- (1) `b' cannot be used because these format letters are also used with the `x' command, where `b' stands for "byte"; *note Examining memory: Memory.. Examining memory ================ You can use the command `x' (for "examine") to examine memory in any of several formats, independently of your program's data types. `x/NFU ADDR' `x ADDR' `x' Use the `x' command to examine memory. N, F, and U are all optional parameters that specify how much memory to display and how to format it; ADDR is an expression giving the address where you want to start displaying memory. If you use defaults for NFU, you need not type the slash `/'. Several commands set convenient defaults for ADDR. N, the repeat count The repeat count is a decimal integer; the default is 1. It specifies how much memory (counting by units U) to display. F, the display format The display format is one of the formats used by `print', `s' (null-terminated string), or `i' (machine instruction). The default is `x' (hexadecimal) initially. The default changes each time you use either `x' or `print'. U, the unit size The unit size is any of `b' Bytes. `h' Halfwords (two bytes). `w' Words (four bytes). This is the initial default. `g' Giant words (eight bytes). Each time you specify a unit size with `x', that size becomes the default unit the next time you use `x'. (For the `s' and `i' formats, the unit size is ignored and is normally not written.) ADDR, starting display address ADDR is the address where you want GDB to begin displaying memory. The expression need not have a pointer value (though it may); it is always interpreted as an integer address of a byte of memory. *Note Expressions: Expressions, for more information on expressions. The default for ADDR is usually just after the last address examined--but several other commands also set the default address: `info breakpoints' (to the address of the last breakpoint listed), `info line' (to the starting address of a line), and `print' (if you use it to display a value from memory). For example, `x/3uh 0x54320' is a request to display three halfwords (`h') of memory, formatted as unsigned decimal integers (`u'), starting at address `0x54320'. `x/4xw $sp' prints the four words (`w') of memory above the stack pointer (here, `$sp'; *note Registers::.) in hexadecimal (`x'). Since the letters indicating unit sizes are all distinct from the letters specifying output formats, you do not have to remember whether unit size or format comes first; either order works. The output specifications `4xw' and `4wx' mean exactly the same thing. (However, the count N must come first; `wx4' does not work.) Even though the unit size U is ignored for the formats `s' and `i', you might still want to use a count N; for example, `3i' specifies that you want to see three machine instructions, including any operands. The command `disassemble' gives an alternative way of inspecting machine instructions; *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 ins