Sometimes we get a bus error or a segmentation fault. But we have no idea where it is happening. The debugger program called gdb can help. Here's how to see where the bug happened in your code.
gdb file [filename] set args [arg1] [arg2] ror
gdb [filename] run arg1 arg2The r command will run the debugger.
GNU Tools gdb(1) NAME gdb - The GNU Debugger SYNOPSIS gdb [-help] [-nx] [-q] [-batch] [-cd=dir] [-f] [-b bps] [-tty=dev] [-s symfile] [-e prog] [-se prog] [-c core] [-x cmds] [-d dir] [prog[core|procID]] DESCRIPTION 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: o Start your program, specifying anything that might affect its behavior. o Make your program stop on specified conditions. o Examine what has happened, when your program has stopped. o 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, C++, and Modula-2. Fortran support will be added when a GNU Fortran compiler is ready. GDB is invoked with the shell command gdb. Once started, it reads commands from the terminal until you tell it to exit with the GDB command quit. You can get online help from gdb itself by using the command help. You can run gdb with no arguments or options; but the most usual way to start GDB is with one argument or two, specify- ing an executable program as the argument: gdb program You can also start with both an executable program and a core file specified: GNU Tools Last change: 4nov1991 1 GNU Tools gdb(1) 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). Here are some of the most frequently needed GDB commands: break [file:]function Set a breakpoint at function (in file). run [arglist] Start your program (with arglist, if specified). bt Backtrace: display the program stack. print expr Display the value of an expression. c Continue running your program (after stopping, e.g. at a breakpoint). next Execute next program line (after stopping); step over any function calls in the line. step Execute next program line (after stopping); step into any function calls in the line. help [name] Show information about GDB command name, or general information about using GDB. quit Exit from GDB. For full details on GDB, see Using GDB: A Guide to the GNU Source-Level Debugger, by Richard M. Stallman and Roland H. Pesch. The same text is available online as the gdb entry in the info program. OPTIONS Any arguments other than options specify an executable file and core file (or process ID); that is, the first argument encountered with no associated option flag is equivalent to a `-se' option, and the second, if any, is equivalent to a `-c' option if it's the name of a file. Many options have both long and short forms; both are shown here. The long GNU Tools Last change: 4nov1991 2 GNU Tools gdb(1) forms are also recognized 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.) All the options and command line arguments you give are pro- cessed in sequential order. The order makes a difference when the `-x' option is used. -help -h List all options, with brief explanations. -symbols=file -s file Read symbol table from file file. -write Enable writing into executable and core files. -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. -command=file -x file Execute GDB commands from file file. -directory=directory GNU Tools Last change: 4nov1991 3 GNU Tools gdb(1) -d directory Add directory to the path to search for source files. -nx -n Do not execute commands from any `.gdbinit' initializa- tion files. Normally, the commands in these files are executed after all the command options and arguments have been processed. -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 `.gdbinit', if not inhibited). 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 mes- sage Program exited normally. (which is ordinarily issued whenever a program running under GDB control terminates) is not issued when run- ning in batch mode. -cd=directory Run GDB using directory as its working directory, instead of the current directory. -fullname -f Emacs sets this option when it runs GDB as a subpro- cess. 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 the program stops). This recognizable format looks like two ` 32' characters, followed by the file GNU Tools Last change: 4nov1991 4 GNU Tools gdb(1) name, line number and character position separated by colons, and a newline. The Emacs-to-GDB interface pro- gram uses the two ` 32' 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. SEE ALSO `gdb' entry in info; Using GDB: A Guide to the GNU Source- Level Debugger, Richard M. Stallman and Roland H. Pesch, July 1991. COPYING Copyright (c) 1991 Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this per- mission notice are preserved on all copies. Permission is granted to copy and distribute modified ver- sions of this manual under the conditions for verbatim copy- ing, provided that the entire resulting derived work is dis- tributed 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 condi- tions for modified versions, except that this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English. GNU Tools Last change: 4nov1991 5