gdb - The GNU Debugger
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.
? 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, specifying an executable program as the argument:
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).
Here are some of the most frequently needed GDB commands:
Set a breakpoint at function (in file).
Start your program (with arglist, if specified).
Display the value of an expression.
next Execute next program line (after stopping); step over any function calls in the line.
look at the program line where it is presently stopped.
type the text of the program in the vicinity of where it is presently stopped.
step Execute next program line (after stopping); step into any function calls in the line.
Show information about GDB command name, or general information about using 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.
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 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 processed in sequential order. The order makes a difference when the ‘-x’ option is used.
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.
‘gdb’ entry in info; Using GDB: A Guide to the GNU Source-Level Debug_ger, Richard M. Stallman and Roland H. Pesch, July 1991.
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 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 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, except that this permission notice may be included in translations approved by the Free Software Foundation instead of in the original English.
Table of Contents