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:
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Start your program, specifying anything that might affect its behavior.
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Make your program stop on specified conditions.
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Examine what has happened, when your program has stopped.
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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:
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). 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.
edit [file:]function
look at the program line where it is presently stopped.
list [file:]function
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.
help [name]
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.