8.1.3 Introduction to GDB Commands
GDB contains a large repertoire of commands.
See Debugging with GDB for extensive documentation on the use
of these commands, together with examples of their use. Furthermore,
the command `help' invoked from within GDB activates a simple help
facility which summarizes the available commands and their options.
In this section we summarize a few of the most commonly
used commands to give an idea of what GDB is about. You should create
a simple program with debugging information and experiment with the use of
these GDB commands on the program as you read through the
- `set args `arguments`'
The arguments list above is a list of arguments to be passed to
the program on a subsequent run command, just as though the arguments
had been entered on a normal invocation of the program. The set args
command is not needed if the program does not require arguments.
The run command causes execution of the program to start from
the beginning. If the program is already running, that is to say if
you are currently positioned at a breakpoint, then a prompt will ask
for confirmation that you want to abandon the current execution and
- `breakpoint `location`'
The breakpoint command sets a breakpoint, that is to say a point at which
execution will halt and GDB will await further
commands. location is
either a line number within a file, given in the format file:linenumber,
or it is the name of a subprogram. If you request that a breakpoint be set on
a subprogram that is overloaded, a prompt will ask you to specify on which of
those subprograms you want to breakpoint. You can also
specify that all of them should be breakpointed. If the program is run
and execution encounters the breakpoint, then the program
stops and GDB signals that the breakpoint was encountered by
printing the line of code before which the program is halted.
- `catch exception `name`'
This command causes the program execution to stop whenever exception
name is raised. If name is omitted, then the execution is
suspended when any exception is raised.
- `print `expression`'
This will print the value of the given expression. Most simple
Ada expression formats are properly handled by GDB, so the expression
can contain function calls, variables, operators, and attribute references.
Continues execution following a breakpoint, until the next breakpoint or the
termination of the program.
Executes a single line after a breakpoint. If the next statement
is a subprogram call, execution continues into (the first statement of)
the called subprogram.
Executes a single line. If this line is a subprogram call, executes and
returns from the call.
Lists a few lines around the current source location. In practice, it
is usually more convenient to have a separate edit window open with the
relevant source file displayed. Successive applications of this command
print subsequent lines. The command can be given an argument which is a
line number, in which case it displays a few lines around the specified one.
Displays a backtrace of the call chain. This command is typically
used after a breakpoint has occurred, to examine the sequence of calls that
leads to the current breakpoint. The display includes one line for each
activation record (frame) corresponding to an active subprogram.
At a breakpoint, GDB can display the values of variables local
to the current frame. The command up can be used to
examine the contents of other active frames, by moving the focus up
the stack, that is to say from callee to caller, one frame at a time.
Moves the focus of GDB down from the frame currently being
examined to the frame of its callee (the reverse of the previous command),
- `frame `n`'
Inspect the frame with the given number. The value 0 denotes the frame
of the current breakpoint, that is to say the top of the call stack.
Kills the child process in which the program is running under GDB.
This may be useful for several purposes:
- It allows you to recompile and relink your program, since on many systems
you cannot regenerate an executable file while it is running in a process.
- You can run your program outside the debugger, on systems that do not
permit executing a program outside GDB while breakpoints are set
- It allows you to debug a core dump rather than a running process.
The above list is a very short introduction to the commands that
GDB provides. Important additional capabilities, including conditional
breakpoints, the ability to execute command sequences on a breakpoint,
the ability to debug at the machine instruction level and many other
features are described in detail in Debugging with GDB.
Note that most commands can be abbreviated
(for example, c for continue, bt for backtrace).