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4.11 Debugging Forks

On most systems, 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 Attach). From that point on you can debug the child process just like any other process which you attached to.

On some systems, GDB provides support for debugging programs that create additional processes using the fork or vfork functions. On GNU/Linux platforms, this feature is supported with kernel version 2.5.46 and later.

The fork debugging commands are supported in native mode and when connected to gdbserver in either target remote mode or target extended-remote mode.

By default, when a program forks, GDB will continue to debug the parent process and the child process will run unimpeded.

If you want to follow the child process instead of the parent process, use the command set follow-fork-mode.

set follow-fork-mode mode

Set the debugger response to a program call of fork or vfork. A call to fork or vfork creates a new process. The mode argument can be:

parent

The original process is debugged after a fork. The child process runs unimpeded. This is the default.

child

The new process is debugged after a fork. The parent process runs unimpeded.

show follow-fork-mode

Display the current debugger response to a fork or vfork call.

On Linux, if you want to debug both the parent and child processes, use the command set detach-on-fork.

set detach-on-fork mode

Tells gdb whether to detach one of the processes after a fork, or retain debugger control over them both.

on

The child process (or parent process, depending on the value of follow-fork-mode) will be detached and allowed to run independently. This is the default.

off

Both processes will be held under the control of GDB. One process (child or parent, depending on the value of follow-fork-mode) is debugged as usual, while the other is held suspended.

show detach-on-fork

Show whether detach-on-fork mode is on/off.

If you choose to set ‘detach-on-fork’ mode off, then GDB will retain control of all forked processes (including nested forks). You can list the forked processes under the control of GDB by using the info inferiors command, and switch from one fork to another by using the inferior command (see Debugging Multiple Inferiors and Programs).

To quit debugging one of the forked processes, you can either detach from it by using the detach inferiors command (allowing it to run independently), or kill it using the kill inferiors command. See Debugging Multiple Inferiors and Programs.

If you ask to debug a child process and a vfork is followed by an exec, GDB executes the new target up to the first breakpoint in the new target. If you have a breakpoint set on main in your original program, the breakpoint will also be set on the child process’s main.

On some systems, when a child process is spawned by vfork, you cannot debug the child or parent until an exec call completes.

If you issue a run command to GDB after an exec call executes, the new target restarts. To restart the parent process, use the file command with the parent executable name as its argument. By default, after an exec call executes, GDB discards the symbols of the previous executable image. You can change this behaviour with the set follow-exec-mode command.

set follow-exec-mode mode

Set debugger response to a program call of exec. An exec call replaces the program image of a process.

follow-exec-mode can be:

new

GDB creates a new inferior and rebinds the process to this new inferior. The program the process was running before the execcall can be restarted afterwards by restarting the original inferior.

For example:

(gdb) info inferiors
(gdb) info inferior
  Id   Description   Executable
* 1    <null>        prog1
(gdb) run
process 12020 is executing new program: prog2
Program exited normally.
(gdb) info inferiors
  Id   Description   Executable
  1    <null>        prog1
* 2    <null>        prog2

same

GDB keeps the process bound to the same inferior. The new executable image replaces the previous executable loaded in the inferior. Restarting the inferior after the exec call, with e.g., the run command, restarts the executable the process was running after the exec call. This is the default mode.

For example:

(gdb) info inferiors
  Id   Description   Executable
* 1    <null>        prog1
(gdb) run
process 12020 is executing new program: prog2
Program exited normally.
(gdb) info inferiors
  Id   Description   Executable
* 1    <null>        prog2

follow-exec-mode is supported in native mode and target extended-remote mode.

You can use the catch command to make GDB stop whenever a fork, vfork, or exec call is made. See Setting Catchpoints.

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