Sending SIGSTOP to a child process stops all execution. C

Question

When I call kill(Child_PID, SIGSTOP); from the parent, I expect the child to halt execution and the parent to continue. Is that the expected behavior or do I have to explicitly declare the SIGSTOP handler in the child? I have searched everywhere and not been able to find this information.

Thanks. Braden

Answer 1

This is the expected behavior.

Use strace your_program to see what's happening.

Answer 2

POSIX says:

The system shall not allow a process to catch the signals SIGKILL and SIGSTOP.

So, the child has no option but to stop - if the signal is sent successfully. And you cannot set a SIGSTOP handler in the child (or parent, or any other) process.

Answer 3

That's the expected behaviour. Quoting from the unix man page:

The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

And the BSD man page mentions that:

The signal() function will fail and no action will take place if one of the following occur:

[EINVAL]           The sig argument is not a valid signal number.
[EINVAL]           An attempt is made to ignore or supply a handler
                   for SIGKILL or SIGSTOP.

Concluding, you're not permitted to install a handler for SIGSTOP. And the process will remain in the suspended state until it receives a SIGCONT.