Block signal from propagating to the inferior when using ptrace

I had put a simple trap instruction to simulate a breakpoint on the inferior, but when this instruction is reached I got a CLD_KILLED instead of CLD_SIGTRAP, like the one below.

--- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_KILLED, si_pid=12668, si_uid=10157, si_status=SIGTRAP, si_utime=2692, si_stime=875}

It seems that gdb is able to prevent SIGTRAP to kill the inferior by using the command "handle SIGSTOP nopass".

How can I do it in C?

Thanks

Solution

According to the man page, https://www.freebsd.org/cgi/man.cgi?query=siginfo&sektion=3&apropos=0&manpath=FreeBSD+7.1-RELEASE

 SIGCHLD
si_pid       child process ID
si_status    exit value or signal; if si_code is equal to
             CLD_EXITED, then it is equal to the exit
             value of the child process, otherwise, it is
             equal to a signal that caused the child
             process to change state.

In your case si_code=CLD_KILLED so the clause si_status [...] is equal to a signal that caused the child to change state.

Thus si_status=SIGTRAP is the information you're looking for.

If you had ptrace your child you would have gotten a SIGTRAP instead.