Re: Should parent's WIFSIGNALED(siginfo->si_status) be true EVEN IF the SIGNAL was caught by the child?

[Michael Kerrisk]

John,

> It is unclear from the various documentions in the kernel and glibc what
> the proper behaviour should be for the case when a child process
> catches a SIGNAL (say for instance, SIGTERM), and then calls exit()
> from within its caught SIGNAL handler.
>
> Since the exit() will cause a SIGCHLD to the parent, and the parent
> (let's say) has a SIGCHLD sigaction (SA_SIGINFO sa_flags set), should
> the parent's WIFSIGNALED(siginfo->si_status) be true?
>
> To recap, the WIFSIGNALED section of the waitpid() manpage says:
>
> WIFSIGNALED(status)
>    returns true if the child process was terminated by a signal.
>
> So the dilemna:  the child caught the signal, so it wasn't terminated by
> a signal, but rather its signal handler (let's say) called exit.
>
> Furthermore:
>
> WTERMSIG(status)
>    returns the number of the signal that caused the  child  process
>    to  terminate. This macro should only be employed if WIFSIGNALED
>    returned true.
>
> Observered behaviour with 2.6.20.6 is that is WIFSIGNALED(status)
> returns true (possibly incorrect), and furthermore, WTERMSIG(status)
> returns the exit(VALUE) VALUE from the child's exit() call, and not the
> SIGNAL (let's say SIGTERM that the child caught).  This may be correct,
> since the siginfo_t * si_status member is:
>
> int      si_status; /* Exit value or signal */
>
> but there's no clarity on which:  exit value or signal.  Since the child
> exited, I'm likely to assume exit status, which is current observed
> behaviour, but then, WIFSIGNALED(status) should be FALSE, which its not
> (observed with 2.6.20.6).
>
> So could someone clarify the kernel's intent?
>
> I can provide a short C program to illustrate above behaviour, if
> needed.  It also could be that I'm just misinterpreting the intent,
> which is why I'm not calling this a bug, despite a possible
> inconsistency in behaviour.

If the child terminated by calling exit(), regardless of whether it
was done from inside a signal handler, then WIFEXITED() should test
true, but WIFSIGNALED() will not.  If you are seeing otherwise, then
show a *short* program that demonstrates the behavior.  (But it seems
unlikely that there would be a kernel bug on this point, so do check
your program carefully!)

Cheers,

Michael
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