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SubjectRe: More SIGCHLD musings
More news. Thanks to all who replied.

When a child dies, SIGCHLD is generated, and the parent process
is woken up. That makes sense.

However, judging from line 671 in kernel/exit.c, when the parent
calls wait4(), the SIGCHLD is discarded!

To me, that is just simply wrong. The signal might be blocked,
and it should be delivered when we unblock it, which is exactly
what happens on the non-Linux systems.

A trace on SunOS shows that SIGCHLD is delivered right after the
final call to sigprocmask(). So SunOS is *not* unblocking the
signal when we call wait(), contrary to what has been suggested.
Ditto for Solaris.

I have received a report (hi Harald :) that SIGCHLD is also
delivered on AIX 3.2.5, AIX 4.1.4, Digital Unix 3.2 and HP/UX
9.something. So Linux is clearly in the minority here.

Blocking SIGCHLD is perfectly legal, e.g. most implementations
of system() apparently do this.

For example, try the following program. On a sane system it
should print `Number of children = 1'. On Linux however, it
prints `Number of children = 0'.
This is wrong, and should be fixed. (By not discarding SIGCHLD
when wait4() is called.)

#include <stdio.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <unistd.h>
#include <stdlib.h>

volatile int numchildren = 0;

static void sigchld()
int pid;
int saved_errno = errno;

while ((pid = waitpid(-1, 0, WNOHANG)) != 0)
if (pid == -1) {
if (errno != EINTR)
} else
errno = saved_errno;

int main(void)
struct sigaction act;

act.sa_handler = sigchld;
act.sa_flags = 0;
sigaction(SIGCHLD, &act, 0);

switch(fork()) {
case -1:
return 1;
case 0:
system("sleep 10");

printf("Number of children = %d\n", numchildren);
return 0;

Michiel Boland <>
University of Nijmegen
The Netherlands

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