Re: wait(2) and SIGCHLD

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

>>> What I observe is that a process that explicitly ignores SIGCHLD
>>> (SIG_IGN), then forks a child which exits, when wait()ing for the
>>> child, gets ECHILD (i.e., wait returns -1 and errno is ECHILD).
>> And the ECHILD return is delayed until all children have terminated
> Huh, I hadn't realized (or expected) that.  So I guess it's wrong to
> implement this by just detaching the child up front...?

I would say so, especially since that would mean the child's parent is
no longer the process that forked it (which could break other use
cases).

> I'm guessing also then that it's the signal setting when the child
> exits that matters; I had always thought it was the signal setting
> when the child was forked.

Oh, interesting point.

Yes, in a test I just did on NetBSD 5.2, it is the signal setting when
the child exits that matters - program below.  (I tried it on 1.4T, but
that's old enough it doesn't have the magic SIG_IGN semantic for
SIGCHLD, not even when set before the child is forked.)  I don't know
whether POSIX specifies that or not.

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

int main(void);
int main(void)
{
 pid_t kid;
 pid_t dead;

 fflush(0);
 kid = fork();
 if (kid == 0)
  { printf("child sleeping\n");
    sleep(5);
    printf("child exiting\n");
    exit(0);
  }
 printf("child %d forked\n",(int)kid);
 printf("parent sleeping\n");
 sleep(1);
 printf("parent ignoring SIGCHLD\n");
 signal(SIGCHLD,SIG_IGN);
 while (1)
  { printf("parent waiting\n");
    dead = wait(0);
    if (dead < 0)
     { if (errno == ECHILD)
	{ printf("wait error: %s\n",strerror(ECHILD));
	  break;
	}
       else
	{ printf("wait error: %s\n",strerror(errno));
	}
     }
    else
     { printf("child %d reaped\n",dead);
     }
  }
 printf("parent exiting\n");
 return(0);
}

> "signals are a semantic cesspool"

That they are.

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