/* Mike Chirico mmc
   CopyRight GPL 2004

   You don't want a zombie process (a child process
   that's kept running until after the parent has
   exited).  And, you don't want to wait in a
   loop taking up valuable cpu resources.

   The program below, waits for a signal telling
   the parent that each child has terminated..ah
   but using a signal is a bad idea, because
   only realtime signals will on.

   sigsuspend(...) blocks and it's run for each
   forked process, or NUM_PROC of them. Ok, that's
   fine for the first signal.

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

#define NUM_PROC 4
sig_atomic_t sigusr1_count = 0;

/* according to recent literature
this function call allows signals to 
be pending, or queued provided real-time
signals are being used.  Currently, I have
2 versions of Linux

2.4.20-28.9 #1 Thu Dec 18 13:45:22 EST 2003

(does not work)
2.4.20-28.8 #1 Thu Dec 18 12:53:39 EST 2003


  /* handler may only execute once
for the first process only, the other
processes are dropped when they are it's a race.  If the first
one finishes you're ok */
void handler (int signal_number)
  fprintf(stderr,"in handler \n");

int main(void)
  struct sigaction sa;
  sigset_t blocked_sigs,no_sigs,all_sigs;
  pid_t p[NUM_PROC],pid;
  int i;


  memset(&sa, 0, sizeof(sa));
  sa.sa_handler = &handler;
  if( sigaction(SIGCHLD,&sa, NULL) == -1) {
    fprintf(stderr,"sigaction error \n");

  for(i=0; i < NUM_PROC; ++i)
      if ( (p[i] = fork()) == 0 ) {
	/* take out sleep and 
	   put your main code here */

  int status;
  while(sigusr1_count < NUM_PROC)
      printf("before: sigsuspend %d\n",sigusr1_count);
      printf("after sigsuspend: %d \n",sigusr1_count);
      /* this is our circuit breaker for safety.
	 if the other processors finish at the same
	 time, they may not queue.  sigusr1_count will 
	 never go above one!  You'll get a zombie
	 process...this protects us, until Linux
	 support realtime signals.  Only realtime
	 signals queue. Ref man 7 signal
        for(i=0; i < NUM_PROC; ++i)
	    if ((sigusr1_count < NUM_PROC) && (i == (NUM_PROC -1))) {
	      fprintf(stderr,"Signal didn't queue\n");


  return 0;


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Chirico img Mike Chirico, a father of triplets (all girls) lives outside of Philadelphia, PA, USA. He has worked with Linux since 1996, has a Masters in Computer Science and Mathematics from Villanova University, and has worked in computer-related jobs from Wall Street to the University of Pennsylvania. His hero is Paul Erdos, a brilliant number theorist who was known for his open collaboration with others.

Mike's notes page is souptonuts. For open source consulting needs, please send an email to All consulting work must include a donation to Logo Logo