Linux Programming Blog

/* The program creates a child process and waits for it to finish. If a timeout
 * elapses the child is killed. Waiting is done using sigtimedwait(). Race
 * condition is avoided by blocking the SIGCHLD signal before fork().
 */
#include <sys/types.h>
#include <sys/wait.h>
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
 
static pid_t fork_child (void)
{
	int p = fork ();
 
	if (p == -1) {
		perror ("fork");
		exit (1);
	}
 
	if (p == 0) {
		puts ("child: sleeping...");
		sleep (10);
		puts ("child: exiting");
		exit (0);
	}
 
	return p;
}
 
int main (int argc, char *argv[])
{
	sigset_t mask;
	sigset_t orig_mask;
	struct timespec timeout;
	pid_t pid;
 
	sigemptyset (&mask);
	sigaddset (&mask, SIGCHLD);
 
	if (sigprocmask(SIG_BLOCK, &mask, &orig_mask) < 0) {
		perror ("sigprocmask");
		return 1;
	}
 
	pid = fork_child ();
 
	timeout.tv_sec = 5;
	timeout.tv_nsec = 0;
 
	do {
		if (sigtimedwait(&mask, NULL, &timeout) < 0) {
			if (errno == EINTR) {
				/* Interrupted by a signal other than SIGCHLD. */
				continue;
			}
			else if (errno == EAGAIN) {
				printf ("Timeout, killing child\n");
				kill (pid, SIGKILL);
			}
			else {
				perror ("sigtimedwait");
				return 1;
			}
		}
 
		break;
	} while (1);
 
	if (waitpid(pid, NULL, 0) < 0) {
		perror ("waitpid");
		return 1;
	}
 
	return 0;
}