Thread: waitpid() non-blocking fork

I have a process that forks a child process that then runs a few syscalls. There is the possibility that the syscalls the child runs could cause it to block while it processes the calls, but I don't want the parent to wait. Further, I want the parent process to allow the child to run for a certain period of time and if the child is still blocked after a specified number of seconds, the parent will kill off the child.

I am using waitpid(pid, &status, WNOHANG) to have the parent process not wait for the child. So I'm not sure what to do next. Do I keep looping back to waitpid() until either the child completes it's tasks and returns status or until the timeout value is reached or how do I loop over checking status of the child while evaluating the timeout?

I looked over the waitpid man page but didn't see

Okay, so as time permits I've been trying to get this going and I'm still kind of confused. Below is the parent portion of the forking code. The issue I'm having is that no matter what I do, when I specify WNOHANG in the waitpid function, it automatically returns a WIFEXITED status of 0. What I want it to do is continue iterating through the do/while loop until either the child exits with status, or waittime equals a predesignated timeout value and the parent hard kills the child.

Right now, the do/while loop exits because waitpid passes a 0 to Stat as the result of specifying NOHANG.

Code:

else {
	printf("The pid of the child is: %d\n", pid);
	int waittime = 0;
	int Stat;
	do {
		wpid = waitpid(pid, &Stat, WNOHANG);

		if (WIFEXITED(Stat)) {
			printf("Child exited, status=%d\n", WEXITSTATUS(Stat));
			break;
		}
		else if (WIFSIGNALED(Stat)) {
			printf("Child %d was terminated with a status of: %d \n", pid, WTERMSIG(Stat));
			break;
		}
		else if (WIFSTOPPED(Stat)) {
			printf("Child stopped (signal %d)\n", WSTOPSIG(Stat));
			break;
		}
		else if (waittime < timeout){
			printf("Parent waiting %d second(s).\n", waittime);
			waittime ++;
			sleep(1);
		}
		else {
			printf("Killing child process\n");
			kill(pid, SIGKILL);	
		}
	} while (!WIFEXITED(Stat) && !WIFSIGNALED(Stat));
}

Thanks Salem,

I try to do all the research before I post, but that line you quoted is towards the bottom of the page and I somehow missed it. Anyway, I have the code working now. For anyone else that may be interested, here's the working code:

Code:

else {
	printf("The pid of the child is: %d\n", pid);
	int waittime = 0;
	int Stat;
	do {
		wpid = waitpid(pid, &Stat, WNOHANG);
		if (wpid == 0) {
			if (waittime < timeout) {
				printf("Parent waiting %d second(s).\n", waittime);
				sleep(1);
				waittime ++;
			}
			else {
				printf("Killing child process\n");
				kill(pid, SIGKILL);	
			}
		}
	} while (wpid == 0 && waittime <= timeout);

	if (WIFEXITED(Stat)) {
		printf("Child exited, status=%d\n", WEXITSTATUS(Stat));
	}
	else if (WIFSIGNALED(Stat)) {
		printf("Child %d was terminated with a status of: %d \n", pid, WTERMSIG(Stat));
	}
}