Waitpid and fork/exec's non-blocking advantage over a syscall?

Question

I always hear that you should never use system() and instead fork/exec because system() blocks the parent process.

If so, am I doing something wrong by calling waitpid(), which also blocks the parent process when I do a fork/exec? Is there a way around calling waitpid...I always thought it was necessary when doing a fork/exec.

pid_t pid = fork();

if (pid == -1)
{
    // failed to fork
} 
else if (pid > 0)
{
    int status;
    waitpid(pid, &status, 0);
}
else 
{
    execve(...);
}

Answer 1

The WNOHANG flag (set in the options argument) will make the call to waitpid() non-blocking.

You'll have to call it periodically to check if the child is finished yet.

Or you could setup SIGCHLD to take care of the children.

Answer 2

If you want to do other stuff whilst the child process is off doing it's thing, you can set up a trap for SIGCHLD that handles the child finishing/exiting. Like in this very simple example.

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>

pid_t pid;
int finished=0;

void zombie_hunter(int sig)
    {
    int status;
    waitpid(pid, &status, 0);
    printf("Got status %d from child\n",status);
    finished=1;
    }

int main(void)
    {
    signal(SIGCHLD,zombie_hunter);

    pid = fork();

    if (pid == -1)
        {
        exit(1);
        } 
    else if (pid == 0)
        {
        sleep(10);
        exit(0);
        }

    while(!finished)
        {
        printf("waiting...\n");
        sleep(1);
        }
    }