Why does WIFEXITED return false after child process exits?

Question

I've got this simple program that creates a child process which immediately calls exit(). So in the parent process I'm expecting WIFEXITED(status) to evaluate as true, but it's not. Instead, WIFSTOPPED(status) evaluates as true and "stopped" is printed. Can anyone explain why I'm getting this behavior? I'm running on OS X and compiling with gcc. Thanks!

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

int main(void)
{
    int pid;
    int status;

    pid = fork();
    if (pid < 0)
        printf("fork failed\n");
    else if (pid == 0)
    {
        wait(&status);
        if (WIFEXITED(status))
            printf("exited\n");
        else if (WIFSTOPPED(status))
            printf("stopped\n");
    }
    else
        exit(0);

    return (0);
}

Answer 1

You have the logic for the child and parent backwards. The parent is exiting immediately and the child is calling wait. Since the child has no children, wait is returning an error (and not touching status) since the child has no children (ECHILD), then you're testing the (uninitialized) value of status and acting on it, resulting in undefined behavior.

Change:

else if (pid == 0)

to:

else if (pid > 0)

and it should work as expected.