int child_pid = fork();

if (child_pid == 0) {   	// I'm the child process.
    printf("I am process #%d\n", getpid());  
    return 0;
} else {			// I'm the parent process.
    printf("I am the parent of process #%d\n", child_pid); 
    return 0;
}

Possible output:
    I am the parent of process 495
    I am process 495

Another less likely but still possible output:
    I am process 456
    I am the parent of process 456