What exactly does fork return?

Question

On success, the PID of the child process is returned in the parent’s thread of execution, and a 0 is returned in the child’s thread of e

Answer 1

Processes are structured in a directed tree where you only know your single-parent (getppid()). In short, fork() returns -1 on error like many other system functions, non-zero value is useful for initiator of the fork call (the parent) to know its new-child pid.

Nothing is as good as example:

/* fork/getpid test */
#include 
#include      /* fork(), getpid() */
#include 

int main(int argc, char* argv[])
{
    int pid;

    printf("Entry point: my pid is %d, parent pid is %d\n",
           getpid(), getppid());

    pid = fork();
    if (pid == 0) {
        printf("Child: my pid is %d, parent pid is %d\n",
               getpid(), getppid());
    }
    else if (pid > 0) {
        printf("Parent: my pid is %d, parent pid is %d, my child pid is %d\n",
               getpid(), getppid(), pid);
    }
    else {
        printf("Parent: oops! can not create a child (my pid is %d)\n",
               getpid());
    }

    return 0;
}

And the result (bash is pid 2249, in this case):

Entry point: my pid is 16051, parent pid is 2249
Parent: my pid is 16051, parent pid is 2249, my child pid is 16052
Child: my pid is 16052, parent pid is 16051

If you need to share some resources (files, parent pid, etc.) between parent and child, look at clone() (for GNU C library, and maybe others)