int main(void)
{
    pid_t    pid;
    if ((pid = fork()) < 0)
        err_sys("fork error");
    else 
        if (pid == 0)
        {
            printf("pid : %d\n", pid);
            printf("child process pid : %d\n", getpid());
        }            
        else 
        {
            printf("pid : %d\n", pid);
            printf("parent process pid : %d\n", getpid());
        }
    exit(0);
}
pid : 11784
parent process pid : 11783

pid : 0
child process pid : 11784