fork and execlp

Question

When I use fork to create a new child process and then call execlp syscall to run a new program in the child. The process ids that I get in the child process after execlp and I get from waitpid syscall after the child terminates are different.

For example, getpid() returns 7103 in the child and waitpid returns 7101 in the parent.

I guess something happens after execlp runs. Can anyone explain this. Thanks.

BTW, I run my code on Fedora.

Here is the code:

/* program parent */

if ((pid = fork()) < 0){
    perror("fork failed");
    exit(2);
}

if (pid == 0){
    // child
    execlp("xterm", "xterm", "-e", "./echo_cli", "127.0.0.1", (char *)0);
    exit(0);
}
/* ... */
// sig_chld handles SIGCHLD singal
void sig_chld(int signo){
    pid_t pid;
    int stat;

    while ((pid = waitpid(-1, &stat, WNOHANG)) > 0){ 
        printf("Child %d terminated\n", pid);
    }   
    return ;
}

/* program echo_cli */

pid = getpid();

Answer 1

You're executing xterm, not echo_cli. Your child's child will of course report a different PID.