waitfg.c

/*
* waitfg - Block until process pid is no longer the foreground process
*/
void waitfg(pid_t pid) {
    sigset_t mask, nomask, old;
    struct job_t *j = getjobpid(jobs, pid);
    sigfillset (&mask);
    sigemptyset (&nomask);
    sigprocmask (SIG_SETMASK, &mask, &old);

/*At this point, all signals are blocked.
There is no way a child can be terminated before
sigsuspend is called, so we're guaranteed to catch
the SIGCHLD now.
However, before calling sigsuspend, I make sure that the
process hasn't terminated quickly.
Without this check, I could wait forever for a signal that will
never arrive.*/

    if (j && j->pid == pid) // job still there?
    while (j->pid == pid && j->state == FG)
        sigsuspend (&nomask); // any signal wakes up the sigsuspend

    sigprocmask (SIG_SETMASK, &old, NULL);
}