/**
 * Demonstration that SIGCHLD signals are not queued.
 * 
 * The program below blocks SIGCHLD, then starts 3 children
 * which exit.  After SIGCHLD is unblocked, only one
 * SIGCHLD is delivered, with the si_pid set to the first child's
 * pid.  Three calls to waitpid() will be necessary to reap each child.
 *
 * Written by Godmar Back for CS 3214, Fall 2010.
 */
#include <signal.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>

static void
sigchld_handler(int signo, siginfo_t *info, void * _ctxt)
{
    char buf[1024];
    snprintf(buf, sizeof buf, "sigchld_handler called si_pid=%d\n", info->si_pid); 
    write(1, buf, strlen(buf));
}

int
main()
{
    int status;
    struct sigaction act;

    act.sa_sigaction = sigchld_handler;
    act.sa_flags = SA_SIGINFO;
    sigemptyset (&act.sa_mask);
    status = sigaction (SIGCHLD, &act, NULL);
    if (status) {
        perror ("sigaction");
        exit (1); 
    }

    // start and stop children #1, #2, #3
    sigset_t block;
    sigemptyset(&block);
    sigaddset(&block, SIGCHLD);
    printf("Parent blocking SIGCHLD\n");
    sigprocmask(SIG_BLOCK, &block, NULL);

    int i = 0;
    for (; i < 3; i++) {
        if (fork() == 0) {
            printf("Child %d exiting\n", getpid());
            exit(0);
        }
    }

    printf("Parent unblocking SIGCHLD\n");
    sigprocmask(SIG_UNBLOCK, &block, NULL);
    sleep(3);
    printf("Parent reaping children\n");
    int rc;
    while ((rc = waitpid(-1, NULL, 0)) > 0)
        printf("Parent reaped child %d\n", rc);

    printf("Parent exiting\n");
    return 0;
}