/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2026.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU General Public License as published by the   *
* Free Software Foundation, either version 3 or (at your option) any      *
* later version. This program is distributed without any warranty.  See   *
* the file COPYING.gpl-v3 for details.                                    *
\*************************************************************************/

/* orphaned_pgrp_SIGHUP.c

   Usage: orphaned_pgrp_SIGHUP {s|p} ...

        (e.g.: orphaned_pgrp_SIGHUP s p p)

   Creates an orphaned process group containing one process for each
   command-line argument. If the command-line argument corresponding to this
   child is 's', then the child stops itself by raising SIGSTOP.  If the
   command-line argument is 'p' then the child does a pause().

   This program can be used to show that when a process group that contains
   stopped children becomes orphaned, then all members of the process group are
   sent a SIGHUP signal, to inform them that they have been disconnected from
   their session, followed by a SIGCONT signal, to ensure that they resume
   execution. Try running the following commands and observing the difference
   in output:

        orphaned_pgrp_SIGHUP s p
        orphaned_pgrp_SIGHUP p p
*/
#define _GNU_SOURCE     /* Get declaration of strsignal() from <string.h> */
#include <string.h>
#include <signal.h>
#include "tlpi_hdr.h"

static void             /* Signal handler */
handler(int sig)
{
    printf("PID=%ld: caught signal %d (%s)\n", (long) getpid(),
            sig, strsignal(sig));       /* UNSAFE (see Section 21.1.2) */
}

int
main(int argc, char *argv[])
{
    int j;
    struct sigaction sa;

    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s {s|p} ...\n", argv[0]);

    setbuf(stdout, NULL);               /* Make stdout unbuffered */

    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;
    sa.sa_handler = handler;
    if (sigaction(SIGHUP, &sa, NULL) == -1)
        errExit("sigaction");
    if (sigaction(SIGCONT, &sa, NULL) == -1)
        errExit("sigaction");

    printf("parent: PID=%ld, PPID=%ld, PGID=%ld, SID=%ld\n",
            (long) getpid(), (long) getppid(),
            (long) getpgrp(), (long) getsid(0));

    /* Create one child for each command-line argument */

    for (j = 1; j < argc; j++) {
        switch (fork()) {
        case -1:
            errExit("fork");

        case 0:         /* Child */
            printf("child:  PID=%ld, PPID=%ld, PGID=%ld, SID=%ld\n",
                    (long) getpid(), (long) getppid(),
                    (long) getpgrp(), (long) getsid(0));

            if (argv[j][0] == 's') {    /* Stop via signal */
                printf("PID=%ld stopping\n", (long) getpid());
                raise(SIGSTOP);
            } else {                    /* Wait for signal */
                alarm(60);              /* So we die if not SIGHUPed */
                printf("PID=%ld pausing\n", (long) getpid());
                pause();
            }

            _exit(EXIT_SUCCESS);

        default:        /* Parent carries on round loop */
            break;
        }
    }

    /* Parent falls through to here after creating all children */

    sleep(3);                           /* Give children a chance to start */
    printf("parent exiting\n");
    exit(EXIT_SUCCESS);                 /* And orphan them and their group */
}