This is pgsjc/orphaned_pgrp_SIGHUP.c (Listing 34-7, page 728), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* 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) */
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 */

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

    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:

        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());
            } else {                    /* Wait for signal */
                alarm(60);              /* So we die if not SIGHUPed */
                printf("PID=%ld pausing\n", (long) getpid());


        default:        /* Parent carries on round loop */

    /* 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 */


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