pgsjc/orphaned_pgrp_SIGHUP.c

+/* 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 */
 }