pgsjc/job_mon.c

+/* job_mon.c
+
+   This program is useful for:
+
+      - demonstrating the order in which the shell creates the processes in a
+        pipeline and how it assigns a process group to these processes, and
+
+      - monitoring some of the job control signals sent to a process (group).
+
+   The program displays its PID, parent PID and process group.
+
+   Try running a pipeline consisting of a series of these commands:
+
+        job_mon | job_mon | job_mon
+
+   This will demonstrate the assignment of process groups and PIDs to
+   each process in the pipeline.
+
+   Try running this pipeline under different shells and also in the background
+   (pipeline &) to see the effect this has.
+
+   You can also try typing control-C (^C) to demonstrate that this is
+   interpreted by the terminal driver as meaning "send a signal to all the jobs
+   in the foreground process group".
+*/
 #define _GNU_SOURCE     /* Get declaration of strsignal() from <string.h> */
 #include <string.h>
 #include <signal.h>
 #include <fcntl.h>
 #include "tlpi_hdr.h"
 
 static int cmdNum;      /* Our position in pipeline */
 
 static void             /* Handler for various signals */
 handler(int sig)
 {
     /* UNSAFE: This handler uses non-async-signal-safe functions
        (fprintf(), strsignal(); see Section 21.1.2) */
 
     if (getpid() == getpgrp())          /* If process group leader */
         fprintf(stderr, "Terminal FG process group: %ld\n",
                 (long) tcgetpgrp(STDERR_FILENO));
     fprintf(stderr, "Process %ld (%d) received signal %d (%s)\n",
                 (long) getpid(), cmdNum, sig, strsignal(sig));
 
     /* If we catch SIGTSTP, it won't actually stop us. Therefore we
        raise SIGSTOP so we actually get stopped. */
 
     if (sig == SIGTSTP)
         raise(SIGSTOP);
 }
 
 int
 main(int argc, char *argv[])
 {
     struct sigaction sa;
 
     sigemptyset(&sa.sa_mask);
     sa.sa_flags = SA_RESTART;
     sa.sa_handler = handler;
     if (sigaction(SIGINT, &sa, NULL) == -1)
         errExit("sigaction");
     if (sigaction(SIGTSTP, &sa, NULL) == -1)
         errExit("sigaction");
     if (sigaction(SIGCONT, &sa, NULL) == -1)
         errExit("sigaction");
 
     /* If stdin is a terminal, this is the first process in pipeline:
        print a heading and initialize message to be sent down pipe */
 
     if (isatty(STDIN_FILENO)) {
         fprintf(stderr, "Terminal FG process group: %ld\n",
                 (long) tcgetpgrp(STDIN_FILENO));
         fprintf(stderr, "Command   PID  PPID  PGRP   SID\n");
         cmdNum = 0;
 
     } else {            /* Not first in pipeline, so read message from pipe */
         if (read(STDIN_FILENO, &cmdNum, sizeof(cmdNum)) <= 0)
             fatal("read got EOF or error");
     }
 
     cmdNum++;
     fprintf(stderr, "%4d    %5ld %5ld %5ld %5ld\n", cmdNum,
             (long) getpid(), (long) getppid(),
             (long) getpgrp(), (long) getsid(0));
 
     /* If not the last process, pass a message to the next process */
 
     if (!isatty(STDOUT_FILENO))   /* If not tty, then should be pipe */
         if (write(STDOUT_FILENO, &cmdNum, sizeof(cmdNum)) == -1)
             errMsg("write");
 
     for (;;)            /* Wait for signals */
         pause();
 }