signals/sig_receiver.c

+/* sig_receiver.c
+
+   Usage: sig_receiver [block-time]
+
+   Catch and report statistics on signals sent by sig_sender.c.
+
+   Note that although we use signal() to establish the signal handler in this
+   program, the use of sigaction() is always preferable for this task.
+*/
 #define _GNU_SOURCE
 #include <signal.h>
 #include "signal_functions.h"           /* Declaration of printSigset() */
 #include "tlpi_hdr.h"
 
 static int sigCnt[NSIG];                /* Counts deliveries of each signal */
 static volatile sig_atomic_t gotSigint = 0;
                                         /* Set nonzero if SIGINT is delivered */
 
 static void
 handler(int sig)
 {
     if (sig == SIGINT)
         gotSigint = 1;
     else
         sigCnt[sig]++;
 }
 
 int
 main(int argc, char *argv[])
 {
     int n, numSecs;
     sigset_t pendingMask, blockingMask, emptyMask;
 
     printf("%s: PID is %ld\n", argv[0], (long) getpid());
 
+    /* Here we use the simpler signal() API to establish a signal handler,
+       but for the reasons described in Section 22.7 of TLPI, sigaction()
+       is the (strongly) preferred API for this task. */
+
     for (n = 1; n < NSIG; n++)          /* Same handler for all signals */
         (void) signal(n, handler);      /* Ignore errors */
 
     /* If a sleep time was specified, temporarily block all signals,
        sleep (while another process sends us signals), and then
        display the mask of pending signals and unblock all signals */
 
     if (argc > 1) {
         numSecs = getInt(argv[1], GN_GT_0, NULL);
 
         sigfillset(&blockingMask);
         if (sigprocmask(SIG_SETMASK, &blockingMask, NULL) == -1)
             errExit("sigprocmask");
 
         printf("%s: sleeping for %d seconds\n", argv[0], numSecs);
         sleep(numSecs);
 
         if (sigpending(&pendingMask) == -1)
             errExit("sigpending");
 
         printf("%s: pending signals are: \n", argv[0]);
         printSigset(stdout, "\t\t", &pendingMask);
 
         sigemptyset(&emptyMask);        /* Unblock all signals */
         if (sigprocmask(SIG_SETMASK, &emptyMask, NULL) == -1)
             errExit("sigprocmask");
     }
 
     while (!gotSigint)                  /* Loop until SIGINT caught */
         continue;
 
     for (n = 1; n < NSIG; n++)          /* Display number of signals received */
         if (sigCnt[n] != 0)
             printf("%s: signal %d caught %d time%s\n", argv[0], n,
                     sigCnt[n], (sigCnt[n] == 1) ? "" : "s");
 
     exit(EXIT_SUCCESS);
 }