signals/sig_receiver.c

This is signals/sig_receiver.c (Listing 20-7, page 414), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* 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);
 }

Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.

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