timers/ptmr_sigev_signal.c

This is timers/ptmr_sigev_signal.c (Listing 23-5, page 500), 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
+/* ptmr_sigev_signal.c
+
+   This program demonstrates the use of signals as the notification mechanism
+   for expirations of a POSIX timer. Each of the program's command-line
+   arguments specifies the initial value and interval for a POSIX timer. The
+   format of these arguments is defined by the function itimerspecFromStr().
+
+   The program establishes a handler for the timer notification signal, creates
+   and arms one timer for each command-line argument, and then pauses. Each
+   timer expiration causes the generation of a signal, and, when invoked, the
+   signal handler displays information about the timer expiration.
+
+   Kernel support for Linux timers is provided since Linux 2.6. On older
+   systems, an incomplete user-space implementation of POSIX timers
+   was provided in glibc.
+*/
 #define _POSIX_C_SOURCE 199309
 #include <signal.h>
 #include <time.h>
 #include "curr_time.h"                  /* Declares currTime() */
 #include "itimerspec_from_str.h"        /* Declares itimerspecFromStr() */
 #include "tlpi_hdr.h"
 
 #define TIMER_SIG SIGRTMAX              /* Our timer notification signal */
 
 static void
 handler(int sig, siginfo_t *si, void *uc)
 {
     timer_t *tidptr;
 
     tidptr = si->si_value.sival_ptr;
 
     /* UNSAFE: This handler uses non-async-signal-safe functions
        (printf(); see Section 21.1.2) */
 
     printf("[%s] Got signal %d\n", currTime("%T"), sig);
     printf("    *sival_ptr         = %ld\n", (long) *tidptr);
     printf("    timer_getoverrun() = %d\n", timer_getoverrun(*tidptr));
 }
 
 int
 main(int argc, char *argv[])
 {
     struct itimerspec ts;
     struct sigaction  sa;
     struct sigevent   sev;
     timer_t *tidlist;
     int j;
 
     if (argc < 2)
         usageErr("%s secs[/nsecs][:int-secs[/int-nsecs]]...\n", argv[0]);
 
     tidlist = calloc(argc - 1, sizeof(timer_t));
     if (tidlist == NULL)
         errExit("malloc");
 
     /* Establish handler for notification signal */
 
     sa.sa_flags = SA_SIGINFO;
     sa.sa_sigaction = handler;
     sigemptyset(&sa.sa_mask);
     if (sigaction(TIMER_SIG, &sa, NULL) == -1)
         errExit("sigaction");
 
     /* Create and start one timer for each command-line argument */
 
     sev.sigev_notify = SIGEV_SIGNAL;    /* Notify via signal */
     sev.sigev_signo = TIMER_SIG;        /* Notify using this signal */
 
     for (j = 0; j < argc - 1; j++) {
         itimerspecFromStr(argv[j + 1], &ts);
 
         sev.sigev_value.sival_ptr = &tidlist[j];
                 /* Allows handler to get ID of this timer */
 
         if (timer_create(CLOCK_REALTIME, &sev, &tidlist[j]) == -1)
             errExit("timer_create");
         printf("Timer ID: %ld (%s)\n", (long) tidlist[j], argv[j + 1]);
 
         if (timer_settime(tidlist[j], 0, &ts, NULL) == -1)
             errExit("timer_settime");
     }
 
     for (;;)                            /* Wait for incoming timer signals */
         pause();
 }

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