timers/demo_timerfd.c

This is timers/demo_timerfd.c (Listing 23-8, page 510), 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
+/* demo_timerfd.c
+
+   Demonstrate the use of the timerfd API, which creates timers whose
+   expirations can be read via a file descriptor.
+
+   This program is Linux-specific. The timerfd API is supported since kernel
+   2.6.25. Library support is provided since glibc 2.8.
+*/
 #include <sys/timerfd.h>
 #include <time.h>
 #include <stdint.h>                     /* Definition of uint64_t */
 #include "itimerspec_from_str.h"        /* Declares itimerspecFromStr() */
 #include "tlpi_hdr.h"
 
 int
 main(int argc, char *argv[])
 {
     struct itimerspec ts;
     struct timespec start, now;
     int maxExp, fd, secs, nanosecs;
     uint64_t numExp, totalExp;
     ssize_t s;
 
     if (argc < 2 || strcmp(argv[1], "--help") == 0)
         usageErr("%s secs[/nsecs][:int-secs[/int-nsecs]] [max-exp]\n", argv[0]);
 
     itimerspecFromStr(argv[1], &ts);
     maxExp = (argc > 2) ? getInt(argv[2], GN_GT_0, "max-exp") : 1;
 
     fd = timerfd_create(CLOCK_REALTIME, 0);
     if (fd == -1)
         errExit("timerfd_create");
 
     if (timerfd_settime(fd, 0, &ts, NULL) == -1)
         errExit("timerfd_settime");
 
     if (clock_gettime(CLOCK_MONOTONIC, &start) == -1)
         errExit("clock_gettime");
 
     for (totalExp = 0; totalExp < maxExp;) {
 
         /* Read number of expirations on the timer, and then display
            time elapsed since timer was started, followed by number
            of expirations read and total expirations so far. */
 
         s = read(fd, &numExp, sizeof(uint64_t));
         if (s != sizeof(uint64_t))
             errExit("read");
 
         totalExp += numExp;
 
         if (clock_gettime(CLOCK_MONOTONIC, &now) == -1)
             errExit("clock_gettime");
 
         secs = now.tv_sec - start.tv_sec;
         nanosecs = now.tv_nsec - start.tv_nsec;
         if (nanosecs < 0) {
             secs--;
             nanosecs += 1000000000;
         }
 
         printf("%d.%03d: expirations read: %llu; total=%llu\n",
                 secs, (nanosecs + 500000) / 1000000,
                 (unsigned long long) numExp, (unsigned long long) totalExp);
     }
 
     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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