This is timers/ptmr_sigev_thread.c (Listing 23-7, page 506), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* ptmr_sigev_thread.c

   This program demonstrates the use of threads 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 creates and arms one timer for each command-line argument.
   The timer notification method is specified as SIGEV_THREAD, causing the
   timer notifications to be delivered via a thread that invokes threadFunc()
   as its start function. The threadFunc() function displays information
   about the timer expiration, increments a global counter of timer expirations,
   and signals a condition variable to indicate that the counter has changed.
   In the main thread, a loop waits on the condition variable, and each time
   the condition variable is signaled, the main thread prints the value of the
   global variable that counts timer expirations.

   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.
#include <signal.h>
#include <time.h>
#include <pthread.h>
#include "curr_time.h"              /* Declares currTime() */
#include "tlpi_hdr.h"
#include "itimerspec_from_str.h"    /* Declares itimerspecFromStr() */

static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

static int expireCnt = 0;           /* Number of expirations of all timers */
static void                         /* Thread notification function */
threadFunc(union sigval sv)
    timer_t *tidptr;
    int s;

    tidptr = sv.sival_ptr;

    printf("[%s] Thread notify\n", currTime("%T"));
    printf("    timer ID=%ld\n", (long) *tidptr);
    printf("    timer_getoverrun()=%d\n", timer_getoverrun(*tidptr));

    /* Increment counter variable shared with main thread and signal
       condition variable to notify main thread of the change. */

    s = pthread_mutex_lock(&mtx);
    if (s != 0)
        errExitEN(s, "pthread_mutex_lock");

    expireCnt += 1 + timer_getoverrun(*tidptr);

    s = pthread_mutex_unlock(&mtx);
    if (s != 0)
        errExitEN(s, "pthread_mutex_unlock");

    s = pthread_cond_signal(&cond);
    if (s != 0)
        errExitEN(s, "pthread_cond_signal");
main(int argc, char *argv[])
    struct sigevent sev;
    struct itimerspec ts;
    timer_t *tidlist;
    int s, 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)

    sev.sigev_notify = SIGEV_THREAD;            /* Notify via thread */
    sev.sigev_notify_function = threadFunc;     /* Thread start function */
    sev.sigev_notify_attributes = NULL;
            /* Could be pointer to pthread_attr_t structure */

    /* Create and start one timer for each command-line argument */

    for (j = 0; j < argc - 1; j++) {
        itimerspecFromStr(argv[j + 1], &ts);

        sev.sigev_value.sival_ptr = &tidlist[j];
                /* Passed as argument to threadFunc() */

        if (timer_create(CLOCK_REALTIME, &sev, &tidlist[j]) == -1)
        printf("Timer ID: %ld (%s)\n", (long) tidlist[j], argv[j + 1]);

        if (timer_settime(tidlist[j], 0, &ts, NULL) == -1)

    /* The main thread waits on a condition variable that is signaled
       on each invocation of the thread notification function. We
       print a message so that the user can see that this occurred. */

    s = pthread_mutex_lock(&mtx);
    if (s != 0)
        errExitEN(s, "pthread_mutex_lock");

    for (;;) {
        s = pthread_cond_wait(&cond, &mtx);
        if (s != 0)
            errExitEN(s, "pthread_cond_wait");
        printf("main(): expireCnt = %d\n", expireCnt);


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