Langue: en

Version: 2009-02-20 (fedora - 01/12/10)

Section: 2 (Appels système)


timer_create - create a POSIX per-process timer


 #include <signal.h>
 #include <time.h>
 int timer_create(clockid_t clockid, struct sigevent *evp,
                  timer_t *timerid);

Link with -lrt.

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

timer_create(): _POSIX_C_SOURCE >= 199309L


timer_create() creates a new per-process interval timer. The ID of the new timer is returned in the buffer pointed to by timerid, which must be a non-NULL pointer. This ID is unique within the process, until the timer is deleted. The new timer is initially disarmed.

The clockid argument specifies the clock that the new timer uses to measure time. It can be specified as one of the following values:

A settable system-wide real-time clock.
A nonsettable monotonically increasing clock that measures time from some unspecified point in the past that does not change after system startup.
CLOCK_PROCESS_CPUTIME_ID (since Linux 2.6.12)
A clock that measures (user and system) CPU time consumed by (all of the threads in) the calling process.
CLOCK_THREAD_CPUTIME_ID (since Linux 2.6.12)
A clock that measures (user and system) CPU time consumed by the calling thread.

As well as the above values, clockid can be specified as the clockid returned by a call to clock_getcpuclockid(3) or pthread_getcpuclockid(3).

The evp argument points to a sigevent structure that specifies how the caller should be notified when the timer expires. This structure is defined something like the following:

 union sigval {
     int   sival_int;
     void *sival_ptr;
 struct sigevent {
     int          sigev_notify;    /* Notification method */
     int          sigev_signo;     /* Timer expiration signal */
     union sigval sigev_value;     /* Value accompanying signal or
                                      passed to thread function */
     void       (*sigev_notify_function) (union sigval);
                    /* Function used for thread
                       notifications (SIGEV_THREAD) */
     void        *sigev_notify_attributes;
                    /* Attributes for notification thread
                       (SIGEV_THREAD) */
     pid_t        sigev_notify_thread_id;
                    /* ID of thread to signal (SIGEV_THREAD_ID) */

Some of these fields may be defined as part of a union: a program should only employ those fields relevant to the value specified in sigev_notify. This field can have the following values:

Don't asynchronously notify when the timer expires. Progress of the timer can be monitored using timer_gettime(2).
Upon timer expiration, generate the signal sigev_signo for the process. If sigev_signo is a real-time signal, then it will be accompanied by the data specified in sigev_value (like the signal-accompanying data for sigqueue(2)). At any point in time, at most one signal is queued to the process for a given timer; see timer_getoverrun(2) for more details.
Upon timer expiration, invoke sigev_notify_function as if it were the start function of a new thread. (Among the implementation possibilities here are that each timer notification could result in the creation of a new thread, or that a single thread is created to receive all notifications.) The function is invoked with sigev_value as its sole argument. If sigev_notify_attributes is not NULL, it should point to a pthread_attr_t structure that defines attributes for the new thread (see pthread_attr_init(3)).
SIGEV_THREAD_ID (Linux-specific)
As for SIGEV_SIGNAL, but the signal is targeted at the thread whose ID is given in sigev_notify_thread_id, which must be a thread in the same process as the caller. The sigev_notify_thread_id field specifies a kernel thread ID, that is, the value returned by clone(2) or gettid(2). This flag is only intended for use by threading libraries.

Specifying evp as NULL is equivalent to specifying a pointer to a sigevent structure in which sigev_notify is SIGEV_SIGNAL, sigev_signo is SIGALRM, and sigev_value.sival_int is the timer ID.


On success, timer_create() returns 0, and the ID of the new timer is placed in *timerid. On failure, -1 is returned, and errno is set to indicate the error.


Temporary error during kernel allocation of timer structures.
Clock ID, sigev_notify, sigev_signo, or sigev_notify_thread_id is invalid.
Could not allocate memory.


This system call is available since Linux 2.6.




A program may create multiple interval timers using timer_create().

Timers are not inherited by the child of a fork(2), and are disarmed and deleted during an execve(2).

The kernel preallocates a "queued real-time signal" for each timer created using timer_create(). Consequently, the number of timers is limited by the RLIMIT_SIGPENDING resource limit (see setrlimit(2)).

The timers created by timer_create() are commonly known as "POSIX (interval) timers". The POSIX timers API consists of the following interfaces:

timer_create(): Create a timer.
timer_settime(2): Arm (start) or disarm (stop) a timer.
timer_gettime(2): Fetch the time remaining until the next expiration of a timer, along with the interval setting of the timer.
timer_getoverrun(2): Return the overrun count for the last timer expiration.
timer_delete(2): Disarm and delete a timer.

Part of the implementation of the POSIX timers API is provided by glibc. In particular:

The functionality for SIGEV_THREAD is implemented within glibc, rather than the kernel.
The timer IDs presented at user level are maintained by glibc, which maps these IDs to the timer IDs employed by the kernel.

The POSIX timers system calls first appeared in Linux 2.6. Prior to this, glibc provided an incomplete userspace implementation (CLOCK_REALTIME timers only) using POSIX threads, and current glibc falls back to this implementation on systems running pre-2.6 Linux kernels.


The program below takes two arguments: a sleep period in seconds, and a timer frequency in nanoseconds. The program establishes a handler for the signal it uses for the timer, blocks that signal, creates and arms a timer that expires with the given frequency, sleeps for the specified number of seconds, and then unblocks the timer signal. Assuming that the timer expired at least once while the program slept, the signal handler will be invoked, and the handler displays some information about the timer notification. The program terminates after one invocation of the signal handler.

In the following example run, the program sleeps for 1 second, after creating a timer that has a frequency of 100 nanoseconds. By the time the signal is unblocked and delivered, there have been around ten million overruns.

 $ ./a.out 1 10
 Establishing handler for signal 34
 Blocking signal 34
 timer ID is 0x804c008
 Sleeping for 1 seconds
 Unblocking signal 34
 Caught signal 34
     sival_ptr = 0xbfb174f4;     *sival_ptr = 0x804c008
     overrun count = 10004886

Program Source

 #include <stdlib.h>
 #include <unistd.h>
 #include <stdio.h>
 #include <signal.h>
 #include <time.h>
 #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                         } while (0)
 static void
 print_siginfo(siginfo_t *si)
     timer_t *tidp;
     int or;
     tidp = si->si_value.sival_ptr;
     printf("    sival_ptr = %p; ", si->si_value.sival_ptr);
     printf("    *sival_ptr = 0x%lx\n", (long) *tidp);
     or = timer_getoverrun(*tidp);
     if (or == -1)
         printf("    overrun count = %d\n", or);
 static void
 handler(int sig, siginfo_t *si, void *uc)
     /* Note: calling printf() from a signal handler is not
        strictly correct, since printf() is not async-signal-safe;
        see signal(7) */
     printf("Caught signal %d\n", sig);
     signal(sig, SIG_IGN);
 main(int argc, char *argv[])
     timer_t timerid;
     struct sigevent sev;
     struct itimerspec its;
     long long freq_nanosecs;
     sigset_t mask;
     struct sigaction sa;
     if (argc != 3) {
         fprintf(stderr, "Usage: %s <sleep-secs> <freq-nanosecs>\n",
     /* Establish handler for timer signal */
     printf("Establishing handler for signal %d\n", SIG);
     sa.sa_flags = SA_SIGINFO;
     sa.sa_sigaction = handler;
     if (sigaction(SIG, &sa, NULL) == -1)
     /* Block timer signal temporarily */
     printf("Blocking signal %d\n", SIG);
     sigaddset(&mask, SIG);
     if (sigprocmask(SIG_SETMASK, &mask, NULL) == -1)
     /* Create the timer */
     sev.sigev_notify = SIGEV_SIGNAL;
     sev.sigev_signo = SIG;
     sev.sigev_value.sival_ptr = &timerid;
     if (timer_create(CLOCKID, &sev, &timerid) == -1)
     printf("timer ID is 0x%lx\n", (long) timerid);
     /* Start the timer */
     freq_nanosecs = atoll(argv[2]);
     its.it_value.tv_sec = freq_nanosecs / 1000000000;
     its.it_value.tv_nsec = freq_nanosecs % 1000000000;
     its.it_interval.tv_sec = its.it_value.tv_sec;
     its.it_interval.tv_nsec = its.it_value.tv_nsec;
     if (timer_settime(timerid, 0, &its, NULL) == -1)
     /* Sleep for a while; meanwhile, the timer may expire
        multiple times */
     printf("Sleeping for %d seconds\n", atoi(argv[1]));
     /* Unlock the timer signal, so that timer notification
        can be delivered */
     printf("Unblocking signal %d\n", SIG);
     if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)


clock_gettime(2), setitimer(2), timer_delete(2), timer_settime(2), timer_getoverrun(2), timerfd_create(2), clock_getcpuclockid(3), pthread_getcpuclockid(3), pthreads(7), signal(7), time(7)


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