sem_timedwait(3p) — Linux manual page


SEM_TIMEDWAIT(3P)       POSIX Programmer's Manual      SEM_TIMEDWAIT(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       sem_timedwait — lock a semaphore

SYNOPSIS         top

       #include <semaphore.h>
       #include <time.h>

       int sem_timedwait(sem_t *restrict sem,
           const struct timespec *restrict abstime);

DESCRIPTION         top

       The sem_timedwait() function shall lock the semaphore referenced
       by sem as in the sem_wait() function. However, if the semaphore
       cannot be locked without waiting for another process or thread to
       unlock the semaphore by performing a sem_post() function, this
       wait shall be terminated when the specified timeout expires.

       The timeout shall expire when the absolute time specified by
       abstime passes, as measured by the clock on which timeouts are
       based (that is, when the value of that clock equals or exceeds
       abstime), or if the absolute time specified by abstime has
       already been passed at the time of the call.

       The timeout shall be based on the CLOCK_REALTIME clock.  The
       resolution of the timeout shall be the resolution of the clock on
       which it is based. The timespec data type is defined as a
       structure in the <time.h> header.

       Under no circumstance shall the function fail with a timeout if
       the semaphore can be locked immediately. The validity of the
       abstime need not be checked if the semaphore can be locked

RETURN VALUE         top

       The sem_timedwait() function shall return zero if the calling
       process successfully performed the semaphore lock operation on
       the semaphore designated by sem.  If the call was unsuccessful,
       the state of the semaphore shall be unchanged, and the function
       shall return a value of -1 and set errno to indicate the error.

ERRORS         top

       The sem_timedwait() function shall fail if:

       EINVAL The process or thread would have blocked, and the abstime
              parameter specified a nanoseconds field value less than
              zero or greater than or equal to 1000 million.

              The semaphore could not be locked before the specified
              timeout expired.

       The sem_timedwait() function may fail if:

              A deadlock condition was detected.

       EINTR  A signal interrupted this function.

       EINVAL The sem argument does not refer to a valid semaphore.

       The following sections are informative.

EXAMPLES         top

       The program shown below operates on an unnamed semaphore. The
       program expects two command-line arguments. The first argument
       specifies a seconds value that is used to set an alarm timer to
       generate a SIGALRM signal. This handler performs a sem_post(3) to
       increment the semaphore that is being waited on in main() using
       sem_timedwait().  The second command-line argument specifies the
       length of the timeout, in seconds, for sem_timedwait().

           #include <unistd.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <semaphore.h>
           #include <time.h>
           #include <assert.h>
           #include <errno.h>
           #include <signal.h>

           sem_t sem;

           static void
           handler(int sig)
               int sav_errno = errno;
               static const char info_msg[] = "sem_post() from handler\n";
               write(STDOUT_FILENO, info_msg, sizeof info_msg - 1);
               if (sem_post(&sem) == -1) {
                   static const char err_msg[] = "sem_post() failed\n";
                   write(STDERR_FILENO, err_msg, sizeof err_msg - 1);
               errno = sav_errno;

           main(int argc, char *argv[])
               struct sigaction sa;
               struct timespec ts;
               int s;

               if (argc != 3) {
                   fprintf(stderr, "Usage: %s <alarm-secs> <wait-secs>\n",

               if (sem_init(&sem, 0, 0) == -1) {

               /* Establish SIGALRM handler; set alarm timer using argv[1] */

               sa.sa_handler = handler;
               sa.sa_flags = 0;
               if (sigaction(SIGALRM, &sa, NULL) == -1) {


               /* Calculate relative interval as current time plus
                  number of seconds given argv[2] */

               if (clock_gettime(CLOCK_REALTIME, &ts) == -1) {
               ts.tv_sec += atoi(argv[2]);

               printf("main() about to call sem_timedwait()\n");
               while ((s = sem_timedwait(&sem, &ts)) == -1 && errno == EINTR)
                   continue;       /* Restart if interrupted by handler */

               /* Check what happened */

               if (s == -1) {
                   if (errno == ETIMEDOUT)
                       printf("sem_timedwait() timed out\n");
               } else
                   printf("sem_timedwait() succeeded\n");

               exit((s == 0) ? EXIT_SUCCESS : EXIT_FAILURE);


       Applications using these functions may be subject to priority
       inversion, as discussed in the Base Definitions volume of
       POSIX.1‐2017, Section 3.291, Priority Inversion.

RATIONALE         top




SEE ALSO         top

       sem_post(3p), sem_trywait(3p), semctl(3p), semget(3p), semop(3p),

       The Base Definitions volume of POSIX.1‐2017, Section 3.291,
       Priority Inversion, semaphore.h(0p), time.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see .

IEEE/The Open Group               2017                 SEM_TIMEDWAIT(3P)

Pages that refer to this page: semaphore.h(0p)time.h(0p)clock_getres(3p)sem_getvalue(3p)sem_init(3p)sem_open(3p)sem_post(3p)sem_trywait(3p)