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 immediately.

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

       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.1‐2008, Section 3.287, 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.1‐2008, Section 3.287, 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, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) 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                 2013                   SEM_TIMEDWAIT(3P)