sem_wait(3) — Linux manual page


sem_wait(3)             Library Functions Manual             sem_wait(3)

NAME         top

       sem_wait, sem_timedwait, sem_trywait - lock a semaphore

LIBRARY         top

       POSIX threads library (libpthread, -lpthread)

SYNOPSIS         top

       #include <semaphore.h>

       int sem_wait(sem_t *sem);
       int sem_trywait(sem_t *sem);
       int sem_timedwait(sem_t *restrict sem,
                         const struct timespec *restrict abs_timeout);

   Feature Test Macro Requirements for glibc (see

           _POSIX_C_SOURCE >= 200112L

DESCRIPTION         top

       sem_wait() decrements (locks) the semaphore pointed to by sem.
       If the semaphore's value is greater than zero, then the decrement
       proceeds, and the function returns, immediately.  If the
       semaphore currently has the value zero, then the call blocks
       until either it becomes possible to perform the decrement (i.e.,
       the semaphore value rises above zero), or a signal handler
       interrupts the call.

       sem_trywait() is the same as sem_wait(), except that if the
       decrement cannot be immediately performed, then call returns an
       error (errno set to EAGAIN) instead of blocking.

       sem_timedwait() is the same as sem_wait(), except that
       abs_timeout specifies a limit on the amount of time that the call
       should block if the decrement cannot be immediately performed.
       The abs_timeout argument points to a timespec(3) structure that
       specifies an absolute timeout in seconds and nanoseconds since
       the Epoch, 1970-01-01 00:00:00 +0000 (UTC).

       If the timeout has already expired by the time of the call, and
       the semaphore could not be locked immediately, then
       sem_timedwait() fails with a timeout error (errno set to

       If the operation can be performed immediately, then
       sem_timedwait() never fails with a timeout error, regardless of
       the value of abs_timeout.  Furthermore, the validity of
       abs_timeout is not checked in this case.

RETURN VALUE         top

       All of these functions return 0 on success; on error, the value
       of the semaphore is left unchanged, -1 is returned, and errno is
       set to indicate the error.

ERRORS         top

       EAGAIN (sem_trywait()) The operation could not be performed
              without blocking (i.e., the semaphore currently has the
              value zero).

       EINTR  The call was interrupted by a signal handler; see

       EINVAL sem is not a valid semaphore.

       EINVAL (sem_timedwait()) The value of abs_timeout.tv_nsecs is
              less than 0, or greater than or equal to 1000 million.

              (sem_timedwait()) The call timed out before the semaphore
              could be locked.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ sem_wait(), sem_trywait(),          │ Thread safety │ MT-Safe │
       │ sem_timedwait()                     │               │         │

STANDARDS         top


HISTORY         top


EXAMPLES         top

       The (somewhat trivial) 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().  The following shows what happens on two
       different runs of the program:

           $ ./a.out 2 3
           About to call sem_timedwait()
           sem_post() from handler
           sem_timedwait() succeeded
           $ ./a.out 2 1
           About to call sem_timedwait()
           sem_timedwait() timed out

   Program source

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

       #include <assert.h>

       sem_t sem;

       #define handle_error(msg) \
           do { perror(msg); exit(EXIT_FAILURE); } while (0)

       static void
       handler(int sig)
           write(STDOUT_FILENO, "sem_post() from handler\n", 24);
           if (sem_post(&sem) == -1) {
               write(STDERR_FILENO, "sem_post() failed\n", 18);

       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("%s() about to call sem_timedwait()\n", __func__);
           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);

SEE ALSO         top

       clock_gettime(2), sem_getvalue(3), sem_post(3), timespec(3),
       sem_overview(7), time(7)

Linux man-pages (unreleased)     (date)                      sem_wait(3)

Pages that refer to this page: prctl(2)sem_close(3)sem_destroy(3)sem_getvalue(3)sem_init(3)sem_open(3)sem_post(3)sem_unlink(3)sem_overview(7)signal(7)