sem_wait(3)                Library Functions Manual                sem_wait(3)

NAME
       sem_wait, sem_timedwait, sem_trywait - lock a semaphore

LIBRARY
       POSIX threads library (libpthread, -lpthread)

SYNOPSIS
       #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 feature_test_macros(7)):

       sem_timedwait():
           _POSIX_C_SOURCE >= 200112L

DESCRIPTION
       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 ETIMEDOUT).

       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
       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
       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 signal(7).

       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.

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

ATTRIBUTES
       For an explanation of the terms used in this section, see
       attributes(7).

       +--------------------------------------------+---------------+---------+
       |Interface                                   | Attribute     | Value   |
       +--------------------------------------------+---------------+---------+
       |sem_wait (), sem_trywait (), sem_timedwait  | Thread safety | MT-Safe |
       |()                                          |               |         |
       +--------------------------------------------+---------------+---------+
STANDARDS
       POSIX.1-2008.

HISTORY
       POSIX.1-2001.

EXAMPLES
       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);
               _exit(EXIT_FAILURE);
           }
       }

       int
       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",
                       argv[0]);
               exit(EXIT_FAILURE);
           }

           if (sem_init(&sem, 0, 0) == -1)
               handle_error("sem_init");

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

           sa.sa_handler = handler;
           sigemptyset(&sa.sa_mask);
           sa.sa_flags = 0;
           if (sigaction(SIGALRM, &sa, NULL) == -1)
               handle_error("sigaction");

           alarm(atoi(argv[1]));

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

           if (clock_gettime(CLOCK_REALTIME, &ts) == -1)
               handle_error("clock_gettime");

           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
                   perror("sem_timedwait");
           } else
               printf("sem_timedwait() succeeded\n");

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

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

Linux man-pages 6.7               2023-10-31                       sem_wait(3)