NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

SEM_WAIT(3)               Linux Programmer's Manual              SEM_WAIT(3)

NAME         top

       sem_wait, sem_timedwait, sem_trywait - lock a semaphore

SYNOPSIS         top

       #include <semaphore.h>

       int sem_wait(sem_t *sem);

       int sem_trywait(sem_t *sem);

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

       Link with -pthread.

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

       sem_timedwait(): _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 structure that specifies an absolute timeout in
       seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000
       (UTC).  This structure is defined as follows:

           struct timespec {
               time_t tv_sec;      /* Seconds */
               long   tv_nsec;     /* Nanoseconds [0 .. 999999999] */
           };

       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         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

       EINTR  The call was interrupted by a signal handler; see signal(7).

       EINVAL sem is not a valid semaphore.

       The following additional error can occur for sem_trywait():

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

       The following additional errors can occur for sem_timedwait():

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

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

ATTRIBUTES         top

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

       ┌───────────────────────────┬───────────────┬─────────┐
       │Interface                  Attribute     Value   │
       ├───────────────────────────┼───────────────┼─────────┤
       │sem_wait(), sem_trywait(), │ Thread safety │ MT-Safe │
       │sem_timedwait()            │               │         │
       └───────────────────────────┴───────────────┴─────────┘

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.

NOTES         top

       A signal handler always interrupts a blocked call to one of these
       functions, regardless of the use of the sigaction(2) SA_RESTART flag.

EXAMPLE         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 <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;

       #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("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
                   perror("sem_timedwait");
           } 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), sem_overview(7),
       time(7)

COLOPHON         top

       This page is part of release 4.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                            2016-03-15                      SEM_WAIT(3)