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

       sem_wait, sem_timedwait, sem_trywait - lock a semaphore

       #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 -lrt or -pthread.

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

       sem_timedwait(): _POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600

       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  argu-
       ment  points  to a structure that specifies an absolute timeout in sec-
       onds and nanoseconds since the Epoch (00:00:00, 1 January 1970).   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_time-
       out.   Furthermore,  the validity of abs_timeout is not checked in this

       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 indi-
       cate the error.

       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.

              The call timed out before the semaphore could be locked.


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

       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_getvalue() from handler; value = 1
           sem_timedwait() succeeded
           $ ./a.out 2 1
           About to call sem_timedwait()
           sem_timedwait() timed out

       The source code of the program is as follows:

       #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);

       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);

       sem_getvalue(3), sem_post(3), sem_overview(7), time(7)

       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at

Linux                             2007-07-26                       SEM_WAIT(3)

Time taken: 0.00030 seconds

Created with the man page lookup class by Andrew Collington,