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       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 *restrict sem,
                         const struct timespec *restrict abs_timeout);

       Link with -pthread.

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

           _POSIX_C_SOURCE >= 200112L


       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

       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

       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.


       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.


       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.

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


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

       │InterfaceAttributeValue   │
       │sem_wait(), sem_trywait(), sem_timedwait()                     │ Thread safety │ MT-Safe │


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


       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);

       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);


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


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