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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 -pthread.
 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
     sem_timedwait(): _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 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, 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_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.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     allbox;  lbw26  lb  lb l l l.  Interface Attribute Value T{ sem_wait(),
     sem_trywait(), sem_timedwait() T}   Thread safety  MT-Safe


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


     The (somewhat trivial) program shown below operates on an unnamed sema-
     phore.   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_timed-
         wait() 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);
         } }
     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",
         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),


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Linux 2017-09-15 SEM_WAIT(3)

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