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



NAME
       sem_wait, sem_timedwait, sem_trywait - lock a semaphore

SYNOPSIS
       #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

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 sig-
       nal 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 spec-
       ifies 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
       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
       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.

CONFORMING TO
       POSIX.1-2001.

NOTES
       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
       The (somewhat trivial) program shown below operates on an unnamed semaphore.  The program expects two command-line  argu-
       ments.   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

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

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2010-02-25                                                SEM_WAIT(3)