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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 -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 signal handler interrupts the call.

       sem_trywait()  is  the  same as sem_wait(), except that if the decrement cannot be immedi-
       ately 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 per-
       formed.	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 struc-
       ture 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 ETIMED-
       OUT).

       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 cur-
	      rently 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 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_timed-
       wait().	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 pro-
       gram:

	   $ ./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
       clock_gettime(2), sem_getvalue(3), sem_post(3), sem_overview(7), time(7)

COLOPHON
       This page is part of release 3.55 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					    2012-05-13				      SEM_WAIT(3)
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