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

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 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
       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 indi-
       cate 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.

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

       +---------------------------+---------------+---------+
       |Interface		   | Attribute	   | Value   |
       +---------------------------+---------------+---------+
       |sem_wait(), sem_trywait(), | Thread safety | MT-Safe |
       |sem_timedwait()		   |		   |	     |
       +---------------------------+---------------+---------+
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008.

EXAMPLE
       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_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 4.10 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-03-13			   SEM_WAIT(3)