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  possi-
       ble 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 argument 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
       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 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_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
", 24);
	   if (sem_post(&sem) == -1) {
	       write(STDERR_FILENO, "sem_post() failed
", 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>
",
		       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()
");
	   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
");
	       else
		   perror("sem_timedwait");
	   } else
	       printf("sem_timedwait() succeeded
");

	   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)