SEM_TIMEDWAIT(3P)					     POSIX Programmer's Manual						 SEM_TIMEDWAIT(3P)

PROLOG

       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

NAME

       sem_timedwait -- lock a semaphore

SYNOPSIS

       #include <semaphore.h>
       #include <time.h>

       int sem_timedwait(sem_t *restrict sem,
	   const struct timespec *restrict abstime);

DESCRIPTION

       The sem_timedwait() function shall lock the semaphore referenced by sem as in the sem_wait() function. However, if the semaphore cannot	be
       locked without waiting for another process or thread to unlock the semaphore by performing a sem_post() function, this wait shall be termi-
       nated when the specified timeout expires.

       The timeout shall expire when the absolute time specified by abstime passes, as measured by the clock on which timeouts are based (that is,
       when  the  value of that clock equals or exceeds abstime), or if the absolute time specified by abstime has already been passed at the time
       of the call.

       The timeout shall be based on the CLOCK_REALTIME clock.	The resolution of the timeout shall be the resolution of the clock on which it	is
       based. The timespec data type is defined as a structure in the <time.h> header.

       Under  no  circumstance shall the function fail with a timeout if the semaphore can be locked immediately. The validity of the abstime need
       not be checked if the semaphore can be locked immediately.

RETURN VALUE

       The sem_timedwait() function shall return zero if the calling process successfully performed the semaphore lock operation on the  semaphore
       designated  by  sem.  If the call was unsuccessful, the state of the semaphore shall be unchanged, and the function shall return a value of
       -1 and set errno to indicate the error.

ERRORS

       The sem_timedwait() function shall fail if:

       EINVAL The process or thread would have blocked, and the abstime parameter specified a nanoseconds field value less than  zero  or  greater
	      than or equal to 1000 million.

       ETIMEDOUT
	      The semaphore could not be locked before the specified timeout expired.

       The sem_timedwait() function may fail if:

       EDEADLK
	      A deadlock condition was detected.

       EINTR  A signal interrupted this function.

       EINVAL The sem argument does not refer to a valid semaphore.

       The following sections are informative.

EXAMPLES

       The  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  sema-
       phore  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().

	   #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;

	   static void
	   handler(int sig)
	   {
	       int sav_errno = errno;
	       static const char info_msg[] = "sem_post() from handler
";
	       write(STDOUT_FILENO, info_msg, sizeof info_msg - 1);
	       if (sem_post(&sem) == -1) {
		   static const char err_msg[] = "sem_post() failed
";
		   write(STDERR_FILENO, err_msg, sizeof err_msg - 1);
		   _exit(EXIT_FAILURE);
	       }
	       errno = sav_errno;
	   }

	   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) {
		   perror("sem_init");
		   exit(EXIT_FAILURE);
	       }

	       /* 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) {
		   perror("sigaction");
		   exit(EXIT_FAILURE);
	       }

	       alarm(atoi(argv[1]));

	       /* Calculate relative interval as current time plus
		  number of seconds given argv[2] */

	       if (clock_gettime(CLOCK_REALTIME, &ts) == -1) {
		   perror("clock_gettime");
		   exit(EXIT_FAILURE);
	       }
	       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);
	   }

APPLICATION USAGE

       Applications using these functions may be subject to priority inversion, as discussed in the Base Definitions volume of POSIX.1-2008,  Sec-
       tion 3.287, Priority Inversion.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       sem_post(), sem_trywait(), semctl(), semget(), semop(), time()

       The Base Definitions volume of POSIX.1-2008, Section 3.287, Priority Inversion, <semaphore.h>, <time.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C)  2013  by	the  Institute	of
       Electrical and Electronics Engineers, Inc and The Open Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Stan-
       dard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE
/The Open Group						       2013							 SEM_TIMEDWAIT(3P)