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



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

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
       terminated 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 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().


           #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\n";
               write(STDOUT_FILENO, info_msg, sizeof info_msg - 1);
               if (sem_post(&sem) == -1) {
                   static const char err_msg[] = "sem_post() failed\n";
                   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>\n",
                       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()\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);
           }

APPLICATION USAGE
       Applications using these functions may be subject to priority
       inversion, as discussed in the Base Definitions volume of POSIX.1-2017,
       Section 3.291, Priority Inversion.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       sem__post(), sem__trywait(), semctl(), semget(), semop(), time()

       The Base Definitions volume of POSIX.1-2017, Section 3.291, Priority
       Inversion, <semaphore.h>, <time.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  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 Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/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                  2017                     SEM_TIMEDWAIT(3)
