# sem_timedwait

## PROLOG



           This manual page is part of the POSIX Programmer's Manual.  The
           Linux implementation of this interface may differ (consult the
           corresponding 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 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.
    


* * *