LinuxGuruz
  • Last 5 Forum Topics
    Replies
    Views
    Last post


The Web Only This Site
  • BOOKMARK

  • ADD TO FAVORITES

  • REFERENCES


  • MARC

    Mailing list ARChives
    - Search by -
     Subjects
     Authors
     Bodies





    FOLDOC

    Computing Dictionary




  • Text Link Ads






  • LINUX man pages
  • Linux Man Page Viewer


    The following form allows you to view linux man pages.

    Command:

    sem_trywait

    
    
    

    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 || _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  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.
                  than or equal to 1000 million.
    
           ETIMEDOUT
                  The call timed out before the semaphore could be locked.
    
    
    

    ATTRIBUTES

       Multithreading (see pthreads(7))
           The  sem_wait(),  sem_trywait(),  and  sem_timedwait()  functions   are
           thread-safe.
    
    
    

    CONFORMING TO

           POSIX.1-2001.
    
    
    

    NOTES

           A signal handler always interrupts a blocked call to one of these func-
           tions, 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_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
               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)
    
    
  • MORE RESOURCE


  • Linux

    The Distributions





    Linux

    The Software





    Linux

    The News



  • MARKETING






  • Toll Free

webmaster@linuxguruz.com
Copyright © 1999 - 2016 by LinuxGuruz