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



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

NAME
       sem_trywait, sem_wait - lock a semaphore (REALTIME)

SYNOPSIS
       #include <semaphore.h>

       int sem_trywait(sem_t *sem);
       int sem_wait(sem_t *sem);


DESCRIPTION
       The sem_trywait() function shall lock the semaphore referenced by sem only if the semaphore is currently not locked; that
       is, if the semaphore value is currently positive. Otherwise, it shall not lock the semaphore.

       The sem_wait() function shall lock the semaphore referenced by sem by performing a semaphore lock operation on that sema-
       phore.  If  the  semaphore  value is currently zero, then the calling thread shall not return from the call to sem_wait()
       until it either locks the semaphore or the call is interrupted by a signal.

       Upon successful return, the state of the semaphore shall be locked and shall remain locked until the sem_post()  function
       is executed and returns successfully.

       The sem_wait() function is interruptible by the delivery of a signal.

RETURN VALUE
       The  sem_trywait() and sem_wait() functions 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_trywait() and sem_wait() functions shall fail if:

       EAGAIN The  semaphore  was  already  locked, so it cannot be immediately locked by the sem_trywait() operation ( sem_try-
              wait() only).

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


       The sem_trywait() and sem_wait() functions may fail if:

       EDEADLK
              A deadlock condition was detected.

       EINTR  A signal interrupted this function.


       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       Applications using these functions may be subject to priority inversion, as discussed in the Base Definitions  volume  of
       IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.

       The  sem_trywait() and sem_wait() functions are part of the Semaphores option and need not be provided on all implementa-
       tions.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       semctl(), semget(), semop(), sem_post(), sem_timedwait(), the Base Definitions  volume  of  IEEE Std 1003.1-2001,  <sema-
       phore.h>

COPYRIGHT
       Portions  of  this  text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
       Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (C) 2001-2003 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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                               SEM_TRYWAIT(3P)