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 semaphore. 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_trywait() 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 implementations.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       semctl() , semget() , semop() , sem_post() , sem_timedwait() , the Base Definitions volume
       of IEEE Std 1003.1-2001, <semaphore.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, Copyright (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.opengroup.org/unix/online.html .