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 .