sem_wait(2)							System Calls Manual						       sem_wait(2)

NAME

       sem_wait, sem_trywait - lock a POSIX semaphore

SYNOPSIS

DESCRIPTION

       is  used to lock a semaphore.  The calling thread will not return from its call to until one of the following events occur: it successfully
       obtains a lock on the semaphore; it is interrupted by a signal or an error condition occurs.

       is used to lock a semaphore, if it is available.  The value of the semaphore sem is checked at some unspecified time during the	call.	If
       the semaphore is available at the time its value is checked, the calling thread will atomically, with respect to the checking of the value,
       lock the semaphore.  The thread will now own a lock on the semaphore; the call will return successfully.  If the semaphore  is  unavailable
       at the time its value is checked, then the call returns -1 with errno set to EAGAIN.

       If  the	specified semaphore referred to by sem is a named semaphore, then this semaphore must have been opened by the calling process with
       The calling process must have both read and write permissions on the semaphore to perform these operations.  The semaphore will	be  locked
       upon successful return and will stay locked until it is explicitly released by a call to

       To use this function, link in the realtime library by specifying on the compiler or linker command line.

EXAMPLES

       The following call to will lock the semaphore sem.

       The following call to will lock the semaphore sem, if it is available.

RETURN VALUE

       A  successful  call  to	will return 0 and the calling thread will then own a lock on the semaphore.  Otherwise, the call to will return -1
       with errno set to the appropriate value of the error condition.

       A successful call to will return 0, if the semaphore was available and the calling thread was able to lock the semaphore.   Otherwise,  the
       call to will return -1 with errno set to the appropriate value of the error condition.

ERRORS

       and fail and do not perform the requested operation if any of the following conditions are encountered:

	      [EPERM]	     The calling process does not have the privileges necessary to lock the semaphore.

	      [EAGAIN]	     The semaphore was not available and hence could not be locked by This error condition only occurs in

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

	      [EINTR]	     The function was interrupted by a signal

SEE ALSO

       sem_post(2), <semaphore.h>.

STANDARDS CONFORMANCE

																       sem_wait(2)