pthread_rwlock_rdlock(3T)												 pthread_rwlock_rdlock(3T)

NAME

       pthread_rwlock_rdlock(), pthread_rwlock_tryrdlock() - lock or attempt to lock a read-write lock for reading

SYNOPSIS

PARAMETERS

	      rwlock	Pointer to the read-write lock to be locked for reading.

DESCRIPTION

       The  function  applies  a  read lock to the read-write lock object referenced by rwlock.  The calling thread shall acquire a read lock if a
       writer does not hold the lock and there are no writers blocked on the lock.  It is unspecified whether the calling thread acquires the lock
       when  a writer does not hold the lock and there are writers waiting for the lock.  If a writer holds the lock, the calling thread shall not
       acquire the read lock.  If the read lock is not acquired, the calling thread blocks (that is, it does not return from the  call)  until	it
       can acquire the lock.  Results are undefined if the calling thread currently owns a write lock on rwlock.

       Implementations shall be allowed to favor writers over readers to avoid writer starvation.

       A thread may hold multiple concurrent locks on rwlock (that is, successfully call the function n times). If so, the thread must perform the
       matching unlocks (that is, it must call the function n times).

       The function applies a read lock as in the function with the exception that the function fails if any thread holds a write lock	on  rwlock
       or there are writers blocked on rwlock.

       Results are undefined if any of these functions are called with an uninitialized read-write lock.

       If  a  signal  is delivered to a thread waiting for a read-write lock, upon return from the signal handler, the thread shall resume waiting
       for the read-write lock as if it was not interrupted.

RETURN VALUE

       Upon successful completion, and return zero. Otherwise, an error number is returned to indicate the error (the variable is not set).

ERRORS

       If any of the following occur, the function returns the corresponding error number:

	      [EBUSY]	     The read-write lock rwlock could not be acquired for reading because a writer holds the lock or was blocked on it.

       For each of the following conditions, if the condition is detected, the and functions return the corresponding error number:

	      [EINVAL]	     The value specified by rwlock does not refer to an initialized read-write lock.

	      [EDEADLK]      The current thread already owns the read-write lock for writing.

	      [EAGAIN]	     The read lock could not be acquired because the maximum number of read locks for rwlock has been exceeded. This error
			     is not detected on HP-UX.

AUTHOR

       and were developed by X/Open.

SEE ALSO

       pthread_rwlock_init(3T), pthread_rwlock_destroy(3T), pthread_rwlock_trywrlock(3T), pthread_rwlock_wrlock(3T), pthread_rwlock_unlock(3T).

STANDARDS CONFORMANCE

								  Pthread Library					 pthread_rwlock_rdlock(3T)