PTHREAD_RWLOCK_RDLOCK(3)				   BSD Library Functions Manual 				  PTHREAD_RWLOCK_RDLOCK(3)

NAME

     pthread_rwlock_rdlock, pthread_rwlock_tryrdlock -- acquire a read/write lock for reading

LIBRARY

     POSIX Threads Library (libpthread, -lpthread)

SYNOPSIS

     #include <pthread.h>

     int
     pthread_rwlock_rdlock(pthread_rwlock_t *lock);

     int
     pthread_rwlock_tryrdlock(pthread_rwlock_t *lock);

DESCRIPTION

     The pthread_rwlock_rdlock() function acquires a read lock on lock provided that lock is not presently held for writing and no writer threads
     are presently blocked on the lock.  If the read lock cannot be immediately acquired, the calling thread blocks until it can acquire the lock.

     The pthread_rwlock_tryrdlock() function performs the same action, but does not block if the lock cannot be immediately obtained (i.e., the
     lock is held for writing or there are waiting writers).

     A thread may hold multiple concurrent read locks.	If so, pthread_rwlock_unlock() must be called once for each lock obtained.

     The results of acquiring a read lock while the calling thread holds a write lock are undefined.

IMPLEMENTATION NOTES

     To prevent writer starvation, writers are favored over readers.

RETURN VALUES

     If successful, the pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions will return zero.  Otherwise an error number will be
     returned to indicate the error.

ERRORS

     The pthread_rwlock_tryrdlock() function will fail if:

     [EBUSY]		The lock could not be acquired because a writer holds the lock or was blocked on it.

     The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions may fail if:

     [EAGAIN]		The lock could not be acquired because the maximum number of read locks against lock has been exceeded.

     [EDEADLK]		The current thread already owns lock for writing.

     [EINVAL]		The value specified by lock is invalid.

     [ENOMEM]		Insufficient memory exists to initialize the lock (applies to statically initialized locks only).

SEE ALSO

     pthread_rwlock_init(3), pthread_rwlock_trywrlock(3), pthread_rwlock_unlock(3), pthread_rwlock_wrlock(3)

STANDARDS

     The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions are expected to conform to Version 2 of the Single UNIX Specification
     (``SUSv2'').

HISTORY

     The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0.

BSD
								  August 4, 1998							       BSD