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pthread_rwlock_tryrdlock(3)

NAME

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

LIBRARY

     Reentrant C Library (libc_r, -lc_r)
     POSIX Threads Library (libpthread, -lpthread)
     1:1 Threading Library (libthr, -lthr)

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 pro-
     vided that lock is not presently held for writing and no writer threads
     are presently blocked on the lock.  If the read lock cannot be immedi-
     ately 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.

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'').

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.

     [EINVAL]		The value specified by lock is invalid.

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

HISTORY

     The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0.

FreeBSD 5.4			August 4, 1998			   FreeBSD 5.4

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