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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

SYNOPSIS
     #include <pthread.h>

     int
     pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);

     int
     pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);

DESCRIPTION
     The pthread_rwlock_rdlock() function acquires a read lock on rwlock, provided that rwlock 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.

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.

     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 rwlock for writing.

     [EINVAL]           The value specified by rwlock 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.

BSD                                            August 4, 1998                                            BSD