pthread_rwlock_rdlock(3P)

NAME
     pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a read-write lock
     object for reading

C SYNOPSIS
     #include <pthread.h>

     int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);

     int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);

DESCRIPTION
     Read-write locks allow a thread to exclusively lock some shared data
     while updating that data, or allow any number of threads to have
     simultaneous read-only access to the data.  The pthread_rwlock_rdlock()
     and pthread_rwlock_tryrdlock() functions apply a read lock to the read-
     write lock object referenced by rwlock.  The calling thread will only
     acquire the lock if a writer does not own the lock.  A thread that calls
     pthread_rwlock_rdlock() will block until it can gain the read-write lock
     for reading, and retains ownership until it calls
     pthread_rwlock_unlock().  Only the thread that locked a read-write lock
     should unlock it.  When unlocked, if there are both readers and writers
     of the same priority waiting for the read-write lock, writers will be
     scheduled before readers.

     A single thread may hold recursive read locks on a read-write lock.  In
     this case there must be a matching number of unlocks by the thread before
     the read-write lock can be released.

     If a thread waiting for a read-write lock receives a signal, upon return
     from the signal handler, the thread resumes waiting for the read-write
     lock as if there was no interrupt.

DIAGNOSTICS
     All of the read-write lock functions return zero if successful;
     otherwise, an error number is returned.

     pthread_rwlock_tryrdlock() can return the following error:

     [EBUSY]        The read-write lock is currently held by a writer.

SEE ALSO
     pthread_rwlock_init(3P), pthread_rwlock_wrlock(3P),
     pthread_rwlock_unlock(3P), pthread_rwlockattr_init(3P),
     pthread_rwlockattr_setpshared(3P).