MirOS Manual: pthread_rwlock_rdlock(3), pthread_rwlock_tryrdlock(3)

PTHREAD_RWLOCK_RDLOCK(3)   BSD Programmer's 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 *lock);

     int
     pthread_rwlock_tryrdlock(pthread_rwlock_t *lock);

DESCRIPTION

     The pthread_rwlock_rdlock() function acquires a read lock on lock provid-
     ed 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 ac-
     quired, 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.

HISTORY

     The pthread_rwlock_rdlock() function first appeared in FreeBSD 3.0 and
     OpenBSD 2.5.

MirOS BSD #10-current           August 4, 1998                               1

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