PTHREAD_RWLOCK_RDLOCK(3C) Standard C Library Functions


pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock or attempt to lock
read-write lock object for reading


cc -mt [ flag... ] file... -lpthread [ library... ]
#include <pthread.h>

int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);

int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);


The pthread_rwlock_rdlock() function applies a read lock to the read-
write lock referenced by rwlock. The calling thread acquires the read
lock if a writer does not hold the lock and there are no writers blocked
on the lock.

The calling thread does not acquire the lock if a writer holds the lock
or if writers of higher or equal priority are blocked on the lock;
otherwise, the calling thread acquires the lock. If the read lock is not
acquired, the calling thread blocks until it can acquire the lock.

A thread can hold multiple concurrent read locks on rwlock (that is,
successfully call the pthread_rwlock_rdlock() function n times). If so,
the thread must perform matching unlocks (that is, it must call the
pthread_rwlock_unlock() function n times).

The maximum number of concurrent read locks that a thread can hold on one
read-write lock is currently set at 100,000, though this number could
change in a future release. There is no imposed limit on the number of
different threads that can apply a read lock to one read-write lock.

The pthread_rwlock_tryrdlock() function applies a read lock like the
pthread_rwlock_rdlock() function, with the exception that the function
fails if the equivalent pthread_rwlock_rdlock() call would have blocked
the calling thread. In no case will the pthread_rwlock_tryrdlock()
function ever block. It always either acquires the lock or fails and
returns immediately.

Results are undefined if any of these functions are called with an
uninitialized read-write lock.

If a signal is delivered to a thread waiting for a read-write lock for
reading, upon return from the signal handler the thread resumes waiting
for the read-write lock for reading as if it was not interrupted.


If successful, the pthread_rwlock_rdlock() function returns 0.
Otherwise, an error number is returned to indicate the error.

The pthread_rwlock_tryrdlock() function returns 0 if the lock for reading
on the read-write lock object referenced by rwlock is acquired.
Otherwise an error number is returned to indicate the error.


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

The read lock could not be acquired because the maximum number
of read locks by the current thread for rwlock has been

The pthread_rwlock_rdlock() function will fail if:

The current thread already owns the read-write lock for

The pthread_rwlock_tryrdlock() function will fail if:

The read-write lock could not be acquired for reading because a
writer holds the lock or a writer with the appropriate priority
was blocked on it.


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Standard |
|MT-Level | MT-Safe |


pthread_rwlock_init(3C), pthread_rwlock_unlock(3C),
pthread_rwlock_wrlock(3C), pthread_rwlockattr_init(3C), attributes(7),