NAME
pthread_rwlock_rdlock, pthread_rwlock_tryrdlock - lock a read-write
lock object 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 shall apply 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.
If the Thread Execution Scheduling option is supported, and the threads
involved in the lock are executing with the scheduling policies
SCHED_FIFO or SCHED_RR, the calling thread shall 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 shall acquire
the lock.
If the Threads Execution Scheduling option is supported, and the
threads involved in the lock are executing with the SCHED_SPORADIC
scheduling policy, the calling thread shall 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 shall acquire the
lock.
If the Thread Execution Scheduling option is not supported, it is
implementation-defined whether the calling thread acquires the lock
when a writer does not hold the lock and there are writers blocked on
the lock. If a writer holds the lock, the calling thread shall not
acquire the read lock. If the read lock is not acquired, the calling
thread shall block until it can acquire the lock. The calling thread
may deadlock if at the time the call is made it holds a write lock.
A thread may hold multiple concurrent read locks on rwlock (that is,
successfully call the pthread_rwlock_rdlock() function n times). If so,
the application shall ensure that the thread performs matching unlocks
(that is, it calls the pthread_rwlock_unlock() function n times).
The maximum number of simultaneous read locks that an implementation
guarantees can be applied to a read-write lock shall be implementation-
defined. The pthread_rwlock_rdlock() function may fail if this maximum
would be exceeded.
The pthread_rwlock_tryrdlock() function shall apply a read lock as in
the pthread_rwlock_rdlock() function, with the exception that the
function shall fail if the equivalent pthread_rwlock_rdlock() call
would have blocked the calling thread. In no case shall 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.
RETURN VALUE
If successful, the pthread_rwlock_rdlock() function shall return zero;
otherwise, an error number shall be returned to indicate the error.
The pthread_rwlock_tryrdlock() function shall return zero if the lock
for reading on the read-write lock object referenced by rwlock is
acquired. Otherwise, an error number shall be returned to indicate the
error.
ERRORS
The pthread_rwlock_tryrdlock() function shall fail if:
EBUSY 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.
The pthread_rwlock_rdlock() and pthread_rwlock_tryrdlock() functions
may fail if:
EINVAL The value specified by rwlock does not refer to an initialized
read-write lock object.
EAGAIN The read lock could not be acquired because the maximum number
of read locks for rwlock has been exceeded.
The pthread_rwlock_rdlock() function may fail if:
EDEADLK
The current thread already owns the read-write lock for writing.
These functions shall not return an error code of [EINTR].
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
Applications using these functions may be subject to priority
inversion, as discussed in the Base Definitions volume of
IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
pthread_rwlock_destroy() , pthread_rwlock_timedrdlock() ,
pthread_rwlock_timedwrlock() , pthread_rwlock_trywrlock() ,
pthread_rwlock_unlock() , pthread_rwlock_wrlock() , the Base
Definitions volume of IEEE Std 1003.1-2001, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .