#### POSIX Threads for Windows – REFERENCE - pthreads-win32

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## 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.

pthreads-win32 does not prefer either writers or readers in acquiring the lock – all threads enter a single prioritised FIFO queue. While this may not be optimally efficient for some applications, it does ensure that one type does not starve the other.

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(3) function n times).

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.

pthreads-win32 does not detect deadlock if the thread already owns the lock for writing.

pthreads-win32 defines _POSIX_READER_WRITER_LOCKS in pthread.h as 200112L to indicate that the reader/writer routines are implemented and may be used.

## 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.

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(3) , pthread_rwlock_timedrdlock(3) , pthread_rwlock_timedwrlock(3) , pthread_rwlock_trywrlock(3) , pthread_rwlock_unlock(3) , pthread_rwlock_wrlock(3) , 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 .

Modified by Ross Johnson for use with pthreads-win32.

Table of Contents

- Name

- Synopsis

- Description

- Return Value

- Errors

- Examples

- Application Usage

- Rationale

- Future Directions

- See Also

- Copyright