pthread_rwlock

Acquire a shared read lock on a read-write lock

Synopsis:

#include <pthread.h>

int pthread_rwlock_rdlock( pthread_rwlock_t* rwl );

Arguments:

rwl: A pointer to a pthread_rwlock_t object that you want to lock for reading.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The pthread_rwlock_rdlock() function acquires a shared lock on the read-write lock referenced by rwl. If the read-write lock is already exclusively locked, the calling thread blocks until the exclusive lock is released.

Note: If another thread is currently waiting to acquire the read-write lock for writing, pthread_rwlock_rdlock() blocks. This can cause a deadlock if a thread attempts to recursively acquire the lock for reading, while another thread is trying to acquire the lock for writing.

If a signal is delivered to a thread waiting to lock a read-write lock, it will resume waiting for the lock after returning from the signal handler.

A thread may hold several read locks on the same read-write lock; it must call pthread_rwlock_unlock() multiple times to release its read lock.

Returns:

EOK: Success.
EAGAIN: On the first use of statically initialized read-write lock, insufficient system resources existed to initialize the read-write lock.
EDEADLK: The calling thread already has an exclusive lock for rwl.
EFAULT: A fault occurred when the kernel tried to access rwl.
EINVAL: The read-write lock rwl is invalid.

Examples:

See “An example of synchronization” in the “Processes and Threads” chapter of Getting Started with QNX Neutrino.

Classification:

POSIX 1003.1

Safety:
Cancellation point	Yes
Interrupt handler	No
Signal handler	Yes
Thread	Yes