Initialize a read-write lock


#include <pthread.h>

int pthread_rwlock_init( 
                 pthread_rwlock_t * rwl,
                 const pthread_rwlockattr_t * attr );


A pointer to a pthread_rwlock_t object that you want to initialize.
NULL, or a pointer to a pthread_rwlockattr_t object that specifies the attributes you want to use for the read-write lock; see pthread_rwlockattr_init().



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


The pthread_rwlock_init() function initializes the read-write lock referenced by rwl with the attributes of attr. You must initialize read-write locks before using them. If attr is NULL, rwl is initialized with the default values for the attributes.

Caution: You should allocate synchronization objects only in normal memory mappings. On certain processors (e.g. some PPC ones), atomic operations such as locking will cause a fault if the control structure is allocated in uncached memory.

Following a successful call to pthread_rwlock_init(), the read-write lock is unlocked, and you can use it in subsequent calls to pthread_rwlock_destroy(), pthread_rwlock_rdlock(), pthread_rwlock_tryrdlock(), pthread_rwlock_trywrlock(), and pthread_rwlock_wrlock(). This lock remains usable until you destroy it by calling pthread_rwlock_destroy().

If the read-write lock is statically allocated, you can initialize it with the default values by setting it to PTHREAD_RWLOCK_INITIALIZER.

More than one thread may hold a shared lock at any time, but only one thread may hold an exclusive lock. This avoids reader and writer starvation during frequent contention by:

Under heavy contention, the lock alternates between a single exclusive lock followed by a batch of shared locks.


Insufficient system resources to initialize the read-write lock.
The read-write lock rwl has been initialized or unsuccessfully destroyed.
A fault occurred when the kernel tried to access rwl or attr.
Invalid read-write lock attribute object attr.


POSIX 1003.1 THR

Cancellation point No
Interrupt handler No
Signal handler Yes
Thread Yes


Beware of priority inversion when using read-write locks. A high-priority thread may be blocked waiting on a read-write lock locked by a low-priority thread.

The microkernel has no knowledge of read-write locks, and therefore can't boost the low-priority thread to prevent the priority inversion.

See also:

pthread_rwlockattr_init(), pthread_rwlock_destroy(), pthread_rwlock_rdlock(), pthread_rwlock_tryrdlock(), pthread_rwlock_trywrlock(), pthread_rwlock_wrlock(), pthread_rwlock_unlock()

Processes and Threads chapter of Getting Started with QNX Neutrino