Initialize a read-write lock
#include <pthread.h> int pthread_rwlock_init( pthread_rwlock_t * rwl, const pthread_rwlockattr_t * attr );
|It's always safe, and typically faster, to assure that rwl is 32-bit aligned.|
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.
|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.
POSIX 1003.1 THR
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.
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