pthread_rwlock

Initialize a read-write lock

Synopsis:

#include <pthread.h>

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

Arguments:

rwl: A pointer to a pthread_rwlock_t object that you want to initialize.
Note: It's always safe, and typically faster, to assure that rwl is 32-bit aligned.
attr: 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().

Library:

libc

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

Description:

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, 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:

favoring blocked readers over writers after a writer has just released an exclusive lock, and
favoring writers over readers when there are no blocked readers

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

Returns:

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

Classification:

POSIX 1003.1

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

Caveats:

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.