pthread_rwlock_init Syntax

#include <pthread.h>

int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock, 
          const pthread_rwlockattr_t *restrict attr);

pthread_rwlock_t  rwlock = PTHREAD_RWLOCK_INITIALIZER;

If attr is NULL, the default read-write lock attributes are used. The effect is the same as passing the address of a default read-write lock attributes object. After the lock is initialized, the lock can be used any number of times without being re-initialized. On successful initialization, the state of the read-write lock becomes initialized and unlocked. Results are undefined if pthread_rwlock_init() is called specifying an already initialized read-write lock. Results are undefined if a read-write lock is used without first being initialized.

In cases where default read-write lock attributes are appropriate, the macro PTHREAD_RWLOCK_INITIALIZER can initialize read-write locks that are statically allocated. The effect is equivalent to dynamic initialization by a call to pthread_rwlock_init() with the parameter attr specified as NULL, except that no error checks are performed.