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man pages section 3: Basic Library Functions

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Updated: Friday, August 13, 2021



pthread_rwlock_init, pthread_rwlock_destroy - initialize or destroy read-write lock object


#include <pthread.h>

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


The pthread_rwlock_init() function initializes the read-write lock referenced by rwlock with the attributes referenced by attr. 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. Once initialized, the lock can be used any number of times without being re-initialized. Upon 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.

If the pthread_rwlock_init() function fails, rwlock is not initialized and the contents of rwlock are undefined.

The pthread_rwlock_destroy() function destroys the read-write lock object referenced by rwlock and releases any resources used by the lock. The effect of subsequent use of the lock is undefined until the lock is re-initialized by another call to pthread_rwlock_init(). An implementation may cause pthread_rwlock_destroy() to set the object referenced by rwlock to an invalid value. Results are undefined if pthread_rwlock_destroy() is called when any thread holds rwlock. Attempting to destroy an uninitialized read-write lock results in undefined behaviour. A destroyed read-write lock object can be re-initialized using pthread_rwlock_init(); the results of otherwise referencing the read-write lock object after it has been destroyed are undefined.

In cases where default read-write lock attributes are appropriate, the macro PTHREAD_RWLOCK_INITIALIZER can be used to 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.

Return Values

If successful, the pthread_rwlock_init() and pthread_rwlock_destroy() functions return 0. Otherwise, an error number is returned to indicate the error.


The pthread_rwlock_init() and pthread_rwlock_destroy() functions will fail if:


The value specified by attr is invalid.


The value specified by rwlock is invalid.


Insufficient memory exists to initialize the rwlock. This error code applies only to scalable rwlocks which require extra memory allocation during initialization.


Example 1 Scalable rwlock

The following example illustrates creation and destruction of the scalable read-write lock.

#include <pthread.h>
main(int argc, char *argv[])
		int                     error;
		pthread_rwlock_t        rwlock;
                pthread_rwlockattr_t rwlattr;

                error = pthread_rwlockattr_init(&rwlattr);
		if (error != 0)
                        return (1);

                error = pthread_rwlockattr_settype_np(&rwlattr,
                if (error != 0)
                        return (2);

		error = pthread_rwlock_init(&rwlock, &rwlattr);
		if (error != 0)
			return (3);

               error = pthread_rwlock_destroy(&rwlock);
                if (error != 0)
                        return (4);

               error = pthread_rwlockattr_destroy(&rwlattr);
                if (error != 0)
                        return (5);
                return (0);

The scalable rwlock pthread_rwlock_init() function must be called to initialize the lock. Static initialization cannot be done for a scalable rwlock as it requires extra memory allocation. Also, after the application is done using the scalable rwlock object, it must call the pthread_rwlock_destroy() function to deallocate memory used by the rwlock object to prevent memory leaks.


See attributes(7) for descriptions of the following attributes:

Interface Stability

See Also

pthread_rwlock_rdlock(3C), pthread_rwlock_unlock(3C), pthread_rwlock_wrlock(3C), pthread_rwlockattr_init(3C), pthread_rwlockattr_getpshared(3C), attributes(7), standards(7)