Read-Write Lock Attributes

Read-write locks permit concurrent reads and exclusive writes to a protected shared resource. The read-write lock is a single entity that can be locked in read or write mode. To modify a resource, a thread must first acquire the exclusive write lock. An exclusive write lock is not permitted until all read locks have been released.

Database access can be synchronized with a read-write lock. Read-write locks support concurrent reads of database records because the read operation does not change the record's information. When the database is to be updated, the write operation must acquire an exclusive write lock.

To change the default read-write lock attributes, you can declare and initialize an attribute object. Often, the read-write lock attributes are set up in one place at the beginning of the application so they can be located quickly and modified easily. The following table lists the functions discussed in this section that manipulate read-write lock attributes.

See Similar Synchronization Functions—Read-Write Locks for the Solaris threads implementation of read-write locks.

Table 4–8 Routines for Read-Write Lock Attributes

Operation	Destination Discussion
Initialize a read-write lock attribute	pthread_rwlockattr_init(3THR)
Destroy a read-write lock attribute	pthread_rwlockattr_destroy(3THR)
Set a read-write lock attribute	pthread_rwlockattr_setpshared(3THR)
Get a read-write lock attribute	pthread_rwlockattr_getpshared(3THR)

Initialize a Read-Write Lock Attribute

pthread_rwlockattr_init(3THR)

#include <pthread.h>

int pthread_rwlockattr_init(pthread_rwlockattr_t *attr);

pthread_rwlockattr_init(3THR) initializes a read-write lock attributes object attr with the default value for all of the attributes defined by the implementation.

Results are undefined if pthread_rwlockattr_init is called specifying an already initialized read-write lock attributes object. After a read-write lock attributes object has been used to initialize one or more read-write locks, any function affecting the attributes object (including destruction) does not affect any previously initialized read-write locks.

Return Values

If successful, pthread_rwlockattr_init() returns zero. Otherwise, an error number is returned to indicate the error.

ENOMEM

Insufficient memory exists to initialize the rwlock attributes object.

Destroy a Read-Write Lock Attribute

pthread_rwlockattr_destroy(3THR)

#include <pthread.h>

int pthread_rwlockattr_destroy(pthread_rwlockattr_t *attr);

pthread_rwlockattr_destroy(3THR) destroys a read-write lock attributes object. The effect of subsequent use of the object is undefined until the object is re-initialized by another call to pthread_rwlockattr_init() An implementation can cause pthread_rwlockattr_destroy() to set the object referenced by attr to an invalid value.

Return Values

If successful, pthread_rwlockattr_destroy() returns zero. Otherwise, an error number is returned to indicate the error.

EINVAL

The value specified by attr is invalid.

Set a Read-Write Lock Attribute

pthread_rwlockattr_setpshared(3THR)

#include <pthread.h>

int pthread_rwlockattr_setpshared(pthread_rwlockattr_t  *attr, 
											int  pshared);

pthread_rwlockattr_setpshared(3THR) sets the process-shared read-write lock attribute.

PTHREAD_PROCESS_SHARED

Permits a read-write lock to be operated on by any thread that has access to the memory where the read-write lock is allocated, even if the read-write lock is allocated in memory that is shared by multiple processes.

PTHREAD_PROCESS_PRIVATE

The read-write lock will only be operated upon by threads created within the same process as the thread that initialized the read-write lock; if threads of differing processes attempt to operate on such a read-write lock, the behavior is undefined. The default value of the process-shared attribute is PTHREAD_PROCESS_PRIVATE.

Return Value

If successful, pthread_rwlockattr_setpshared() returns zero. Otherwise, an error number is returned to indicate the error.

EINVAL

The value specified by attr or pshared is invalid.

Get a Read-Write Lock Attribute

pthread_rwlockattr_getpshared(3THR)

#include <pthread.h>

int pthread_rwlockattr_getpshared(const pthread_rwlockattr_t  *attr, 
											int *pshared);

pthread_rwlockattr_getpshared(3THR) gets the process-shared read-write lock attribute.

pthread_rwlockattr_getpshared() obtains the value of the process-shared attribute from the initialized attributes object referenced by attr.

Return Value

If successful, pthread_rwlockattr_getpshared() returns zero. Otherwise, an error number is returned to indicate the error.

EINVAL

The value specified by attr or pshared is invalid.