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. Set up at the beginning of the application makes the attributes easier to locate and modify. The following table lists the functions discussed in this section that manipulate read-write lock attributes.
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pthread_rwlockattr_init(3C) initializes a read-write lock attributes object attr with the default value for all of the attributes defined by the implementation.
#include <pthread.h> int pthread_rwlockattr_init(pthread_rwlockattr_t *attr);
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 initializes one or more read-write locks, any function that affects the object, including destruction, does not affect previously initialized read-write locks.
If successful, pthread_rwlockattr_init() returns zero. Otherwise, an error number is returned to indicate the error.
ENOMEM
Description: Insufficient memory exists to initialize the read-write attributes object.
pthread_rwlockattr_destroy(3C) destroys a read-write lock attributes object.
#include <pthread.h> int pthread_rwlockattr_destroy(pthread_rwlockattr_t *attr);
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.
If successful, pthread_rwlockattr_destroy() returns zero. Otherwise, an error number is returned to indicate the error.
EINVAL
Description: The value specified by attr is invalid.
pthread_rwlockattr_setpshared(3C) sets the process-shared read-write lock attribute.
#include <pthread.h> int pthread_rwlockattr_setpshared(pthread_rwlockattr_t *attr, int pshared);
The pshared lock attribute has one of the following values:
PTHREAD_PROCESS_SHARED
Description: 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. Operation on the read-write lock is permitted even if the lock is allocated in memory that is shared by multiple processes.
PTHREAD_PROCESS_PRIVATE
Description: The read-write lock is only 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.
If successful, pthread_rwlockattr_setpshared() returns zero. Otherwise, an error number is returned to indicate the error.
EINVAL
Description: The value specified by attr or pshared is invalid.
pthread_rwlockattr_getpshared(3C) gets the process-shared read-write lock attribute.
#include <pthread.h>
int pthread_rwlockattr_getpshared(const pthread_rwlockattr_t *restrict attr,
int *restrict pshared);pthread_rwlockattr_getpshared() obtains the value of the process-shared attribute from the initialized attributes object referenced by attr.
If successful, pthread_rwlockattr_getpshared() returns zero. Otherwise, an error number is returned to indicate the error.
EINVAL
Description: The value specified by attr or pshared is invalid.