StaticRwLock
The #GStaticRWLock struct represents a read-write lock. A read-write lock can be used for protecting data that some portions of code only read from, while others also write. In such situations it is desirable that several readers can read at once, whereas of course only one writer may write at a time.
Take a look at the following example: |[ GStaticRWLock rwlock = G_STATIC_RW_LOCK_INIT; GPtrArray *array;
gpointer my_array_get (guint index) { gpointer retval = NULL;
if (!array)
return NULL;
g_static_rw_lock_reader_lock (&rwlock);
if (index < array->len)
retval = g_ptr_array_index (array, index);
g_static_rw_lock_reader_unlock (&rwlock);
return retval;}
void my_array_set (guint index, gpointer data) { g_static_rw_lock_writer_lock (&rwlock);
if (!array)
array = g_ptr_array_new ();
if (index >= array->len)
g_ptr_array_set_size (array, index + 1);
g_ptr_array_index (array, index) = data;
g_static_rw_lock_writer_unlock (&rwlock);} ]|
This example shows an array which can be accessed by many readers (the my_array_get() function) simultaneously, whereas the writers (the my_array_set() function) will only be allowed once at a time and only if no readers currently access the array. This is because of the potentially dangerous resizing of the array. Using these functions is fully multi-thread safe now.
Most of the time, writers should have precedence over readers. That means, for this implementation, that as soon as a writer wants to lock the data, no other reader is allowed to lock the data, whereas, of course, the readers that already have locked the data are allowed to finish their operation. As soon as the last reader unlocks the data, the writer will lock it.
Even though #GStaticRWLock is not opaque, it should only be used with the following functions.
All of the g_static_rw_lock_* functions can be used even if g_thread_init() has not been called. Then they do nothing, apart from g_static_rw_lock_*_trylock, which does nothing but returning true.
A read-write lock has a higher overhead than a mutex. For example, both g_static_rw_lock_reader_lock() and g_static_rw_lock_reader_unlock() have to lock and unlock a #GStaticMutex, so it takes at least twice the time to lock and unlock a #GStaticRWLock that it does to lock and unlock a #GStaticMutex. So only data structures that are accessed by multiple readers, and which keep the lock for a considerable time justify a #GStaticRWLock. The above example most probably would fare better with a #GStaticMutex.
Constructors
Allocate a new StaticRwLock.
Allocate a new StaticRwLock using the provided AutofreeScope.
Functions
Locks @lock for reading. There may be unlimited concurrent locks for reading of a #GStaticRWLock at the same time. If @lock is already locked for writing by another thread or if another thread is already waiting to lock @lock for writing, this function will block until
Tries to lock @lock for reading. If @lock is already locked for writing by another thread or if another thread is already waiting to lock @lock for writing, immediately returns false. Otherwise locks
Unlocks @lock. If a thread waits to lock @lock for writing and all locks for reading have been unlocked, the waiting thread is woken up and can lock @lock for writing.
Locks @lock for writing. If @lock is already locked for writing or reading by other threads, this function will block until @lock is completely unlocked and then lock @lock for writing. While this functions waits to lock @lock, no other thread can lock @lock for reading. When @lock is locked for writing, no other thread can lock
Tries to lock @lock for writing. If @lock is already locked (for either reading or writing) by another thread, it immediately returns false. Otherwise it locks @lock for writing and returns true. This lock has to be unlocked by g_static_rw_lock_writer_unlock().
Unlocks @lock. If a thread is waiting to lock @lock for writing and all locks for reading have been unlocked, the waiting thread is woken up and can lock @lock for writing. If no thread is waiting to lock @lock for writing, and some thread or threads are waiting to lock @lock for reading, the waiting threads are woken up and can lock @lock for reading.