StaticRWLock

Warning: StaticRWLock is deprecated since 2.32. Use RWLock.

The StaticRWLock 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 StaticRWLock 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 reader_lock and reader_unlock have to lock and unlock a StaticMutex, so it takes at least twice the time to lock and unlock a StaticRWLock that it does to lock and unlock a StaticMutex . So only data structures that are accessed by multiple readers, and which keep the lock for a considerable time justify a StaticRWLock. The above example most probably would fare better with a StaticMutex.