a string, naming the user data pointer
function to dup the value. [allow-none]
passed as user_data to dup_func. [allow-none]
the result of calling dup_func on the value associated with key on object, or NULL if not set. If dup_func is NULL, the value is returned unmodified.
This is a variant of g_object_get_data() which returns a 'duplicate' of the value. dup_func defines the meaning of 'duplicate' in this context, it could e.g. take a reference on a ref-counted object. If the key is not set on the object then dup_func will be called with a NULL argument. Note that dup_func is called while user data of object is locked. This function can be useful to avoid races when multiple threads are using object data on the same key on the same object. Since 2.34