Gets the size of the currently allocated data area (available from g_memory_output_stream_get_data()).
You probably don’t want to use this function on resizable streams.
g_memory_output_stream_get_data_size() instead. For resizable
streams the size returned by this function is an implementation
detail and may be change at any time in response to operations on the stream.
If the stream is fixed-sized (ie: no realloc was passed to
g_memory_output_stream_new()) then this is the maximum size of the
stream and further writes will return
In any case, if you want the number of bytes currently written to the stream, use g_memory_output_stream_get_data_size().