The data is owned by the caller of the function.
The return location for a recoverable error.
The argument can be
If the return location is not
NULL, then you must initialize it to a
The argument will left initialized to
NULLby the virtual function if there are no errors.
In case of error, the argument will be set to a newly allocated
GError; the caller will take ownership of the data, and be responsible for freeing it.