NULL is OK, both for passing and for returning.
Parameter points to an array of items.
This parameter is a 'user_data', for callbacks; many bindings can pass NULL here.
This parameter is a 'destroy_data', for callbacks.
Generics and defining elements of containers and arrays.
Parameter for input and for returning results. Default is transfer full.
NULL may be passed as the value in, out, in-out; or as a return value.
NULL may be passed instead of a pointer to a location.
Parameter for returning results. Default is transfer full.
Out parameter, where caller must allocate storage.
The callback is valid until the GDestroyNotify argument is called.
Exposed in C code, not necessarily available in other languages.
Free data after the code is done.
Don't free data after the code is done.