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NULL is OK, both for passing and for returning.
This parameter is a 'user_data', for callbacks; many bindings can pass NULL here.
Generics and defining elements of containers and arrays.
NULL must not be passed as the value in, out, in-out; or as a return value.
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.
The callback is valid until first called.
The caller owns the data container, but not the data inside it.
The caller owns the data, and is responsible for free it.
The data is owned by the callee, which is responsible of freeing it.