Parameter for input and for returning results. Default is transfer full.

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.

Parameter for returning results. Default is transfer full.

The callback is valid until first called.

Exposed in C code, not necessarily available in other languages.

The caller owns the data, and is responsible for free it.

The data is owned by the callee, which is responsible of freeing it.