NULL is OK, both for passing and for returning.
Rename the original symbol's name to SYMBOL.
The callback is valid until first called.
The callback is valid only during the call to the method.
The caller owns the data, and is responsible for free it.
The data is owned by the callee, which is responsible of freeing it.
Unstable interfaces are experimental or transitional. They are typically used to give outside developers early access to new or rapidly changing technology, or to provide an interim solution to a problem where a more general solution is anticipated. No claims are made about either source or binary compatibility from one minor release to the next. The Unstable interface level is a warning that these interfaces are subject to change without warning and should not be used in unbundled products. Given such caveats, customer impact need not be a factor when considering incompatible changes to an Unstable interface in a major or minor release. Nonetheless, when such changes are introduced, the changes should still be mentioned in the release notes for the affected release.