SDKs may have attached [[license]]s that make them unsuitable for [[Free software]] development.
An SDK for an operating system add-on (for instance, [[QuickTime]] for [[MacOSMac OS]]) may include the add-on software itself, to be used for development purposes, if not necessarily for redistribution. An interesting situation arises here between platforms where it is possible to develop applications that can at least start up on a system configuration without the add-on installed, and use a [[Gestalt (MacOSMac OS)|Gestalt]]-style run-time ''environment query'' to determine if the add-on is present, and ones where the application will simply fail to start. In other words, is it possible to build a single binary that will run on configurations with and without the add-on present, albeit operating with reduced functionality in the latter situation.
