|ext:stream-file-descriptor stream direction||[Generic Function]|
Return the underlying file descriptor for the stream in the given direction, or nil if not applicable. If a stream has multiple input or output file descriptors then the descriptor blocking the most recent operation on the stream is returned. Implements the function sys:fd-stream-fd. The file descriptor based streams include local file streams, network socket streams, and SSL streams. Methods are provided for composite streams to return the source or target stream descriptors. When appropriate, user defined composite streams should provide a method to return the source or target stream file descriptor. A default method is provided for the class stream that returns nil.