User file stream length callback function.
QWORD CALLBACK FileLenProc(
The length of the file in bytes. Returning 0 for a buffered file stream makes BASS stream the file in blocks and is equivalent to using the BASS_STREAM_BLOCK flag in the BASS_StreamCreateFileUser call.
This function is called first thing, and only the once with buffered streams. With unbuffered streams, it may be called again when testing for EOF (end of file), allowing the file to grow in size.
BASS_StreamCreateFileUser, BASS_FILEPROCS structure