extern long midi_loop_start;
extern long midi_loop_end;
The loop start and end points, set by the play_looped_midi() function. These may safely be altered while the music is playing, but you should be sure they are always set to sensible values (start < end). If you are changing them both at the same time, make sure to alter them in the right order in case a MIDI interrupt happens to occur in between your two writes! Setting these values to -1 represents the start and end of the file respectively.
The man page midi_loop_end(3) is an alias of midi_loop_start(3).