Lesson 22 · End-of-track - Build a MIDI Parser

End-of-track

Every track ends with one required meta event - end-of-track - and it must land exactly at the track's byte length. Today you detect it and use it to stop the event loop cleanly.

The goal

Detect the end-of-track meta event and stop parsing exactly at the track's length.

Start here - the target

TO DO

Scenario: End-of-track terminates the track at its chunk length

Givena track body ending in the bytes 0xFF 0x2F 0x00, with the body length matching the bytes exactly

Whenthe event loop reaches that event

Thenit is recognised as end-of-track and parsing stops

Andthe reader position equals the track body length - no bytes are read past the end-of-track

Background

Every MTrk must finish with an end-of-track meta event: 0xFF 0x2F 0x00 - type 0x2F, length 0, no payload. It is not decoration; it is the official signal that the track is over, and a well-formed file places it so the reader’s position lands exactly at the track body’s length. That equality is a strong correctness check: if you stop before the end there are trailing bytes you mishandled, and if you read past it your event decoding drifted.

Until now the event loop stopped when the bytes ran out; from here it should stop when it sees end-of-track, which is more precise and matches how a real file is framed. Detecting it cleanly is what lets the next chapter parse a complete track - tempo and time signature in a conductor track, notes in another - and trust that each track begins and ends where the chunk says it does.

Make it work

// meta type 0x2F, length 0: the required last event of every track
if metaType == 0x2F {
  // record an EndOfTrack event and break out of the event loop
}

CheckpointDONE

You can detect end-of-track and stop the loop. Commit and stop here.