Check whether an MP4, MOV or M4A file keeps its index at the end, and write a copy with the…

Guide ID: art_01M7BKH3VMENG4T1N7AGGEG0HB

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Guide title

Check whether an MP4, MOV or M4A file keeps its index at the end, and write a copy with the index first (ffmpeg -movflags +faststart), with a check that the streams are unchanged

Coverage

For anyone who puts an MP4, MOV or M4A on a web page, share link or server and wants to know whether the file's index (the moov box) is at the end, where ffmpeg normally writes it, and to write a copy with the index first without re-encoding. A PowerShell script lists the file's top-level boxes, tests what ffprobe and ffmpeg can read from the first half of the media data, and writes the copy with -movflags +faststart, then checks that the streams are identical, the length and tags are kept and the partial-file test passes. It ran on generated MP4, MOV and M4A files with ffmpeg 9.0.2, including ten refusal cases; no browser, player or server was tested.

Example

A 20 s MP4 made by ffmpeg had its 16,472-byte index in the last bytes of 2,103,268; ffprobe found no stream in its first 1,051,634 bytes (moov atom not found). The copy had the index at bytes 32 to 16,504, the same size and identical packets in both streams, and ffprobe found both streams and decoded a frame from the first 50.4 %. On a file with a chapter list, plain -map 0 wrote the chapter track twice.

Useful starting points

• Read the box order first: moov before mdat means the index is already first.
• Add -movflags +faststart when you write the file, or run a second -c copy pass with it.
• The flag does nothing for fragmented MP4: ffmpeg accepts it silently and still writes fragments.
• On a file with chapters, check the stream count after -map 0: the chapter track can be written twice.