get chunks of a mp4 in ffmpeg

C
ffmpeg -i input.mp4 -c copy -map 0 -segment_time 00:20:00 -f segment -reset_timestamps 1 output%03d.mp4
ffmpeg -i input.mp4 -c copy -map 0 -segment_time 00:20:00 -f segment output%03d.mp4

Source

Also in C: