How to output fragmented mp4 with ffmpeg?

ffmpeg -i infile.avi out.mp4 outputs non-fragmented MP4.

How do I obtain fragmented mp4?

Update A fragmented mp4 file is internally divided into several back-to-back chunks or MPEG-4 movie fragments. Each chunk has its own moof atom - so there are several moof atoms interleaved in the file instead of a single moov at the end as in the case of an unfragmented mp4. This makes it easier to stream over slow networks where buffering is involved

There are several tools like mp4box that convert a normal mp4 to a fragmented one. Unfortunately we cannot use something like this

ffmpeg <options to output mp4> | mp4box

since ffmpeg does not produce seekable output while producing mp4 containers.

This should do the trick:

ffmpeg -re -i infile.ext -g 52 \
-c:a aac -b:a 64k -c:v libx264 -b:v 448k \
-f mp4 -movflags frag_keyframe+empty_moov \
output.mp4

• frag_keyframe causes fragmented output,
• empty_moov will cause output to be 100% fragmented; without this the first fragment will be muxed as a short movie (using moov) followed by the rest of the media in fragments,
• -re is useful when live streaming (use input media frame rate), do not use it if you are creating a file,
• -g 52 forces (at least) every 52nd frame to be a keyframe

To calculate a healthy keyframe interval please see the paragraphs about fragment sizes in the docs of my streaming server. - You can also consider using WebM which is a free alternative to H.264 (and has better support on some platforms than fragmented mp4).

Important note: FFMpeg's muxer will set the Duration in both tkhd and mdhd atoms to 0xffffffff for each track. This causes problems in some players (for example Quicktime will not play such files). You should find a tool and change it to zero (0x00000000).

UPDATE: Considering a fragmented MP4 as ISMV (Smooth Streaming) file. The new version FFMPEG 0.10, since January, 27, 2012 is able to mux to this format.

$ ffmpeg -h muxer=ismv
ismv muxer AVOptions:
-movflags          <flags> E.... MOV muxer flags
   rtphint                 E.... Add RTP hint tracks
   empty_moov              E.... Make the initial moov atom empty (not supported by QuickTime)
   frag_keyframe           E.... Fragment at video keyframes
   separate_moof           E.... Write separate moof/mdat atoms for each track
   frag_custom             E.... Flush fragments on caller requests
   isml                    E.... Create a live smooth streaming feed (for pushing to a publishing point)
-moov_size         <int>   E.... maximum moov size so it can be placed at the begin
-rtpflags          <flags> E.... RTP muxer flags
   latm                    E.... Use MP4A-LATM packetization instead of MPEG4-GENERIC for AAC
   rfc2190                 E.... Use RFC 2190 packetization instead of RFC 4629 for H.263
   skip_rtcp               E.... Don't send RTCP sender reports
-skip_iods         <int>   E.... Skip writing iods atom.
-iods_audio_profile <int>  E.... iods audio profile atom.
-iods_video_profile <int>  E.... iods video profile atom.
-frag_duration     <int>   E.... Maximum fragment duration
-min_frag_duration <int>   E.... Minimum fragment duration
-frag_size         <int>   E.... Maximum fragment size
-ism_lookahead     <int>   E.... Number of lookahead entries for ISM files