python - ffmpeg hangs when run in background

Question

If I run ffmpeg as follows:

ffmpeg -i H264-media-4.264 4.avi

It works OK (i.e. 4.avi created OK). However, if I try to run it in background:

ffmpeg -i H264-media-4.264 4.avi &

it hangs! (and 4.avi never created) Any Idea?

---

Note: The problem is an isolation of of similar problem in python when trying to run it as subprocess and there it hanged as well: ff.py includes

ps = subprocess.Popen(ffmpeg_list, stderr=subprocess.STDOUT,stdout = subprocess.PIPE)

and running ./ff.py runs OK, ./ff.py & hangs too.

• System: CentOS 6.6
• ffmpeg: 0.10.2

Results from successful run:

 ffmpeg version 0.10.2 Copyright (c) 2000-2012 the FFmpeg developers
 built on Mar 20 2012 04:34:50 with gcc 4.4.6 20110731 (Red Hat 4.4.6-3)
  configuration: --prefix=/usr --libdir=/usr/lib64 --shlibdir=/usr/lib64 --mandir=/usr/share/man --enable-shared --enable-runtime-cpudetect --enable-gpl --enable-version3 --enable-postproc --enable-avfilter --enable-pthreads --enable-x11grab --enable-vdpau --disable-avisynth --enable-frei0r --enable-libopencv --enable-libdc1394 --enable-libdirac --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --extra-cflags='-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic -fPIC' --disable-stripping
  libavutil      51. 35.100 / 51. 35.100
  libavcodec     53. 61.100 / 53. 61.100
  libavformat    53. 32.100 / 53. 32.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 61.100 /  2. 61.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  6.100 /  0.  6.100
  libpostproc    52.  0.100 / 52.  0.100
[h264 @ 0x24eac00] max_analyze_duration 5000000 reached at 5000000
[h264 @ 0x24eac00] Estimating duration from bitrate, this may be inaccurate
Input #0, h264, from 'H264-media-4.264':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: h264 (Baseline), yuv420p, 640x480 [SAR 1:1 DAR 4:3], 25 fps, 25 tbr, 1200k tbn, 50 tbc
[buffer @ 0x24efa60] w:640 h:480 pixfmt:yuv420p tb:1/1000000 sar:1/1 sws_param:
[mpeg4 @ 0x24eb540] removing common factors from framerate
Output #0, avi, to '4.avi':
  Metadata:
    ISFT            : Lavf53.32.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 640x480 [SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (h264 -> mpeg4)
Press [q] to stop, [?] for help
frame= 2324 fps=477 q=31.0 Lsize=    2603kB time=00:01:32.96 bitrate= 229.4kbits/s    
video:2542kB audio:0kB global headers:0kB muxing overhead 2.409572%

Results from hanged run:

ffmpeg version 0.10.2 Copyright (c) 2000-2012 the FFmpeg developers
  built on Mar 20 2012 04:34:50 with gcc 4.4.6 20110731 (Red Hat 4.4.6-3)
  configuration: --prefix=/usr --libdir=/usr/lib64 --shlibdir=/usr/lib64 --mandir=/usr/share/man --enable-shared --enable-runtime-cpudetect --enable-gpl --enable-version3 --enable-postproc --enable-avfilter --enable-pthreads --enable-x11grab --enable-vdpau --disable-avisynth --enable-frei0r --enable-libopencv --enable-libdc1394 --enable-libdirac --enable-libgsm --enable-libmp3lame --enable-libnut --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libxavs --enable-libxvid --extra-cflags='-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic -fPIC' --disable-stripping
  libavutil      51. 35.100 / 51. 35.100
  libavcodec     53. 61.100 / 53. 61.100
  libavformat    53. 32.100 / 53. 32.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 61.100 /  2. 61.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0.  6.100 /  0.  6.100
  libpostproc    52.  0.100 / 52.  0.100


[5]+  Stopped                 ffmpeg -i H264-media-4.264 4.avi

Answer 1

It hangs because after a certain point it can not write to it's output pipe any longer.

When you run a process it has 3 opened pipes for: stdin, stdout and stderr. A pipe has a memory buffer ( 4KB on Linux ) that can hold up to a certain amount of data and the next write operation will pause until some data is read from the other side of the pipe.

Since you never read from stdout and stderr of your child process and FFMpeg outputs quite a lot it will hang at some point.

As explained in the comment above you can simply redirect your ffmpeg output to /dev/null using:

ffmpeg .... > /dev/null 2>&1 < /dev/null

In this case ffmpeg will never output enough data to have the pipe 'hang'.

Another option would be to just close stdin, stdout and stderr of your child process as soon as you launch it.

And yet another option would be to actually read ( and optionally discard ) everything on stdout and stderr of your child process.