问题描述
如果我按如下方式运行 ffmpeg:
If I run ffmpeg as follows:
ffmpeg -i H264-media-4.264 4.avi
它工作正常(即 4.avi 创建正常).但是，如果我尝试在后台运行它:
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 &
它挂了！(和 4.avi 从未创建)有什么想法吗?
it hangs! (and 4.avi never created)Any Idea?
注意:问题是在尝试将其作为子进程运行时，python 中类似问题的隔离，并且它也挂了:ff.py 包括
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)
并且运行 ./ff.py 运行正常，./ff.py & 也会挂起.
and running ./ff.py runs OK, ./ff.py & hangs too.
- 系统:CentOS 6.6
- ffmpeg:0.10.2
成功运行的结果:
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%
挂起运行的结果:
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
推荐答案
它挂起是因为在某个点之后它无法再写入它的输出管道.
It hangs because after a certain point it can not write to it's output pipe any longer.
当你运行一个进程时，它有 3 个打开的管道:stdin、stdout 和 stderr.管道有一个内存缓冲区(Linux 上为 4KB)，可以容纳一定数量的数据，下一次写操作将暂停，直到从管道的另一端读取一些数据.
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.
由于您从未从子进程的 stdout 和 stderr 中读取数据，并且 FFMpeg 输出很多，因此它会在某些时候挂起.
Since you never read from stdout and stderr of your child process and FFMpeg outputs quite a lot it will hang at some point.
如上面评论中所述，您可以简单地使用以下命令将 ffmpeg 输出重定向到/dev/null:
As explained in the comment above you can simply redirect your ffmpeg output to /dev/null using:
ffmpeg .... > /dev/null 2>&1 < /dev/null
在这种情况下，ffmpeg 永远不会输出足够的数据来让管道挂起".
In this case ffmpeg will never output enough data to have the pipe 'hang'.
另一种选择是在您启动子进程后立即关闭它的 stdin、stdout 和 stderr.
Another option would be to just close stdin, stdout and stderr of your child process as soon as you launch it.
另一种选择是实际读取(并可选择丢弃)子进程的 stdout 和 stderr 上的所有内容.
And yet another option would be to actually read ( and optionally discard ) everything on stdout and stderr of your child process.
这篇关于ffmpeg 在后台运行时挂起的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！