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| -rw-r--r-- | doc/faq.texi | 65 | ||||
| -rw-r--r-- | doc/ffmpeg.texi | 1 |
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diff --git a/doc/faq.texi b/doc/faq.texi index dcaf89c6d2..73624c647e 100644 --- a/doc/faq.texi +++ b/doc/faq.texi @@ -501,6 +501,71 @@ ffmpeg -i ega_screen.nut -vf setdar=4/3 ega_screen_anamorphic.nut ffmpeg -i ega_screen.nut -aspect 4/3 -c copy ega_screen_overridden.nut @end example +@anchor{background task} +@section How do I run ffmpeg as a background task? + +ffmpeg normally checks the console input, for entries like "q" to stop +and "?" to give help, while performing operations. ffmpeg does not have a way of +detecting when it is running as a background task. +When it checks the console input, that can cause the process running ffmpeg +in the background to suspend. + +To prevent those input checks, allowing ffmpeg to run as a background task, +use the @url{ffmpeg.html#stdin-option, @code{-nostdin} option} +in the ffmpeg invocation. This is effective whether you run ffmpeg in a shell +or invoke ffmpeg in its own process via an operating system API. + +As an alternative, when you are running ffmpeg in a shell, you can redirect +standard input to @code{/dev/null} (on Linux and Mac OS) +or @code{NUL} (on Windows). You can do this redirect either +on the ffmpeg invocation, or from a shell script which calls ffmpeg. + +For example: + +@example +ffmpeg -nostdin -i INPUT OUTPUT +@end example + +or (on Linux, Mac OS, and other UNIX-like shells): + +@example +ffmpeg -i INPUT OUTPUT </dev/null +@end example + +or (on Windows): + +@example +ffmpeg -i INPUT OUTPUT <NUL +@end example + +@section How do I prevent ffmpeg from suspending with a message like @emph{suspended (tty output)}? + +If you run ffmpeg in the background, you may find that its process suspends. +There may be a message like @emph{suspended (tty output)}. The question is how +to prevent the process from being suspended. + +For example: + +@example +% ffmpeg -i INPUT OUTPUT &> ~/tmp/log.txt & +[1] 93352 +% +[1] + suspended (tty output) ffmpeg -i INPUT OUTPUT &> +@end example + +The message "tty output" notwithstanding, the problem here is that +ffmpeg normally checks the console input when it runs. The operating system +detects this, and suspends the process until you can bring it to the +foreground and attend to it. + +The solution is to use the right techniques to tell ffmpeg not to consult +console input. You can use the +@url{ffmpeg.html#stdin-option, @code{-nostdin} option}, +or redirect standard input with @code{< /dev/null}. +See FAQ +@ref{background task, @emph{How do I run ffmpeg as a background task?}} +for details. + @chapter Development @section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat? diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi index 9a90d7327a..d767e48c44 100644 --- a/doc/ffmpeg.texi +++ b/doc/ffmpeg.texi @@ -470,6 +470,7 @@ the encoding process. It is made of "@var{key}=@var{value}" lines. @var{key} consists of only alphanumeric characters. The last key of a sequence of progress information is always "progress". +@anchor{stdin option} @item -stdin Enable interaction on standard input. On by default unless standard input is used as an input. To explicitly disable interaction you need to specify |