ndrean
Can't make erlexec work with stdin as input
I want to transform programmatically an image with FFmpeg. I know you have nice libraries Image, Vix, Stb_image but I want to use the capacity of FFmpeg to take buffer as input. I don’t see how System.cmd nor Port can accept inputs from stdin so I sought for something.
I found Porcelain and erlexec. I saw the last commit was 4 years ago for Porcelaine. Not saying it’s bad but erlexec has its last commit 2 months ago.
I can do it with Porcelaine but not with erlexec and I don’t see why. Maybe someone has some experience with it.
file = File.read!("img.png")
porcelaine = Porcelain.spawn("ffmpeg", ["-i", "-", "img.jpg"], in: :receive, out: :stream)
{:input, data} = Porcelain.Process.send_input(porcelaine, file)
File.write!("img.jpg", file)
#=> success
but not erlexec: “bad arguments”.
{:ok, pid, os_pid} = :exec.run("ffmpeg -i - img.jpg", [:stdin, stdout: "img2.jpg"])
:ok = :exec.send(os_pid, file)
[error] GenServer :exec terminating
** (stop) :einval
Last message: {:EXIT, #Port<0.6>, :einval}
Is it related to the way you pass strings to the function?
Marked As Solved
saleyn
I see. The issue is that ffmpeg needs to know that the input sent to it is complete. Otherwise, it’ll sit waiting for more data.
When you use pipes in the shell, the head of the pipe (cat img.png) closes the pipe upon sending the entire content of the file, which helps ffmpeg detect the eof.
In case of erlexec you need to tell the child process that there’s an eof condition like this:
> f = File.read!("test.png")
> {:ok, _, i} = :exec.run_link("ffmpeg -y -i - img.jpg", [:stdin, :stderr])
> :exec.send(i, f)
> :exec.send(i, :eof)
> flush # This will print out the output of ffmpeg written to stderr and sent to the caller's terminal
This will fix your issue.
It is illustrated here.
Also Liked
akash-akya
@ndrean I created Exile & ExCmd specifically to support moving large amount of data between external process. If you are interested you can check them out.
I created a Livebook sometime back using ffmpeg : Community Livebooks Thread - Share Yours! - #4 by akash-akya
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