CODEWITHSUNDEEP
linuxpython-3.xshellsubprocess
mkiever
mkiever

Reputation: 891

why does my ffmpeg process wait forever without stdin parameter to subprocess.run?

I am using the python subprocess module to call ffmpeg like this:

def ffmpeg_wait(self, fname, args):
    ffmpeg = subprocess.run([
        'ffmpeg', '-v', 'quiet', '-i', fname, *args],
        stdin=subprocess.DEVNULL,
        stdout=subprocess.PIPE, stderr=subprocess.PIPE,
        universal_newlines=True)
    print('terminated')
    (out,err) = (ffmpeg.stdout, ffmpeg.stderr)
    return out + err

This works fine. An example call:

self.ffmpeg_wait(fname,
    ['-r', str(sps), '-s', '320x240', '-f', 'image2', 'tmp/%06d.png'])

What I want to know is why it doesn't work when I omit the stdin parameter. In this case the ffmpeg process waits forever iff I start my program as a background process in the shell. Running in the foreground works.

I guess this has something to do with how the shell handles stdio channels and how they are inherited by subprocesses, but I cannot figure it out. I tried (unsuccessfully) to re-establish this behaviour in the shell (using redirection).

Any explanation or documentation pointers?

PS: ffprobe does not do this. It always works even without stdin parameter. So it's also something in ffmpeg which triggers this behaviour.

Upvotes: 1

Views: 608

Answers (1)

elixenide
elixenide

Reputation: 44851

If you don’t tell it somehow to ignore stdin, it expects the image itself (not just a file name) to come from stdin, as if you sat there and typed in the image byte by byte. This is essential to “piping” commands in *nix systems, but it means that many commands will appear to hang if you don’t explicitly direct them to some input stream other than stdin.

Upvotes: 1

Related Questions