Why do two sub-processes stop each other from working?

Question

I'm having a problem getting two subprocesses to run together.

The first subprocesss is a transcode of a video stream:

subprocess.Popen("ffmpeg -i input output", shell=True)

I need this to run in the background of my program, transcoding video from my IP camera into a mjpeg stream.

The second subprocess is the Openalpr Daemon that looks at the mjpeg stream and returns car license plates it sees in the stream.

 subprocess.Popen("alprd -f", shell=True)

Here is a sample piece of python test code that tries to run both subprocesses:

import subprocess
subprocess.Popen("ffmpeg -i input output", shell=True)
subprocess.Popen("alprd -f", shell=True)

When i do this, the ffmpeg transcoding works fine, i can view the transcoded mjpeg stream and i can see ffmpegs verbose output in the console. However, the alprd daemon seems to not return any number plates as expected. In fact, i can't see any output from alprd in the console window.

If i run the above code with just one subprocess it works e.g.

import subprocess
subprocess.Popen("ffmpeg -i input output", shell=True)

works fine, as does:

import subprocess
subprocess.Popen("alprd -f", shell=True)

If i run either of the two working code snippets above - and at the same time run the other command in a separate linux terminal, it all works.

I'm clearly not understanding something with subprocesses, They are clearly conflicting with each other, but Can anyone explain what is happening and how to resolve the problem?

Thanks!

Answer 1

It's likely that ffmpeg and alprd are both trying to interact with the same stdin/stdout file handles. To solve this, create separate pipes for one or both subprocesses to use as stdin/stdout. Then they can interact with them without interfering with each other.

import subprocess

with open('ffmpeg-output.txt', 'w') as ffmpeg_output:
    ffmpeg = subprocess.Popen(
        ['ffmpeg', '-i', 'input', 'output'],
        stdin=subprocess.PIPE,
        stdout=ffmpeg_output,
        stderr=subprocess.STDOUT)
    # We won't be sending any input into ffmpeg's stdin, so close it.
    ffmpeg.stdin.close()

    # alprd inherits stdin, stdout, and stderr from the current process.
    alprd = subprocess.Popen(['alprd', '-f'])

    # Wait for the subprocesses to finish.
    ffmpeg.wait()
    alprd.wait()