Why process output reader returns null data? When same code in other process works correct

Question

I try to get output from ffmpeg process but cant get output. In another processes and commands it works correctly but output returns immideately when start!

        using (var process = new Process())
        {
            process.StartInfo = new ProcessStartInfo()
            {
                FileName = LinkHelper.IPFS_PATH,
                Arguments = cmd,
                UseShellExecute = false,
                CreateNoWindow = true,
                RedirectStandardOutput = true
            };

            process.ErrorDataReceived += FfmpegErrorRecieved;
            process.Start();

            using (StreamReader reader = process.StandardOutput)
            {
                string output = await reader.ReadToEndAsync();
                Console.WriteLine(output);
            }               
            process.WaitForExit();
        }

Before output handle!

Update: As szatmary said ffmpeg use output error instead standard output, so when you initialize process.StandartInfo don't forget initialize property "RedirectStandardError" to TRUE!

Here is correct code:

private async Task DetectFFmpegCamerasAsync()
    {
        var cmd = "-list_devices true -f dshow -i dummy";
        using (var process = new Process())
        {
            process.StartInfo = new ProcessStartInfo()
            {
                FileName = LinkHelper.FFMPEG_PATH,
                Arguments = cmd,
                UseShellExecute = false,
                CreateNoWindow = true,
                RedirectStandardError = true
            };

            process.Start();

            using (StreamReader reader = process.StandardError)
            {
                string output = await reader.ReadToEndAsync();
                Console.WriteLine($"Camera detection output: \n {output}");
            }               
            process.WaitForExit();
        }
    }

zx485 · Accepted Answer · 2019-08-22 17:32:05Z

2

ffmpeg writes its output to stderr and not stdout.
Therefore you must read from standard error instead of standard out.

So use the following lines instead:

using (StreamReader reader = process.StandardError)
{
    string output = await reader.ReadToEndAsync();
    Console.WriteLine(output);
}

edited Aug 22, 2019 at 17:32

zx485

29.1k28 gold badges55 silver badges65 bronze badges

answered Aug 22, 2019 at 15:37

szatmary

31.3k8 gold badges49 silver badges58 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

szatmary Over a year ago

Agree to disagree. I believe the user is smart enough to use this information to fix the issue.

zx485 Over a year ago

Agreed. But then your answer would have been missing the appropriate code sample to show this difference. So I edited your answer. I hope that you agree with the edit.

demoncrate · Accepted Answer · 2019-08-22 12:42:30Z

0

After

process.Start();

Add

process.BeginOutputReadLine();

answered Aug 22, 2019 at 12:42

demoncrate

3902 silver badges14 bronze badges

Collectives™ on Stack Overflow

Why process output reader returns null data? When same code in other process works correct

2 Answers 2

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related