Reading console output from mplayer to parse track's position/length - linux

Question

When you run mplayer, it will display the playing track's position and length (among some other information) through, what I'd assume is, stdout.
Here's a sample output from mplayer:
MPlayer2 2.0-728-g2c378c7-4+b1 (C) 2000-2012 MPlayer Team
Cannot open file '/home/pi/.mplayer/input.conf': No such file or directory
Failed to open /home/pi/.mplayer/input.conf.
Cannot open file '/etc/mplayer/input.conf': No such file or directory
Failed to open /etc/mplayer/input.conf.
Playing Bomba Estéreo - La Boquilla [Dixone Remix].mp3.
Detected file format: MP2/3 (MPEG audio layer 2/3) (libavformat)
[mp3 # 0x75bc15b8]max_analyze_duration 5000000 reached
[mp3 # 0x75bc15b8]Estimating duration from bitrate, this may be inaccurate
[lavf] stream 0: audio (mp3), -aid 0
Clip info:
album_artist: Bomba Estéreo
genre: Latin
title: La Boquilla [Dixone Remix]
artist: Bomba Estéreo
TBPM: 109
TKEY: 11A
album: Unknown
date: 2011
Load subtitles in .
Selected audio codec: MPEG 1.0/2.0/2.5 layers I, II, III [mpg123]
AUDIO: 44100 Hz, 2 ch, s16le, 320.0 kbit/22.68% (ratio: 40000->176400)
AO: [pulse] 44100Hz 2ch s16le (2 bytes per sample)
Video: no video
Starting playback...
A: 47.5 (47.4) of 229.3 (03:49.3) 4.1%
The last line (A: 47.5 (47.4) of 229.3 (03:49.3) 4.1%) is what I'm trying to read but, for some reason, it's never received by the Process.OutputDataReceived event handler.
Am I missing something? Is mplayer using some non-standard way of outputting the "A:" line to the console?
Here's the code in case it helps:
Public Overrides Sub Play()
player = New Process()
player.EnableRaisingEvents = True
With player.StartInfo
.FileName = "mplayer"
.Arguments = String.Format("-ss {1} -endpos {2} -volume {3} -nolirc -vc null -vo null ""{0}""",
tmpFileName,
mTrack.StartTime,
mTrack.EndTime,
100)
.CreateNoWindow = False
.UseShellExecute = False
.RedirectStandardOutput = True
.RedirectStandardError = True
.RedirectStandardInput = True
End With
AddHandler player.OutputDataReceived, AddressOf DataReceived
AddHandler player.ErrorDataReceived, AddressOf DataReceived
AddHandler player.Exited, Sub() KillPlayer()
player.Start()
player.BeginOutputReadLine()
player.BeginErrorReadLine()
waitForPlayer.WaitOne()
KillPlayer()
End Sub
Private Sub DataReceived(sender As Object, e As DataReceivedEventArgs)
If e.Data = Nothing Then Exit Sub
If e.Data.Contains("A: ") Then
' Parse the data
End If
End Sub

Answer 1

Apparently, the only solution is to run mplayer in "slave" mode, as explained here: http://www.mplayerhq.hu/DOCS/tech/slave.txt
In this mode we can send commands to mplayer (via stdin) and the response (if any) will be sent via stdout.
Here's a very simple implementation that displays mplayer's current position (in seconds):
using System;
using System.Threading;
using System.Diagnostics;
using System.Collections.Generic;
namespace TestMplayer {
class MainClass {
private static Process player;
public static void Main(string[] args) {
String fileName = "/home/pi/Documents/Projects/Raspberry/RPiPlayer/RPiPlayer/bin/Electronica/Skrillex - Make It Bun Dem (Damian Marley) [Butch Clancy Remix].mp3";
player = new Process();
player.EnableRaisingEvents = true;
player.StartInfo.FileName = "mplayer";
player.StartInfo.Arguments = String.Format("-slave -nolirc -vc null -vo null \"{0}\"", fileName);
player.StartInfo.CreateNoWindow = false;
player.StartInfo.UseShellExecute = false;
player.StartInfo.RedirectStandardOutput = true;
player.StartInfo.RedirectStandardError = true;
player.StartInfo.RedirectStandardInput = true;
player.OutputDataReceived += DataReceived;
player.Start();
player.BeginOutputReadLine();
player.BeginErrorReadLine();
Thread getPosThread = new Thread(GetPosLoop);
getPosThread.Start();
}
private static void DataReceived(object o, DataReceivedEventArgs e) {
Console.Clear();
Console.WriteLine(e.Data);
}
private static void GetPosLoop() {
do {
Thread.Sleep(250);
player.StandardInput.Write("get_time_pos" + Environment.NewLine);
} while(!player.HasExited);
}
}
}

Answer 2

I found the same problem with another application that works more or less in a similar way (dbPowerAmp), in my case, the problem was that the process output uses Unicode encoding to write the stdout buffer, so I have to set the StandardOutputEncoding and StandardError to Unicode to be able start reading.
Your problem seems to be the same, because if "A" cannot be found inside the output that you published which clearlly shows that existing "A", then probably means that the character differs when reading in the current encoding that you are using to read the output.
So, try setting the proper encoding when reading the process output, try setting them to Unicode.
ProcessStartInfo.StandardOutputEncoding
ProcessStartInfo.StandardErrorEncoding

Answer 3

Using "read" instead of "readline", and treating the input as binary, will probably fix your problem.
First off, yes, mplayer slave mode is probably what you want. However, if you're determined to parse the console output, it is possible.
Slave mode exists for a reason, and if you're half serious about using mplayer from within your program, it's worth a little time to figure out how to properly use it. That said, I'm sure there's situations where the wrapper is the appropriate approach. Maybe you want to pretend that mplayer is running normally, and control it from the console, but secretly monitor file position to resume it later? The wrapper might be easier than translating all of mplayers keyboard commands into slave mode commands?
Your problem is likely that you're trying to use "readline" from within python on an endless line. That line of output contains \r instead of \n as the line separator, so readline will treat it as a single endless line. sed also fails this way, but other commands (such as grep) treat \r as \n under some circumstances.
Handling of \r is inconsistent, and can't be relied on. For instance, my version of grep treats \r as \n when matching IF output is a console, and uses \n to seperate the output. But if output is a pipe, it treats it as any other character.
For instance:
mplayer TMBG-Older.mp3 2>/dev/null | tr '\r' '\n' | grep "^A: " | sed 's/^A: *\([0-9.]*\) .*/\1/' | tail -n 1
I'm using "tr" here to force it to '\n', so other commands in the pipe can deal with it in a consistent manner.
This pipeline of commands outputs a single line, containing ONLY the ending position in seconds, with decimal point. But if you remove the "tr" command from this pipe, bad things happen. On my system, it shows only "0.0" as the position, as "sed" doesn't deal well with the '\r' line separators, and ALL the position updates are treated as the same line.
I'm fairly sure python doesn't handle \r well either, and that's likely your problem. If so, using "read" instead of "readline" and treating it like binary is probably the correct solution.
There are other problems with this approach though. Buffering is a big one. ^C causes this command to output nothing, mplayer must quit gracefully to show anything at all, as pipelines buffers things, and buffers get discarded on SIGINT.
If you really wanted to get fancy, you could probably cat several input sources together, tee the output several ways, and REALLY write a wrapper around mplayer. A wrapper that's fragile, complicated, and might break every time mplayer is updated, a user does something unexpected, or the name of the file being played contains something weird, SIGSTOP or SIGINT. And probably other things that I haven't though of.