[Solved] Tprocess, detach program, background, Omxplayer, UDP socket

[chrnobel]

Hi.

I am trying to make an embedded video player on the Raspberry Pi, which I am going to start over the network.

Using Synapse, I can easily create a program that listens on a socket, and writes what is in the buffer like this:

Code: Pascal  [Select][+][-]

1. var
2.   x:integer;
3.  
4. procedure MulticastRecvTest;
5. var
6.   rcvsock:TUDPBlockSocket;
7.   buf:string;
8. begin
9.   rcvsock:=TUDPBlockSocket.Create;
10.   try
11.     rcvsock.createsocket;
12.     rcvsock.Bind('0.0.0.0','22401');
13.     rcvsock.AddMulticast('224.0.0.100');
14.     buf:=rcvsock.RecvPacket (60000);
15.     writeln(buf);
16.     writeln(DateTimeToTimeStamp(Now).Time);
17.   finally
18.     rcvsock.free;
19.   end;
20. end;
21.  
22. begin
23.   for x:=1 to 10 do
24.   MulticastRecvTest;
25. end.      
26.  

Works like a charm, loops ten times and output result if anything is send to the socket.

Then I want to combine it with this program, which can be run from commandline, and starts a omxplayer, showing the video, and detaches:

Code: Pascal  [Select][+][-]

1. procedure runshow;
2. var
3.   AProcess: TProcess;
4.  
5. begin
6.    AProcess := TProcess.Create(nil);  
7.    try
8.      AProcess.Executable:='omxplayer';
9.      AProcess.Parameters.Add('/home/admin/videos/Engl.mov');
10.      AProcess.Options := [];
11.      AProcess.execute;
12.    finally
13.     Aprocess.free;
14.    end;
15. end;
16.  
17. begin
18.   runshow;
19. end.
20.  

This also works as intended, executing the program (as root, due to Omxplayer, but this is no problem, as it is intended to work from startup) starts Omxplayer, and then detaches stdin as expected.

I still gets messages to stdout and stderr - this is a minor problem I have to find a solution to.

However my big problem is, if I try to combine those two program like this:

Code: Pascal  [Select][+][-]

1. var
2.   x:integer;
3.  
4. procedure MulticastRecvTest;
5. var
6.   rcvsock:TUDPBlockSocket;
7.   AProcess: TProcess;
8.   buf:string;
9. begin
10.   rcvsock:=TUDPBlockSocket.Create;
11.   try
12.     rcvsock.createsocket;
13.     rcvsock.Bind('0.0.0.0','22401');
14.     rcvsock.AddMulticast('224.0.0.100');
15.     buf:=rcvsock.RecvPacket (60000);
16.     if buf='start' then
17.     begin
18.       AProcess := TProcess.Create(nil);
19.       try
20.         AProcess.Executable:='omxplayer';
21.         AProcess.Parameters.Add('/home/admin/videos/Engl.mov');
22.         AProcess.Options := [];
23.         AProcess.execute;
24.       finally
25.         Aprocess.free;
26.       end;
27.     end;
28.   finally
29.     rcvsock.free;
30.   end;
31. end;
32.  
33. begin
34.   for x:=1 to 10 do
35.   MulticastRecvTest;
36. end.
37.  

It does not detach the Omxplayer.

What am I doing wrong?

[Zaxxon]

what happens if you try

Code: Pascal  [Select][+][-]

1.         AProcess.Executable:='nohup';
2.         AProcess.Parameters.Add('omxplayer');
3.         AProcess.Parameters.Add('/home/admin/videos/Engl.mov');
4.         AProcess.Options := [];
5.  

?

[chrnobel]

what happens if you try

Code: Pascal  [Select][+][-]

1.         AProcess.Executable:='nohup';
2.         AProcess.Parameters.Add('omxplayer');
3.         AProcess.Parameters.Add('/home/admin/videos/Engl.mov');
4.         AProcess.Options := [];
5.  

?

Tprocess can not do any piping, so starting piping omxplayer into nohup is not a solution as far as I understand.

However it gave me an idea, so  tried this instead:

Firstly I created a small script, that I was sure did piping and detaching as expected:

Code: Pascal  [Select][+][-]

1. #!/bin/bash
2. nohup omxplayer /home/admin/videos/Engl.mov < /dev/null > /dev/null 2>&1 &
3. exit;

Then I tried with RunCommand instead of using TProcess, as it can handle scripts:

Code: Pascal  [Select][+][-]

1. var
2.   x:integer;
3.   s:ansistring;
4.  
5. procedure MulticastRecvTest;
6. var
7.   rcvsock:TUDPBlockSocket;
8.   buf:string;
9. begin
10.   rcvsock:=TUDPBlockSocket.Create;
11.   try
12.     rcvsock.createsocket;
13.     rcvsock.Bind('0.0.0.0','22401');
14.     rcvsock.AddMulticast('224.0.0.100');
15.     buf:=rcvsock.RecvPacket (60000);
16.     if buf='stop' then
17.         fpsystem('killall -9 omxplayer.bin');
18.     if buf='start' then
19.         runcommand('/bin/bash',['scrstart','&'],s);    
20.   finally
21.     rcvsock.free;
22.   end;
23. end;
24.  
25. begin
26.   for x:=1 to 10 do
27.     MulticastRecvTest;
28. end.

However, the result remains the same, the program do not detach as expected, and does not react to new commands (start/stop) send to it.

But if I removed the starting of an external process out of the socket try-finally loop, and executed that afterwards, then it works:

Code: Pascal  [Select][+][-]

1. var
2.   x:integer;
3.   s:ansistring;
4.  
5. procedure MulticastRecvTest;
6. var
7.   rcvsock:TUDPBlockSocket;
8.   buf:string;
9. begin
10.   rcvsock:=TUDPBlockSocket.Create;
11.   try
12.     rcvsock.createsocket;
13.     rcvsock.Bind('0.0.0.0','22401');
14.     rcvsock.AddMulticast('224.0.0.100');
15.     buf:=rcvsock.RecvPacket (60000);
16.   finally
17.     rcvsock.free;
18.   end;
19.     if buf='stop' then
20.         fpsystem('killall -9 omxplayer.bin');
21.     if buf='start' then
22.         runcommand('/bin/bash',['scrstart','&'],s);
23. end;

Now I do only have one small issue, because even though I am able to detach the omxplayer from my program, and stdin, stdout, strerr is send to null, the program is still owner of omxplayer, so if I stop the program, it also stops the omxplayer.

This is a little bit odd?

Actually I do have two issues, but this a little OT, as I also would like to pipe keyboard commands like 'p' for pause into omxplayer - I can do that on Raspbian, but I use Alpine Linux, and for some strange reason it does not work there (neither do DBUS).
Alpine Linux is really good, but unfortunately there is no forum, which is a shame.

[Zaxxon]

what happens if you try

Code: Pascal  [Select][+][-]

1.         AProcess.Executable:='nohup';
2.         AProcess.Parameters.Add('omxplayer');
3.         AProcess.Parameters.Add('/home/admin/videos/Engl.mov');
4.         AProcess.Options := [];
5.  

?

Tprocess can not do any piping, so starting piping omxplayer into nohup is not a solution as far as I understand.

Are you sure?
I've used nohup to run vlc to open mp3 files, with tprocess. And that worked with no problem.

[winni]

If you set the right options it will work:

Code: Pascal  [Select][+][-]

1. AProcess.Options := [poUsePipes]

Winni

[chrnobel]

If you set the right options it will work:

Code: Pascal  [Select][+][-]

1. AProcess.Options := [poUsePipes]

Winni

I will give it a try, maybe I can also control the pausing of the Omxplayer this way, as discussed in another thread, but I am not having to high expectations.

[chrnobel]

This is really really strange.

First of all, I have made a simple shell script, just for testing purposes:

Code: Pascal  [Select][+][-]

1. #!/bin/sh
2. echo "start script"
3. sleep 2
4. echo "2"
5. sleep 2
6. echo "4"
7. sleep 2
8. echo "6"
9. sleep 2
10. echo "8"
11. sleep 2
12. echo "10"
13. sleep 2
14. echo "12"
15. sleep 2
16. echo "end script"

When executed from console, no problem, it counts for 14 seconds.

I can also call it from at small program, then I start the script by use of TProcess, here made with at loop that rapidly starts my script ten times, meaning ten running processes:

Code: Pascal  [Select][+][-]

1. var
2.   x:integer;
3.  
4. procedure waitforcall;
5. var
6.   AProcess: TProcess;
7. begin
8.   writeln('udp start');
9.   AProcess := TProcess.Create(nil);  
10.   try
11.     AProcess.InheritHandles := False;
12.     AProcess.Executable:='/bin/sh';
13.     AProcess.Parameters.Add('-c');
14.     Aprocess.Parameters.Add('./startshow');
15.     AProcess.Options:=[];
16.     AProcess.execute;
17.   finally
18.     Aprocess.free;
19.   end;
20.   writeln('udp end');
21. end;
22.  
23. begin
24.   for x:=1 to 10 do
25.     waitforcall;
26. end.

So far so good.

Then I want to control this via a multicast, so I created this UDP Multicast listener:

Code: Pascal  [Select][+][-]

1. procedure waitforcall;
2. var
3.   rcvsock:TUDPBlockSocket;
4.   buf:string;
5.  
6. begin
7.   rcvsock:=TUDPBlockSocket.Create;
8.   try
9.     rcvsock.createsocket;
10.     rcvsock.Bind('0.0.0.0','22401');
11.     rcvsock.AddMulticast('239.66.16.10');
12.     buf:=rcvsock.RecvPacket (60000);
13.     if buf='startshow' then
14.       writeln(buf);
15.   finally
16.     rcvsock.free;
17.   end;
18. end;
19.  
20. begin
21.   while true do
22.     waitforcall;
23. end.

This also works totally as expected, as fast as I can send "startshow" telegrams over the UDP socket, is writes it on the screen.

However, if I combine the two program like this:

Code: Pascal  [Select][+][-]

1. procedure waitforcall;
2. var
3.   rcvsock:TUDPBlockSocket;
4.   buf:string;
5.   AProcess: TProcess;
6.  
7. begin
8.   rcvsock:=TUDPBlockSocket.Create;
9.   try
10.     rcvsock.createsocket;
11.     rcvsock.Bind('0.0.0.0','22401');
12.     rcvsock.AddMulticast('239.66.16.10');
13.     buf:=rcvsock.RecvPacket (60000);
14.     if buf='startshow' then
15.     begin
16.       writeln('udp start');
17.       AProcess := TProcess.Create(nil);  
18.       try
19.         AProcess.InheritHandles := False;
20.         AProcess.Executable:='/bin/sh';
21.         AProcess.Parameters.Add('-c');
22.         Aprocess.Parameters.Add('./startshow');
23.         AProcess.Options:=[];
24.         AProcess.execute;
25.       finally
26.         Aprocess.free;
27.       end;
28.       writeln('udp end');
29.     end;
30.   finally
31.     rcvsock.free;
32.   end;
33. end;
34.  
35. begin
36.   while true do
37.     waitforcall;
38. end.

Then the fun begins, because I would expect that for every time I send my UDP telegram, it should execute the startshow command, and start it as a new detached process - but no, it has to wait 14 seconds before it is ready for the next command.

And setting the recsock timeout to a lower value makes no difference (and using poUsePipes just sends the output from script into the blue, the problem remains the same, it takes 14 seconds).

The really, really odd thing is, that it actually outputs:

Udp start
Udp end
Script start
2
....
12
Script end

Which means that the script actually is detached, but still it blocks?

So why is it falling over its own feets?

[winni]

Hi!

I think Tprocess blocks 12 seconds.
Use TAsyncProcess instead.

Winni

[chrnobel]

I think Tprocess blocks 12 seconds.

No, as my read example show, it runs the procedure ten times without blocking

Use TAsyncProcess instead.

TAsyncProcess is a LCL component, and it have to be pure FPC.

BUT I have found out what the problem is - it is not legal / bad practice to nest try sentences.

So instead I moved the finally to right after buf have been filled, and then it works:

Code: Pascal  [Select][+][-]

1. procedure waitforcall;
2. var
3.    rcvsock:TUDPBlockSocket;
4.    buf:string;
5.    AProcess: TProcess;
6.    
7. begin
8.    rcvsock:=TUDPBlockSocket.Create;
9.    try
10.      rcvsock.createsocket;
11.      rcvsock.Bind('0.0.0.0','22401');
12.      rcvsock.AddMulticast('239.66.16.10');
13.      buf:=rcvsock.RecvPacket (60000);
14.   finally
15.      rcvsock.free;
16.   end;
17.   if buf='startshow' then
18.   begin
19.      writeln('udp start');
20.      AProcess := TProcess.Create(nil);  
21.      try
22.        AProcess.InheritHandles := False;
23.        AProcess.Executable:='/bin/sh';
24.        AProcess.Parameters.Add('-c');
25.        Aprocess.Parameters.Add('./startshow');
26.        AProcess.Options:=[];
27.        AProcess.execute;
28.      finally
29.        Aprocess.free;
30.      end;
31.      writeln('udp end');
32.   end;
33. end;
34.  

[chrnobel]

I discovered that the above solution revealed another problem: a trail of defunct processes.

So I had to come up with another solution, and ended actually up (again) with fpsystem - but the important part is that the call is outside the try loop:

Code: Pascal  [Select][+][-]

1. begin
2.   rcvsock:=TUDPBlockSocket.Create;
3.   try
4.     rcvsock.createsocket;
5.     rcvsock.Bind('0.0.0.0','22401');
6.     rcvsock.AddMulticast('239.66.16.10');
7.     buf:=rcvsock.RecvPacket (6000);
8.   finally
9.     rcvsock.free;
10.   end;
11.   if buf='startshow' then
12.   fpsystem('/bin/sh ./startshow &');
13.   if buf='pauseshow' then
14.   fpsystem('/bin/sh ./pauseshow &');
15. end;
16.