Re: [fpc-pascal] How to poll for a byte in Input?

[tcoq]

Chad, 
Thank you very much. I will try your approach, as I'm interested in the piping.

I also did some experimenting with SetConsoleMode, and also with 
SetFileAttributes, and with ReadFileEx. But no answer there. Still blocks. 

Best regards,
Thierry

----- Mail Original -----
De: "Chad Berchek" <ad...@vobarian.com>
À: "FPC-Pascal users discussions" <fpc-pascal@lists.freepascal.org>
Envoyé: Samedi 10 Décembre 2011 04h37:51 GMT +01:00 Amsterdam / Berlin / Berne 
/ Rome / Stockholm / Vienne
Objet: Re: [fpc-pascal] How to poll for a byte in Input?

On 12/9/2011 9:44 AM, tcoq wrote:
> Hello,
> I'm trying to poll the input stream to see whether there is a new character 
> available, without blocking my software.

Maybe TInputPipeStream.NumBytesAvailable can help. You can find it in 
the documentation of the Pipes unit.

I tried an experiment to make sure it would work. I included the code 
for the experiment below. It works when stdin is from another process. 
It does not seem to work if stdin is from the console and I'm not sure 
why. I tried adding the Windows API call SetConsoleMode(StdInputHandle, 
1) to disable line-input mode but it didn't seem to make any difference.

Regarding WaitForSingleObject, there are some comments on the MSDN 
documentation page regarding the use of this function for console input. 
In particular, one observation was that if the console is in line-input 
mode, WaitForSingleObject can return but ReadConsole will still block 
until a carriage return is entered. Perhaps the same is true for 
ReadFile. I have not tested this. See:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms687032%28v=vs.85%29.aspx

Code for the test: There are two programs. You can run WriteToPipe to 
see writing to the stdin stream of one process from another process. You 
can also test piped input using TestPipes by typing a command like:

type TestPipes.pas | TestPipes

However this does not seem to work:

TestPipes < TestPipes.pas

I don't know why exactly it doesn't appear to work with redirected input 
from a file or from the console. By the way, I'm using Windows XP Pro 
64-bit. Here is the test code.

----- WriteToPipe.pas -----

program WriteToPipe;

{$MODE ObjFpc}

uses
   SysUtils,
   Process,
   Windows;

var
   AProc: TProcess;
   AString: string;
   ACount: Integer;
begin
   WriteLn('This will take about 20 seconds.');
   AProc := TProcess.Create(nil);
   try
     AProc.CommandLine := 'TestPipes.exe';
     AProc.Options := [poUsePipes];
     AProc.Execute;
     try
       for ACount := 0 to 5 do
       begin
         AString := IntToStr(ACount);
         AProc.Input.WriteBuffer(AString[1], Length(AString));
         Sleep (3000);
       end;
     finally
       AProc.Terminate(0);
     end;
   finally
     AProc.Free;
   end;
end.

----- End -----

----- TestPipes.pas -----

program TestPipes;

{$MODE ObjFpc}

uses
   Classes,
   Pipes,
   SysUtils,
   Windows;

var
   AStream: TInputPipeStream;
   AString: string;
   AFile: TextFile;
begin
   Assign(AFile, 'output.txt');
   Rewrite(AFile);
   try
     AStream := TInputPipeStream.Create(StdInputHandle);
     while True do
     begin
       if AStream.NumBytesAvailable > 0 then
       begin
         SetLength(AString, AStream.NumBytesAvailable);
         AStream.ReadBuffer(AString[1], Length(AString));
         WriteLn(AFile, 'I just got this: ', AString);
       end else
       begin
         WriteLn(AFile, 'Waiting...');
         Sleep(1000);
       end;
     end;
   finally
     Close(AFile);
   end;
end.

----- End -----
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