Indy10, Bit messaging between TIdTCPClient and TIdTCPServer.
lazarus / 38-Networking and Web Programming

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shyub · 2022-03-04 16:22

The server receives messages from the client, but the client either
does not accept or hangs.
The sniffer shows that a transmission from the server is in progress.

Server:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Button2Click(Sender: TObject);var  Buf: TIdBytes=nil;begin 
SetLength(Buf, 5);  Buf[0]:=$01; Buf[1]:=$10; Buf[2]:=$00;
Buf[3]:=$22; Buf[4]:=$33;  MCon.Connection.IOHandler.Write(Buf,
5);end;The client does not receive anything:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Timer1Timer(Sender: TObject);var AByte: Byte; Buf:
TIdBytes=nil; n: Integer; S: String='HEX: ';begin 
n:=IdTCPClient1.IOHandler.InputBuffer.Size;  if n>0 then begin   
AByte:=IdTCPClient1.IOHandler.ReadByte;    S:=S+IntToHex(AByte, 2)+'
';    IdTCPClient1.IOHandler.ReadBytes(Buf, 4, False);    for n:=0 to
Length(Buf)-1 do      S:=S+IntToHex(Buf[n], 2)+' ';  end;end;Client
freezes:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Timer1Timer(Sender: TObject);var Buf: TIdBytes=nil; n:
Integer; S: String='HEX: ';begin  if
IdTCPClient1.IOHandler.InputBufferIsEmpty then begin    if
IdTCPClient1.IOHandler.CheckForDataOnSource(1000) then begin     
Timer1.Enabled:=false;      IdTCPClient1.IOHandler.ReadBytes(Buf, 5,
False);      for n:=0 to Length(Buf)-1 do        S:=S+IntToHex(Buf[n],
2)+' ';      Timer1.Enabled:=true;    end;  end; end;I am attaching
the source code:

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rvk · 2022-03-04 17:12

Quote from: shyub on March 04, 2022, 05:22:00 pmCode: Pascal
 [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i   if
IdTCPClient1.IOHandler.InputBufferIsEmpty then begin    if
IdTCPClient1.IOHandler.CheckForDataOnSource(1000) then begin     
Timer1.Enabled:=false;      IdTCPClient1.IOHandler.ReadBytes(Buf, 5,
False); Just a small question/observation:
Why are you continuing to check for data (CheckForDataOnSource) when
you just established that the inputbuffer is empty
(InputBufferIsEmpty) ?

In that case the Timer1Timer will hang on CheckForDataOnSource for
1000ms before it times out.
In that time the Timer1 might fire again and check for data again
(because you disable the timer in the wrong place).

Perhaps you meant something like this:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Timer1Timer(Sender: TObject);var Buf: TIdBytes=nil; n:
Integer; S: String='HEX: ';begin  if NOT
IdTCPClient1.IOHandler.InputBufferIsEmpty then // NOT will make sure
to only do this when there is data  begin    Timer1.Enabled:=false; //
directly disable, don't wait for reading of the line, there is already
data so disable the timer    try      if
IdTCPClient1.IOHandler.CheckForDataOnSource(1000) then      begin    
  IdTCPClient1.IOHandler.ReadBytes(Buf, 5, False);        for n:=0 to
Length(Buf)-1 do          S:=S+IntToHex(Buf[n], 2)+' ';      end;   
finally      Timer1.Enabled:=true;    end;  end;end;
BTW: I would never do this in a TTimer but in TThread but that is
somewhat harder to program and needs some knowledge of threads.

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Remy Lebeau · 2022-03-04 17:43

Quote from: shyub on March 04, 2022, 05:22:00 pmServer:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i
MCon.Connection.IOHandler.Write(Buf, 5);
That is not really thread-safe.  Where is MCon coming from?  You
really should avoid writing any data to a TIdContext from outside of
the OnExecute event, especially if the event sends its own data.  You
can easily corrupt your communications this way.

Quote from: shyub on March 04, 2022, 05:22:00 pmThe client does not
receive anything:

That is because in that example, you are never attempting to read from
the socket, so the InputBuffer.Size is always 0.

Quote from: shyub on March 04, 2022, 05:22:00 pmClient freezes:

The only problem I see with that example is that
CheckForDataOnSource() may receive fewer than 5 bytes, and then
ReadBytes() will have to block waiting for the remaining bytes to
arrive.  But as long as the server is always sending 5 bytes at a
time, that really shouldn't be an issue, unless you are running on a
really slow network.

Quote from: rvk on March 04, 2022, 06:12:55 pmWhy are you continuing
to check for data (CheckForDataOnSource) when you just established
that the inputbuffer is empty (InputBufferIsEmpty) ?

Because CheckForDataOnSource() performs an actual read of the socket
and puts any received bytes into the InputBuffer.

Quote from: rvk on March 04, 2022, 06:12:55 pmPerhaps you meant
something like this:

That is incorrect usage.  I would suggest something more like this
instead:

Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Timer1Timer(Sender: TObject);var  Buf: TIdBytes=nil;  n:
Integer;  S: String;begin  if
IdTCPClient1.IOHandler.InputBufferIsEmpty then begin    if not
IdTCPClient1.IOHandler.CheckForDataOnSource(0) then begin     
IdTCPClient1.IOHandler.CheckForDisconnect;      Exit;    end;  end; 
Timer1.Enabled := false;  IdTCPClient1.IOHandler.ReadBytes(Buf, 5,
False);  S := 'HEX: ';  for n := 0 to Length(Buf)-1 do    S := S +
IntToHex(Buf[n], 2) + ' ';  Timer1.Enabled := true;end;
Quote from: rvk on March 04, 2022, 06:12:55 pmBTW: I would never do
this in a TTimer but in TThread

Agreed.  However, the use of InputBufferIsEmpty() and
CheckForDataOnSource() can makes this somewhat safe in the main UI
thread, by reducing the time spent waiting for bytes.  Though, the
ReadBytes() could still block, which would not be good, but the use of
TIdAntiFreeze can mitigate that.

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rvk · 2022-03-04 18:13

Quote from: Remy Lebeau on March 04, 2022, 06:43:35 pmQuote from: rvk
on March 04, 2022, 06:12:55 pmWhy are you continuing to check for data
(CheckForDataOnSource) when you just established that the inputbuffer
is empty (InputBufferIsEmpty) ?
Because CheckForDataOnSource() performs an actual read of the socket
and puts any received bytes into the InputBuffer.
Woops, my bad   I thought InputBufferIsEmpty was more on the raw side
to check if there is data on the line (I work mostly with Synapse   ).
In that case indeed my code doesn't make sense.

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shyub · 2022-03-04 18:54

Many thanks!
And what does "0" mean in "CheckForDataOnSource(0)" and in what cases
should "False" or "True" be specified in "ReadBytes(Buf, -1, False)"?

In a real program, packets will be from 7 to 50 bytes.
I did it like this, it works:
Code: Pascal  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i procedure
TForm1.Timer1Timer(Sender: TObject);var AByte: Byte; Buf:
TIdBytes=nil; n: Integer; S: String='HEX: ';begin  if
IdTCPClient1.IOHandler.InputBufferIsEmpty then begin // Наличие данных
в буфере.    // Перенос байтов из сокета во входной буффер.    if not
IdTCPClient1.IOHandler.CheckForDataOnSource(0) then begin     
IdTCPClient1.IOHandler.CheckForDisconnect;      Exit;    end;  end; 
Timer1.Enabled:=false;   AByte:=IdTCPClient1.IOHandler.ReadByte; 
S:=S+IntToHex(AByte, 2)+'_';   IdTCPClient1.IOHandler.ReadBytes(Buf,
-1, False);  for n:=0 to Length(Buf)-1 do    S:=S+IntToHex(Buf[n],
2)+' ';  Edit1.Text:=S;  Timer1.Enabled:=true;end;

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Remy Lebeau · 2022-03-07 19:15

Quote from: shyub on March 04, 2022, 07:54:35 pmAnd what does "0" mean
in "CheckForDataOnSource(0)"

It is a timeout in milliseconds for how long CheckForDataOnSource()
waits for any new bytes to be available on the underlying socket.  If
any bytes arrive in that timeframe, they are read from the socket and
put into the IOHandler.InputBuffer.

Quote from: shyub on March 04, 2022, 07:54:35 pmand in what cases
should "False" or "True" be specified in "ReadBytes(Buf, -1, False)"?

That is the AAppend parameter.  When True, the bytes read are appended
to the end of the destination array, preserving any existing bytes
already present in the array.  When False, any existing bytes in the
array (up to the specified count) are overwritten with the bytes read.

Quote from: shyub on March 04, 2022, 07:54:35 pmIn a real program,
packets will be from 7 to 50 bytes.
I did it like this, it works:

What is the purpose of the lead byte?  You are not using it to help
you decide how many bytes to read.  You are passing -1 for the
AByteCount parameter of ReadBytes(), so it is going to return to you
whatever arbitrary bytes happen to be available at that exact moment. 
Are you sure that is what you really want?  If the Sender sends 7
bytes, how do you know to read 7 bytes?  If the Sender sends 50 bytes,
how do you know to read 50 bytes?  The answer is - you don't, because
you didn't design your communication to work that way.