How to: read a fixed number of bytes from a TCP socket in Delphi and Free Pascal

Full source code now available on GitHub. Covering Internet Direct (Indy), Ararat Synapse, and Synapse SynCrtSock.

This is the first part in a series which explores basic TCP socket use cases and presents them in minimal examples, with useful comments regarding not-so-obvious requirements and pitfalls (a.k.a surprising results).

The clients are tested with a small console application (see below).

program FixedLengthClient;

uses
  ClientMainIndy10,
  SysUtils;

const
  CONTENT_LENGTH = 8192;
  SERVER_HOST = '127.0.0.1';
  SERVER_PORT = 30000;

  procedure Test(AExpectedLength: Integer);
  var
    Response: TBytes;
  begin
    WriteLn(Format('try to read %d bytes from %s:%d',
      [AExpectedLength, SERVER_HOST, SERVER_PORT]));
    Response := Read(SERVER_HOST, SERVER_PORT, AExpectedLength);
    WriteLn(Format('received %d bytes', [Length(Response)]));
  end;

begin
  try
    Test(CONTENT_LENGTH);
    Test(CONTENT_LENGTH - 1);
    Test(CONTENT_LENGTH + 1); // (surprise me)
  except
    on E: Exception do
    begin
      WriteLn(E.Message);
    end;
  end;
  ReadLn;
end. 

The first client uses Internet Direct (Indy). Here is the source code:

(*
  This Source Code Form is subject to the terms of the Mozilla Public
  License, v. 2.0. If a copy of the MPL was not distributed with this
  file, You can obtain one at http://mozilla.org/MPL/2.0/.
*)

unit ClientMainIndy10;

interface

uses
  SysUtils;

function Read(AHost: string; APort: Integer; ALength: Integer): TBytes;

implementation

uses
  IdTcpClient, IdGlobal, Classes;

function Read(AHost: string; APort: Integer; ALength: Integer): TBytes;
var
   Client: TIdTCPClient;
begin
   SetLength(Result, ALength);
   Client := TIdTCPClient.Create;
   try
     Client.Host := AHost;
     Client.Port := APort;
     Client.Connect;
     Client.IOHandler.ReadBytes(Result, Length(Result), False);
   finally
     Client.Free;
   end;
end;

end.

Notes

• Included is a test server which responds wiith a fixed-length byte array. The test server requires Indy for compilation.
• Credits: partially inspired by the thread https://en.delphipraxis.net/topic/7935-dl-a-file-from-the-web/
• Fork me on GitHub: https://github.com/michaelJustin/tcpclient-test

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