Start two processes and connect them with a pipe in Delphi

Question

I need to launch two external programs in my program and connect the STDOUT of the first one to the STDIN of the second program. How can you achieve this in Delphi (RAD Studio 2

Answer 1

Here is the corrected code to work in Delphi XE. The CommandLine Strings must be variables and also defined above the ExecutePiped function.

    program Parent;

    {$APPTYPE CONSOLE}

    uses
      Windows, SysUtils, Classes;

    var cmd1, cmd2 :string;

    function ExecutePiped(CommandLine1: string; CommandLine2: string):string;
    var
      StartupInfo1, StartupInfo2 : TStartupInfo;
      ProcessInfo1, ProcessInfo2 : TProcessInformation;
      SecurityAttr               : TSecurityAttributes;
      PipeRead, PipeWrite        : THandle;
      Handle                     : Boolean;
      WorkDir                    : String;
    begin
      PipeWrite := 0;
      PipeRead  := 0;
      try
        SecurityAttr.nLength              := SizeOf(SecurityAttr);
        SecurityAttr.bInheritHandle       := True;
        SecurityAttr.lpSecurityDescriptor := nil;

        CreatePipe(PipeRead, PipeWrite, @SecurityAttr, 0);

        FillChar(StartupInfo1, SizeOf(TStartupInfo), 0);
        StartupInfo1.cb          := SizeOf(TStartupInfo);
        StartupInfo1.dwFlags     := STARTF_USESHOWWINDOW or STARTF_USESTDHANDLES;
        StartupInfo1.wShowWindow := SW_HIDE;
        StartupInfo1.hStdInput   := GetStdHandle(STD_INPUT_HANDLE);
        StartupInfo1.hStdOutput  := PipeWrite;
        StartupInfo1.hStdError   := GetStdHandle(STD_ERROR_HANDLE);

        FillChar(StartupInfo2, SizeOf(TStartupInfo), 0);
        StartupInfo2.cb          := SizeOf(TStartupInfo);
        StartupInfo2.dwFlags     := STARTF_USESHOWWINDOW or STARTF_USESTDHANDLES;
        StartupInfo2.wShowWindow := SW_HIDE;
        StartupInfo2.hStdInput   := PipeRead;
        StartupInfo2.hStdOutput  := GetStdHandle(STD_OUTPUT_HANDLE);
        StartupInfo2.hStdError   := GetStdHandle(STD_ERROR_HANDLE);

        FillChar(ProcessInfo1, SizeOf(TProcessInformation), 0);
        FillChar(ProcessInfo2, SizeOf(TProcessInformation), 0);

        WorkDir := '';

        Handle := CreateProcess(nil, PChar(CommandLine2), nil, nil, True, 0, nil, PChar(WorkDir), StartupInfo2, ProcessInfo2);
        Handle := CreateProcess(nil, PChar(CommandLine1), nil, nil, True, 0, nil, PChar(WorkDir), StartupInfo1, ProcessInfo1);

        WaitForSingleObject(ProcessInfo2.hProcess, INFINITE);

      finally

        if PipeRead              <> 0 then CloseHandle(PipeRead);
        if PipeWrite             <> 0 then CloseHandle(PipeWrite);

        if ProcessInfo2.hThread  <> 0 then CloseHandle(ProcessInfo2.hThread);
        if ProcessInfo2.hProcess <> 0 then CloseHandle(ProcessInfo2.hProcess);

        if ProcessInfo1.hThread  <> 0 then CloseHandle(ProcessInfo1.hThread);
        if ProcessInfo1.hProcess <> 0 then CloseHandle(ProcessInfo1.hProcess);

      end;

    end;

    procedure Main;
    begin
      cmd1 := '"child1.exe"';
      cmd2 := '"child2.exe"';
      ExecutePiped(cmd1, cmd2);
    end;

    begin
      try
        Main;
      except
        on E: Exception do
        begin
          ExitCode := 1;
          Writeln(Error, Format('[%s] %s', [E.ClassName, E.Message]));
        end;
      end;
    end.

To test I have modified Child2.pas to write the received text into a file.

    program Child2;

    {$APPTYPE CONSOLE}

    uses
    Windows, SysUtils, Classes;

    procedure Main;
    var S: string;
        OutFile : TextFile;
    begin
      AssignFile(OutFile, 'test.txt');
      Rewrite(OutFile);
      while not Eof(Input) do
      begin
        Readln(S);
        Writeln(OutFile,S);
        //if S <> '' then OutputDebugString(PChar(S));
      end;
      CloseFile(OutFile);
    end;

    begin
      try
        Main;
      except
        on E: Exception do
        begin
          ExitCode := 1;
          Writeln(ErrOutput, Format('[%s] %s', [E.ClassName, E.Message]));
        end;
      end;
    end.