How do I write to StdOut from a GUI app started from the command line?

Question

I am writing a standard windows app in Delphi 7.

If I was writing a console app, I can call the following to output to the cmd line or output file.

write         


        

Answer 1

I found this very complete article about the whole issue: http://www.boku.ru/2016/02/28/posting-to-console-from-gui-app/

I made a unit to do the AttachConsole, hook the exception handler to mirror messages to console.

To use it, you only need to call ATTACH in your code. It is best to make attaching a commandline option e.g -console

if FindCmdLineSwitch('console',true) then AttachConsole(true,true);

This is for a gui application, and when using this, you must use START /W to launch your program is you expect it to be blocking on the commandline/batch e.g. start /w myprogram.exe -console

One handy benefit is that you can launch it standalone with a console if you want, and get to see all the error messages in the console.

unit ConsoleConnector;
// Connects the/a console to a GUI program
// Can hook exception handler to mirror messages to console.
// To use it, you only need to call ATTACH
// best to make attaching a commandline option e.g -console
//    if FindCmdLineSwitch('console',true) then AttachConsole(true,true);
// When using this, you will use START to launch your program e.g.
// start /w myprogram.exe -console
// creates Console var at end in initialise/finalise - you might want to do this explicitly in your own program instead.
// see: http://www.boku.ru/2016/02/28/posting-to-console-from-gui-app/

//sjb 18Nov16

interface
uses sysutils,forms;

type
  TConsoleConnector = class
  private
    OldExceptionEvent:TExceptionEvent;
    Hooked:boolean;
    BlockApplicationExceptionHandler:boolean; //errors ONLY to console, no error messageboxes blocking program
    procedure DetachErrorHandler;
    procedure GlobalExceptionHandler(Sender: TObject; E: Exception);
    procedure HookExceptionHandler;
  public
    IsAttached:boolean;

    function Attach(
        CreateIfNeeded:boolean=true; //Call ALLOCCONSOLE if no console to attach to
        HookExceptions:boolean=false;  //Hook Application.OnException to echo all unhandled exceptions to console
        OnlyToConsole:boolean=false  // Suppresses exception popups in gui, errors only go to console
        ):boolean;
    procedure Detach;            //detach and unhook
    procedure writeln(S:string); //only writes if console is attached
    procedure ShowMessage(S:string); //Popup ShowMessage box and mirror to console. Obeys OnlyToConsole
  end;

  var Console:TConsoleConnector;

implementation

uses Windows,dialogs;

//winapi function
function AttachConsole(dwProcessId: Int32): boolean; stdcall; external kernel32 name 'AttachConsole';

function TConsoleConnector.Attach(CreateIfNeeded:boolean=true;HookExceptions:boolean=false;OnlyToConsole:boolean=false):boolean;
begin
  IsAttached:=AttachConsole(-1);
  if not IsAttached and CreateIfNeeded
    then begin
      IsAttached:=AllocConsole;
    end;
  result:=IsAttached;
  if HookExceptions then HookExceptionHandler;
end;

procedure TConsoleConnector.Detach;
begin
  FreeConsole;
  IsAttached:=false;
  DetachErrorHandler;
end;

procedure TConsoleConnector.WriteLn(S:string);
begin
  if IsAttached then system.writeln(S);
end;
procedure TConsoleConnector.ShowMessage(S:string);
begin
  self.Writeln(S);
  if BlockApplicationExceptionHandler then exit;
  dialogs.ShowMessage(S);
end;
procedure TConsoleConnector.GlobalExceptionHandler(Sender: TObject; E: Exception);
begin
  self.Writeln(E.Message);
  if BlockApplicationExceptionHandler then exit;
  if assigned(OldExceptionEvent) //i.e there was an old event before we hooked it
    then OldExceptionEvent(Sender,E)
    else Application.ShowException(E);
end;

procedure TConsoleConnector.HookExceptionHandler;
begin
  OldExceptionEvent:=Application.OnException;
  Application.OnException:=GlobalExceptionHandler;
  Hooked:=true;
end;

procedure TConsoleConnector.DetachErrorHandler;
begin
  if Hooked //I have hooked it
    then begin
      Application.OnException:=OldExceptionEvent;
      OldExceptionEvent:=nil;
      Hooked:=false;
    end;
end;

initialization
  Console:=TconsoleConnector.create;
finalization
  Console.Detach;
  Console.Destroy;
end.