Waiting till an application ends

Copyright © 2000 Ernesto De Spirito

WaitForSingleObject

If we ever need to run an external application and wait until it terminates, then we would have to do without ShellExecute and resort to more basic functions, like CreateProcess, WaitForSingleObject and CloseHandle.

uses Forms, Windows;

procedure TForm1.Button1Click(Sender: TObject);
var
  proc_info: TProcessInformation;
  startinfo: TStartupInfo;
  ExitCode: longword;
begin
  // Initialize the structures
  FillChar(proc_info, sizeof(TProcessInformation), 0);
  FillChar(startinfo, sizeof(TStartupInfo), 0);
  startinfo.cb := sizeof(TStartupInfo);

  // Attempts to create the process
  if CreateProcess('c:\windows\notepad.exe', nil, nil,
      nil, false, NORMAL_PRIORITY_CLASS, nil, nil,
       startinfo, proc_info) <> False then begin
    // The process has been successfully created
    // No let's wait till it ends...
    WaitForSingleObject(proc_info.hProcess, INFINITE);
    // Process has finished. Now we should close it.
    GetExitCodeProcess(proc_info.hProcess, ExitCode);  // Optional
    CloseHandle(proc_info.hThread);
    CloseHandle(proc_info.hProcess);
    Application.MessageBox(
      PChar(Format('Notepad finished! (Exit code=%d)', [ExitCode])),
      'Info', MB_ICONINFORMATION);
  end else begin
    // Failure creating the process
    Application.MessageBox('Couldn''t execute the '
      + 'application', 'Error', MB_ICONEXCLAMATION);
  end;//if
end;

Problems

We have experienced problems with WaitForSingleObject in combination with certain applications and we had to finish them with the task manager. Other applications apparently fail to finish correctly and WaitForSingleObject never returns. Appart from that, an application does not respond to events while it waits and this means that for example the forms won't repaint...

A possible workaround to these problems is checking the status of the launched application (calling GetExitCodeProcess) at intervals (in a timer event).

uses Forms, Windows, StdCtrls, ExtCtrls;

type
TForm1 = class(TForm)
  btnExecute: TButton;
  btnCancel: TButton;
  Timer1: TTimer;
  procedure FormCreate(Sender: TObject);
  procedure btnExecuteClick(Sender: TObject);
  procedure Timer1Timer(Sender: TObject);
  procedure btnCancelClick(Sender: TObject);
  procedure FormCloseQuery(Sender: TObject;
    var CanClose: Boolean);
private
  proc_info: TProcessInformation;
  startinfo: TStartupInfo;
  ExitCode: LongWord;
end;

implementation

procedure TForm1.FormCreate(Sender: TObject);
begin
  btnCancel.Enabled := False;
  Timer1.Enabled := False;
  Timer1.Interval := 200;
end;

procedure TForm1.btnExecuteClick(Sender: TObject);
begin
  FillChar(proc_info, sizeof(TProcessInformation), 0);
  FillChar(startinfo, sizeof(TStartupInfo), 0);
  startinfo.cb := sizeof(TStartupInfo);
  if CreateProcess(nil, 'c:\windows\notepad.exe', nil,
      nil, false, CREATE_DEFAULT_ERROR_MODE
      + NORMAL_PRIORITY_CLASS, nil, nil, startinfo,
      proc_info) then begin
    btnExecute.Enabled := False;
    btnCancel.Enabled := True;
    Timer1.Enabled := True;
  end else begin
    CloseHandle(proc_info.hProcess);
    Application.MessageBox('Couldn''t execute the '
      + 'application', 'Error', MB_ICONEXCLAMATION);
  end;
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  Timer1.Enabled := False;
  if GetExitCodeProcess(proc_info.hProcess, ExitCode)
  then
    if ExitCode = STILL_ACTIVE then
      Timer1.Enabled := True
    else begin
      btnCancel.Enabled := False;
      btnExecute.Enabled := True;
      CloseHandle(proc_info.hProcess);
    end
  else begin
    btnCancel.Enabled := False;
    btnExecute.Enabled := True;
    TerminateProcess(proc_info.hProcess, 0);
    CloseHandle(proc_info.hProcess);
  end;
end;

procedure TForm1.btnCancelClick(Sender: TObject);
begin
  Timer1.Enabled := False;
  if Application.MessageBox('You should try to finish'
  + ' the application normally.'#13#13'¿Terminate it '
  + 'anyway?', 'Warning', MB_YESNO + MB_DEFBUTTON2 +
  MB_ICONQUESTION + MB_TASKMODAL) = ID_YES then begin
    TerminateProcess(proc_info.hProcess, 0);
    CloseHandle(proc_info.hProcess);
    btnCancel.Enabled := False;
    btnExecute.Enabled := True;
  end else begin
    Timer1.Enabled := True;
  end;
end;

procedure TForm1.FormCloseQuery(Sender: TObject;
  var CanClose: Boolean);
begin
  if btnCancel.Enabled then begin
    btnCancelClick(Sender);
    if btnCancel.Enabled then CanClose := False;
  end;
end;