Allow only 3 instances of an application using semaphores

Question

I am trying to implement a simple routine using semaphores that will allow me to run only 3 instances of the application. I could use 3 mutexes but that is not a nice approach i tried this so far

var
  hSem:THandle;
begin
  hSem := CreateSemaphore(nil,3,3,'MySemp3');
  if hSem = 0 then
  begin
    ShowMessage('Application can be run only 3 times at once');
    Halt(1);
  end;

How can i do this properly ?

Answer 1

Always make sure that you release a semaphore because this is not done automatically if your application dies.

program Project72;

{$APPTYPE CONSOLE}

uses
  Windows,
  SysUtils;

var
  hSem: THandle;

begin
  try
    hSem := CreateSemaphore(nil, 3, 3, 'C15F8F92-2620-4F3C-B8EA-A27285F870DC/myApp');
    Win32Check(hSem <> 0);
    try
      if WaitForSingleObject(hSem, 0) <> WAIT_OBJECT_0 then
        Writeln('Cannot execute, 3 instances already running')
      else begin
        try
          // place your code here
          Writeln('Running, press Enter to stop ...');
          Readln;
        finally ReleaseSemaphore(hSem, 1, nil); end;
      end;
    finally CloseHandle(hSem); end;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
end.

Answer 2

You could use TJclAppInstances from the JCL.

Answer 3

1. You must try to see if it was created
2. You must use one of the wait function to see if you can get a count
3. At the end, you must release the lock & handle so that it can work the next time user close and open your app

Cheers

var
  hSem: THandle;
begin
  hSem := OpenSemaphore(nil, SEMAPHORE_ALL_ACCESS, True, 'MySemp3');
  if hSem = 0 then
    hSem := CreateSemaphore(nil, 3, 3,'MySemp3');

  if hSem = 0 then
  begin
    ShowMessage('... show the error');
    Halt(1);
    Exit;     
  end;

  if WaitForSingleObject(hSem, 0) <> WAIT_OBJECT_0 then
  begin
    CloseHandle(hSem);
    ShowMessage('Application can be runed only 3 times at once');
    Halt(1);
    Exit; 
  end;

  try   
    your application.run codes..

  finally
    ReleaseSemaphore(hSem, 1, nil);
    CloseHandle(hSem);
  end;