Avoiding two processes of the same application in Pascal

Question

What is the most viable way to identify the processes of my application in memory and close them if there is more than one execution. I use the Lazarus IDE platform (looks like Delphi), which supports the object-oriented Pascal programming language.

OBS: Even if there are different process names I can identify my application.

Answer 1

The component Uniqueinstance can do this job for you, to use it just drop a component in the main form, manipulate the property Identifier(used to identify your application) and activate it.

You can also do this using the function CreateMutex() for identify the application, if the function succeeds the return value will be identity otherwise a null value.

If such an identifier already exists before calling the function, the return will be the identifier for the existing object, in which case when calling the function GetLastError(), the return value will be ERROR_ALREADY_EXISTS. That in practice would be something like this:

var
  mutex: THandle;
  ID: string;
begin
  ID:= 'MyAppUniqueID';
  mutex := CreateMutex(nil, False, PChar(ID));

  if GetLastError = ERROR_ALREADY_EXISTS then begin
    Application.Terminate;
  end;
end;

The code snippet above can be used in the event OnCreate() of the main form. To release the identifier you can use the function CloseHandle() at the event OnClose() or OnDestroy(), for this the variable mutex would have to be a global variable.

Answer 2

The function is used CreateSemaphore windows. If the function GetLastError say that there is already a semaphore with that name, it is already running. Otherwise, no.

Relevant part of the function documentation:

Return value

If the Function succeeds, the Return value is a Handle to the Semaphore Object. If the named Semaphore Object existed before the Function call, the Function Returns a Handle to the existing Object and Getlasterror Returns ERROR_ALREADY_EXISTS. If the Function fails, the Return value is NULL. To get Extended error information, call Getlasterror.

Example of use:

repeat
sleep(10000);
createsemaphore("umnomequalquer");
until getlasterror <> ERROR_ALREADY_EXISTS;