How do you only allow one instance of an app?

Sorry, I'm not all that good at windows programming here.
But if I am programming for windows 2000/XP the traditional way, how would I prevent the user from initiating multiple instances of my application?

try

#include <windows.h>

HWND FindWindow(
LPCTSTR lpClassName, // pointer to class name
LPCTSTR lpWindowName // pointer to window name
);

at startup of your programs try this

if it returns a window then the app is running close the program.

Try this at the begining of your WinMain function

Code:

---

hMapping = CreateFileMapping((HANDLE)0xffffffff, NULL, PAGE_READONLY, 0, 32, szAppName);

if (hMapping)                // Check to see if app is already running, if so exit
{
  if (GetLastError() == ERROR_ALREADY_EXISTS)
  {
      sprintf(sBuffer,"%s is already running.",szAppName);
      MessageBox(NULL, sBuffer,  "Error", MB_ICONERROR | MB_OK);
      ExitProcess(1);
    }
}
else
{
  MessageBox(NULL, "Error creating file mapping.", "Error",MB_ICONERROR| MB_OK);
  ExitProcess(1);
}

---

the normal way of doing this is with mutexes.
go to msdn and do a search for CreateMutex()

There's quite a good article here -

http://www.codeproject.com/cpp/avoidmultinstance.asp

about avoiding multiple instances.

Simplified way to do it, add the following to the begining of your WinMain.

HANDLE hMutex;

hMutex = CreateMutex(NULL, FALSE, "myAppMutex");

if (hMutex) {

if (GetLastError() == ERROR_ALREADY_EXISTS) {

return 0;

}
}

This is just a REAL SIMPLE way of not starting the app if its already running.

Thanks for all the help. I went with a FindWindow solution but set my class name to some really long and unique name. I know that this was discredited as having race conditions, but in all fairness, i think the likelihood of such conditions ever arising to be slim to nill. And I don't want to spend that much time on such a simple task.

Ofcourse, all help was appreciated.