Single Instance Application - The simple solution

• Download demo project - 19 Kb
• Download the source - 2 Kb

Introduction

Here is one more solution to solve the one instance application issue. The main distinguishing feature of this implementation is simplicity. It's not bound to MFC and can be used in pure Win32 application, no matter GUI or console. But MFC version is also present. There are two demo projects (MFC and Win32) to demonstrate how it works. Try to start several instances to see what happens.

How does this work

The first instance of application creates file mapping, registers window message and save its thread ID in the file mapping. The second instance opens the file mapping, copies its command line to the file mapping and posts registered window message to the first instance thread. The posted message notifies the first instance that command line of the second instance is available. So, it uses one file mapping and one registered windows message. Implementation is very lightweight - you don't have to pay much for such a common task.

Background

I wrote this in response to Single Instance Application by Justin Hallet . I found his implementation too complicated (one mutex, one thread, one file mapping, two events - whoa, this is overhead!). Indeed, because such a code keeps the computer running needlessly, it also needlessly increases the entropy of the universe, thereby hastening said heat death. In short, such a code is quite environmentally unfriendly and should be considered a threat to our species :). (Copyright © 2002 Herb Sutter )

Implementation presented here is made as simple as possible.

Using the code with MFC

It's very simple to use it with MFC.

1. Derive your application class from CSingleInstanceApp instead of CWinApp. Do not forget to include SingleInstanceApp.h in your application class header.

class CMyApp : public /*CWinApp*/ CSingleInstanceApp
{
    // ...
};

2. Replace all references to CWinApp with CSingleInstanceApp in your CSingleInstanceApp-derived class.
3. Add the code to the CMyApp constructor:

CMyApp::CMyApp() :
CSingleInstanceApp(TEXT("{49E4CD26-9489-4474-91B0-2A30F6CA3097}"))

Please note, that argument to CSingleInstanceApp constructor must be the pointer to the system unique string (it is used to name the file mapping and to register the window message). Replace it with newly generated GUID.

4. Add the check to InitInstance method of your application class.

BOOL CMyApp::InitInstance()
{
    if (!AmITheFirst())
        return false;
    // ...

5. Optional. Override the CSingleInstanceApp::OnSecondInstance method to get notified when the second instance is being tried to start. You can use here the CSingleInstanceApp::GetSecondInstanceCmdLine() method to get the second instance command line.

Using the code without MFC

It's also very simple.

1. Define UINT variable and SingleInstanceApp instance like this:

UINT uSecondInstanceMessage;
SingleInstanceApp single(TEXT(
            "{B0C9E56F-687A-4c1f-9862-2F4D0F748827}"), 
            uSecondInstanceMessage);

These two must be accessible from scope of the application message loop. The easiest way to do this is to define it at the global scope.

2. Having single defined at the global scope add the check at your WinMain function:

int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, 
        LPSTR lpCmdLine, int nCmdShow)
{
    if (!single.AmITheFirst())
        return false;
    // ...

3. Optional. Registered message posted by PostThreadMessage() function can't be dispatched to a window procedure. So, add the code to your application message loop to get notified when the second instance is being tried to start. You can use here the SingleInstanceApp::GetSecondInstanceCmdLine() method to get the second instance command line:

while (GetMessage(&msg, NULL, 0, 0))
{
    if (uSecondInstanceMessage == msg.message)
    {
        // add your code here
        continue;
    }
    // ...

That's all folks :)