Microsoft KB Archive/124134

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Allowing Only One Application Instance on Win32s

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Q124134

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The information in this article applies to:


 * Microsoft Win32s versions 1.3, 1.30a, 1.3c

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SUMMARY
The entry point for both Windows-based and Win32-based applications is:

  int WinMain( hInstance, hPrevInstance, lpszCmdLine, nCmdShow )

HINSTANCE hInstance;       /* Handle of current instance  */ HINSTANCE hPrevInstance;   /* Handle of previous instance */ LPSTR lpszCmdLine;         /* Address of command line     */ int nCmdShow;              /* Show state of window        */ You can allow only one instance of your Windows-based application to run at a time by using hPrevInstance to determine if there is already an existing application instance; then exit the process if there is one. If there is no previous instance, hPrevInstance is NULL.

However, in a Win32-based application, hPrevInstance is always NULL. Therefore, you cannot determine if another instance of your application has been started simply by examining hPrevInstance. This article gives you a method you can use.

MORE INFORMATION
Use one of the following four methods to determine if there is an existing application instance on Win32s:


 * Synchronize with a named object, such as a file mapping.
 * Synchronize with a global atom.
 * Synchronize with a private message.
 * Use FindWindow to check for the application.

Using a File Mapping
Using a file mapping works well on any Win32 platform. The global atom is a cheaper resource, whereas a file mapping will cost a page of memory. A private message is good if you want to inform the first instance that the user attempted to start a second instance, and then let it handle the request -- post a message, become the active application, and so on.

NOTE: You need to clean up before terminating the second instance. FindWindow doesn't require cleanup, but this method will not work as reliably in a preemptive multitasking environment, such as Windows NT, because you can get in a race condition.

The following code fragment demonstrates how a file mapping can be used to allow only one instance of a Win32-based application. This code should avoid any race conditions. Place this code at the beginning of WinMain.

The code creates a file mapping named MyTestMap using CreateFileMapping. If MyTestMap already exists, then you know that there is already a running instance of this application. A similar technique would be used with a global atom.

Sample Code
  HANDLE hMapping;

hMapping = CreateFileMapping( (HANDLE) 0xffffffff,                                NULL,                                 PAGE_READONLY,                                 0,                                 32,                                 "MyTestMap" ); if( hMapping ) {     if( GetLastError == ERROR_ALREADY_EXISTS ) {     //       // Display something that tells the user // the app is already running. //         MessageBox( NULL, "Application is running.", "Test", MB_OK ); ExitProcess(1); }  }   else {  //    // Some other error; handle error. //      MessageBox( NULL, "Error creating mapping", "Test", MB_OK ); ExitProcess(1); } Additional query words:

Keywords : _IK kbOSWin32s

Issue type :

Technology : kbWin32sSearch kbWin32s130 kbWin32s130a kbWin32s130c