Microsoft Windows Caches Five Display Contexts

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0

The Microsoft Windows graphical environment maintains an internal cache of five display contexts (DCs) which are shared by all applications in the system. An application retrieves one of these DCs when it calls the BeginPaint(), GetDC(), or GetWindowDC() functions. When an application retrieves a DC, Windows marks the DC as in use and does not allow another application to retrieve it. When an application has finished using a DC, it must return the DC to the cache which makes the DC available to another application. Two APIs return a DC to the cache: EndPaint() function (to match BeginPaint()) and the ReleaseDC() function (to match GetDC() or GetWindowDC()).

If an application fails to return DCs to the DC cache correctly, the cache becomes empty, and subsequent calls to BeginPaint(), GetDC(), and GetWindowDC() fail. Because most applications do not check to be sure that the DC they retrieve is valid, they attempt to use an invalid DC in a graphics device interface (GDI) call, which causes a FatalExit error.

To avoid this problem, be sure to return DCs to the cache correctly and check the value returned by BeginPaint(), GetDC(), and GetWindowDC() to ensure that a valid DC is available.