The WinMain application entry point

Every Windows program includes an entry-point function named either WinMain or wWinMain. The following code shows the signature for wWinMain:

int WINAPI wWinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PWSTR pCmdLine, int nCmdShow);

The four wWinMain parameters are as follows:

  • hInstance is the handle to an instance or handle to a module. The operating system uses this value to identify the executable or EXE when it's loaded in memory. Certain Windows functions need the instance handle, for example to load icons or bitmaps.
  • hPrevInstance has no meaning. It was used in 16-bit Windows, but is now always zero.
  • pCmdLine contains the command-line arguments as a Unicode string.
  • nCmdShow is a flag that indicates whether the main application window is minimized, maximized, or shown normally.

The function returns an int value. The operating system doesn't use the return value, but you can use the value to pass a status code to another program.

A calling convention, such as WINAPI, defines how a function receives parameters from the caller. For example, the calling convention defines the order that parameters appear on the stack. Make sure to declare your wWinMain function as shown in the preceding example.

The WinMain function is the same as wWinMain, except the command-line arguments are passed as an ANSI string. The Unicode string is preferred. You can use the ANSI WinMain function even if you compile your program as Unicode. To get a Unicode copy of the command-line arguments, call the GetCommandLine function. This function returns all of the arguments in a single string. If you want the arguments as an argv-style array, pass this string to CommandLineToArgvW.

How does the compiler know to invoke wWinMain instead of the standard main function? What actually happens is that the Microsoft C runtime library (CRT) provides an implementation of main that calls either WinMain or wWinMain.

The CRT does some more work inside main. For example, it calls any static initializers before wWinMain. Although you can tell the linker to use a different entry-point function, you should use the default if you link to the CRT. Otherwise, the CRT initialization code is skipped, with unpredictable results such as global objects not being initialized correctly.

The following code shows an empty WinMain function:

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
    PSTR lpCmdLine, int nCmdShow)
    return 0;

Now that you have the entry point and understand some of the basic terminology and coding conventions, you're ready to Create your first Windows program.