Winmain Explained

The WinMain function is the conventional name for the user-provided entry point for a Windows-based application.
 The name WinMain is used by convention by many programming frameworks. Depending on the programming framework, the call to the WinMain function can be preceded and followed by additional activities specific to that framework.
Your WinMain should initialize the application, display its main window, and enter a message retrieval-and-dispatch loop that is the top-level control structure for the remainder of the application's execution. Terminate the message loop when it receives a WM_QUIT message. At that point, your WinMain should exit the application, returning the value passed in the WM_QUIT message's wParam parameter. If WM_QUIT was received as a result of calling PostQuitMessage, the value of wParam is the value of the PostQuitMessage function's nExitCode parameter. For more information, see Creating a Message Loop.

Note:
ANSI applications can use the lpCmdLine parameter of the WinMain function to access the command-line string, excluding the program name. Note that lpCmdLine uses the LPSTR data type instead of the LPTSTR data type. This means that WinMain cannot be used by Unicode programs. The GetCommandLineW function can be used to obtain the command line as a Unicode string. Some programming frameworks might provide an alternative entry point that provides a Unicode command line. For example, the Microsoft Visual Studio C++ complier uses the name wWinMain for the Unicode entry point.

Syntax

int WINAPI WinMain(      
    HINSTANCE hInstance,
    HINSTANCE hPrevInstance,
    LPSTR lpCmdLine,
    int nCmdShow
);

Parameters

hInstance

[in] Handle to the current instance of the application.

hPrevInstance

[in] Handle to the previous instance of the application. This parameter is always NULL. If you need to detect whether another instance already exists, create a uniquely named mutex using the CreateMutex function. CreateMutex will succeed even if the mutex already exists, but the function will return ERROR_ALREADY_EXISTS. This indicates that another instance of your application exists, because it created the mutex first. However, a malicious user can create this mutex before you do and prevent your application from starting. To prevent this situation, create a randomly named mutex and store the name so that it can only be obtained by an authorized user. Alternatively, you can use a file for this purpose. To limit your application to one instance per user, create a locked file in the user's profile directory.

lpCmdLine

[in] Pointer to a null-terminated string specifying the command line for the application, excluding the program name. To retrieve the entire command line, use the GetCommandLine function.

nCmdShow

[in] Specifies how the window is to be shown. This parameter can be one of the following values.

SW_HIDE

Hides the window and activates another window.

SW_MAXIMIZE

Maximizes the specified window.

SW_MINIMIZE

Minimizes the specified window and activates the next top-level window in the Z order.

SW_RESTORE

Activates and displays the window. If the window is minimized or maximized, the system restores it to its original size and position. An application should specify this flag when restoring a minimized window.

SW_SHOW

Activates the window and displays it in its current size and position.

SW_SHOWMAXIMIZED

Activates the window and displays it as a maximized window.

SW_SHOWMINIMIZED

Activates the window and displays it as a minimized window.

SW_SHOWMINNOACTIVE

Displays the window as a minimized window. This value is similar to SW_SHOWMINIMIZED, except the window is not activated.

SW_SHOWNA

Displays the window in its current size and position. This value is similar to SW_SHOW, except the window is not activated.

SW_SHOWNOACTIVATE

Displays a window in its most recent size and position. This value is similar to SW_SHOWNORMAL, except the window is not actived.

SW_SHOWNORMAL

Activates and displays a window. If the window is minimized or maximized, the system restores it to its original size and position. An application should specify this flag when displaying the window for the first time.

Return Value

If the function succeeds, terminating when it receives a WM_QUIT message, it should return the exit value contained in that message's wParam parameter. If the function terminates before entering the message loop, it should return zero.