Windows API Development

1. Handles

A handle is a generic pointer or identifier that refers to a system resource. It's an abstract way for the operating system to manage objects like windows, files, threads, and processes. You don't work directly with the resource itself, but rather with its handle.

• Handles are opaque (you cannot inspect their internal structure).
• They are typically of type HANDLE.
• You obtain handles by calling specific API functions (e.g., CreateFile, CreateWindowEx).
• You must close handles when they are no longer needed to free up system resources (e.g., using CloseHandle).

Example:

HANDLE hFile = CreateFile(
    L"C:\\example.txt",
    GENERIC_READ,
    FILE_SHARE_READ,
    NULL,
    OPEN_EXISTING,
    FILE_ATTRIBUTE_NORMAL,
    NULL
);

if (hFile != INVALID_HANDLE_VALUE) {
    // Use the file handle...
    CloseHandle(hFile); // Release the resource
}
            

2. Messages and Message Loops

Windows is a message-driven operating system. Applications interact with the system and other applications by sending and receiving messages. A message loop is a fundamental construct in every Win32 application that processes these messages.

• Messages are defined by numerical constants (e.g., WM_PAINT, WM_KEYDOWN).
• The GetMessage function retrieves messages from the application's message queue.
• The TranslateMessage and DispatchMessage functions process and dispatch these messages to the appropriate window procedure.

The typical message loop:

MSG msg;
while (GetMessage(&msg, NULL, 0, 0)) {
    TranslateMessage(&msg);
    DispatchMessage(&msg);
}
            

3. Window Procedures (WNDPROC)

A WNDPROC is a callback function that handles messages sent to a specific window. When a message is dispatched, the system calls the window's associated WNDPROC to process it.

• It receives the window handle (HWND), message identifier (UINT), and message parameters (WPARAM, LPARAM).
• It must process known messages and pass unknown messages to the DefWindowProc function.

Signature:

LRESULT CALLBACK WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam);
            

4. Structures and Data Types

The Win32 API extensively uses structures to pass complex data between the application and the operating system. These structures define various properties, parameters, and states of system objects.

• Common examples include RECT (for rectangles), POINT (for coordinates), and CREATESTRUCT.
• Data types like DWORD, LPVOID, BOOL are standard.
• Understanding the layout and purpose of these structures is crucial for correct API usage.

5. Threading and Processes

Win32 applications can leverage multithreading and multiprocessing to improve performance and responsiveness.

• A process is an instance of a running program.
• A thread is the smallest unit of execution within a process.
• You can create threads using CreateThread and manage processes using functions like CreateProcess.
• Synchronization mechanisms like mutexes, semaphores, and critical sections are vital for safe multithreaded programming.

6. Resource Management

Efficiently managing system resources (memory, handles, GDI objects) is paramount.

• Memory management: Using functions like LocalAlloc, GlobalAlloc, and more modern HeapAlloc.
• GDI objects: Handles to device contexts (HDC), pens (HPEN), brushes (HBRUSH), fonts (HFONT), bitmaps (HBITMAP) must be selected and deleted appropriately.