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WNDPROC callback function (winuser.h)

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A callback function, which you define in your application, that processes messages sent to a window. The WNDPROC type defines a pointer to this callback function. The WndProc name is a placeholder for the name of the function that you define in your application.

Syntax

WNDPROC Wndproc;

LRESULT Wndproc(
  HWND unnamedParam1,
  UINT unnamedParam2,
  WPARAM unnamedParam3,
  LPARAM unnamedParam4
)
{...}

Parameters

unnamedParam1

Type: HWND

A handle to the window. This parameter is typically named hWnd.

unnamedParam2

Type: UINT

The message. This parameter is typically named uMsg.

For lists of the system-provided messages, see System-defined messages.

unnamedParam3

Type: WPARAM

Additional message information. This parameter is typically named wParam.

The contents of the wParam parameter depend on the value of the uMsg parameter.

unnamedParam4

Type: LPARAM

Additional message information. This parameter is typically named lParam.

The contents of the lParam parameter depend on the value of the uMsg parameter.

Return value

Type: LRESULT

The return value is the result of the message processing, and depends on the message sent.

Remarks

If your application runs on a 32-bit version of Windows operating system, uncaught exceptions from the callback will be passed onto higher-level exception handlers of your application when available. The system then calls the unhandled exception filter to handle the exception prior to terminating the process. If the PCA is enabled, it will offer to fix the problem the next time you run the application.

However, if your application runs on a 64-bit version of Windows operating system or WOW64, you should be aware that a 64-bit operating system handles uncaught exceptions differently based on its 64-bit processor architecture, exception architecture, and calling convention. The following table summarizes all possible ways that a 64-bit Windows operating system or WOW64 handles uncaught exceptions.

Behavior type How the system handles uncaught exceptions
1 The system suppresses any uncaught exceptions.
2 The system first terminates the process, and then the Program Compatibility Assistant (PCA) offers to fix it the next time you run the application. You can disable the PCA mitigation by adding a Compatibility section to the application manifest.
3 The system calls the exception filters but suppresses any uncaught exceptions when it leaves the callback scope, without invoking the associated handlers.

The following table shows how a 64-bit version of the Windows operating system, and WOW64, handles uncaught exceptions. Notice that behavior type 2 applies only to the 64-bit version of the Windows 7 operating system and later.

Operating system WOW64 64-bit Windows
Windows XP 3 1
Windows Server 2003 3 1
Windows Vista 3 1
Windows Vista SP1 1 1
Windows 7 and later 1 2

Note

On Windows 7 with SP1 (32-bit, 64-bit, or WOW64), the system calls the unhandled exception filter to handle the exception prior to terminating the process. If the Program Compatibility Assistant (PCA) is enabled, then it will offer to fix the problem the next time you run the application.

If you need to handle exceptions in your application, you can use structured exception handling to do so. For more information on how to use structured exception handling, see Structured exception handling.

Requirements

Requirement Value
Minimum supported client Windows 2000 Professional [desktop apps only]
Minimum supported server Windows 2000 Server [desktop apps only]
Header winuser.h (include windows.h)

See also