CallWndProc callback function

Article
03/30/2018

An application-defined or library-defined callback function used with the SetWindowsHookEx function. The system calls this function before calling the window procedure to process a message sent to the thread.

The HOOKPROC type defines a pointer to this callback function. CallWndProc is a placeholder for the application-defined or library-defined function name.

Syntax

LRESULT CALLBACK CallWndProc(
  _In_ int    nCode,
  _In_ WPARAM wParam,
  _In_ LPARAM lParam
);

Parameters

nCode [in]
Type: int

Specifies whether the hook procedure must process the message. If nCode is HC_ACTION, the hook procedure must process the message. If nCode is less than zero, the hook procedure must pass the message to the CallNextHookEx function without further processing and must return the value returned by CallNextHookEx.
wParam [in]
Type: WPARAM

Specifies whether the message was sent by the current thread. If the message was sent by the current thread, it is nonzero; otherwise, it is zero.
lParam [in]
Type: LPARAM

A pointer to a CWPSTRUCT structure that contains details about the message.

Return value

Type: ****

Type: LRESULT

If nCode is less than zero, the hook procedure must return the value returned by CallNextHookEx.

If nCode is greater than or equal to zero, it is highly recommended that you call CallNextHookEx and return the value it returns; otherwise, other applications that have installed WH_CALLWNDPROC hooks will not receive hook notifications and may behave incorrectly as a result. If the hook procedure does not call CallNextHookEx, the return value should be zero.

Remarks

The CallWndProc hook procedure can examine the message, but it cannot modify it. After the hook procedure returns control to the system, the message is passed to the window procedure.

An application installs the hook procedure by specifying the WH_CALLWNDPROC hook type and a pointer to the hook procedure in a call to the SetWindowsHookEx function.

Requirements

Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Header	Winuser.h (include Windows.h)

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