HOOKPROC callback function (winuser.h)

Article
02/22/2024

An application-defined or library-defined callback function used with the SetWindowsHookEx function. The system calls this function after the SendMessage function is called. The hook procedure can examine the message; it cannot modify it.

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

Syntax

HOOKPROC Hookproc;

LRESULT Hookproc(
       int code,
  [in] WPARAM wParam,
  [in] LPARAM lParam
)
{...}

Parameters

code

[in] wParam

Type: WPARAM

Specifies whether the message is sent by the current process. If the message is sent by the current process, it is nonzero; otherwise, it is NULL.

[in] lParam

Type: LPARAM

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

Return value

Type: LRESULT

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

If nCode is greater than or equal to zero, it is highly recommended that you call CallNextHookEx function and return the value it returns; otherwise, other applications that have installed WH_CALLWNDPROCRET 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

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

Requirements

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