The SetWindowsHookEx function installs an application-defined hook procedure into a hook chain. You would install a hook procedure to monitor the system for certain types of events. These events are associated either with a specific thread or with all threads in the same desktop as the calling thread.

Syntax

HHOOK SetWindowsHookEx(      
    int idHook,
    HOOKPROC lpfn,
    HINSTANCE hMod,
    DWORD dwThreadId
);

Parameters

idHook

[in] Specifies the type of hook procedure to be installed. This parameter can be one of the following values.

WH_CALLWNDPROC

Installs a hook procedure that monitors messages before the system sends them to the destination window procedure. For more information, see the CallWndProc hook procedure.

WH_CALLWNDPROCRET

Installs a hook procedure that monitors messages after they have been processed by the destination window procedure. For more information, see the CallWndRetProc hook procedure.

WH_CBT

Installs a hook procedure that receives notifications useful to a computer-based training (CBT) application. For more information, see the CBTProc hook procedure.

WH_DEBUG

Installs a hook procedure useful for debugging other hook procedures. For more information, see the DebugProc hook procedure.

WH_FOREGROUNDIDLE

Installs a hook procedure that will be called when the application's foreground thread is about to become idle. This hook is useful for performing low priority tasks during idle time. For more information, see the ForegroundIdleProc hook procedure.

WH_GETMESSAGE

Installs a hook procedure that monitors messages posted to a message queue. For more information, see the GetMsgProc hook procedure.

WH_JOURNALPLAYBACK

Installs a hook procedure that posts messages previously recorded by a WH_JOURNALRECORD hook procedure. For more information, see the JournalPlaybackProc hook procedure.

WH_JOURNALRECORD

Installs a hook procedure that records input messages posted to the system message queue. This hook is useful for recording macros. For more information, see the JournalRecordProc hook procedure.

WH_KEYBOARD

Installs a hook procedure that monitors keystroke messages. For more information, see the KeyboardProc hook procedure.

WH_KEYBOARD_LL

Windows NT/2000/XP: Installs a hook procedure that monitors low-level keyboard input events. For more information, see the LowLevelKeyboardProc hook procedure.

WH_MOUSE

Installs a hook procedure that monitors mouse messages. For more information, see the MouseProc hook procedure.

WH_MOUSE_LL

Windows NT/2000/XP: Installs a hook procedure that monitors low-level mouse input events. For more information, see the LowLevelMouseProc hook procedure.

WH_MSGFILTER

Installs a hook procedure that monitors messages generated as a result of an input event in a dialog box, message box, menu, or scroll bar. For more information, see the MessageProc hook procedure.

WH_SHELL

Installs a hook procedure that receives notifications useful to shell applications. For more information, see the ShellProc hook procedure.

WH_SYSMSGFILTER

Installs a hook procedure that monitors messages generated as a result of an input event in a dialog box, message box, menu, or scroll bar. The hook procedure monitors these messages for all applications in the same desktop as the calling thread. For more information, see the SysMsgProc hook procedure.

lpfn

[in] Pointer to the hook procedure. If the dwThreadId parameter is zero or specifies the identifier of a thread created by a different process, the lpfn parameter must point to a hook procedure in a DLL. Otherwise, lpfn can point to a hook procedure in the code associated with the current process.

hMod

[in] Handle to the DLL containing the hook procedure pointed to by the lpfn parameter. The hMod parameter must be set to NULL if the dwThreadId parameter specifies a thread created by the current process and if the hook procedure is within the code associated with the current process.

dwThreadId

[in] Specifies the identifier of the thread with which the hook procedure is to be associated. If this parameter is zero, the hook procedure is associated with all existing threads running in the same desktop as the calling thread.

Return Value

If the function succeeds, the return value is the handle to the hook procedure.

If the function fails, the return value is NULL. To get extended error information, call GetLastError.

Remarks

SetWindowsHookEx can be used to inject a DLL into another process. A 32-bit DLL cannot be injected into a 64-bit process, and a 64-bit DLL cannot be injected into a 32-bit process. If an application requires the use of hooks in other processes, it is required that a 32-bit application call SetWindowsHookEx to inject a 32-bit DLL into 32-bit processes, and a 64-bit application call SetWindowsHookEx to inject a 64-bit DLL into 64-bit processes. The 32-bit and 64-bit DLLs must have different names.

An error may occur if the hMod parameter is NULL and the dwThreadId parameter is zero or specifies the identifier of a thread created by another process.

Calling the CallNextHookEx function to chain to the next hook procedure is optional, but it is highly recommended; otherwise, other applications that have installed hooks will not receive hook notifications and may behave incorrectly as a result. You should call CallNextHookEx unless you absolutely need to prevent the notification from being seen by other applications.

Before terminating, an application must call the UnhookWindowsHookEx function to free system resources associated with the hook.

The scope of a hook depends on the hook type. Some hooks can be set only with global scope; others can also be set for only a specific thread, as shown in the following table.

Hook	Scope
WH_CALLWNDPROC	Thread or global
WH_CALLWNDPROCRET	Thread or global
WH_CBT	Thread or global
WH_DEBUG	Thread or global
WH_FOREGROUNDIDLE	Thread or global
WH_GETMESSAGE	Thread or global
WH_JOURNALPLAYBACK	Global only
WH_JOURNALRECORD	Global only
WH_KEYBOARD	Thread or global
WH_KEYBOARD_LL	Global only
WH_MOUSE	Thread or global
WH_MOUSE_LL	Global only
WH_MSGFILTER	Thread or global
WH_SHELL	Thread or global
WH_SYSMSGFILTER	Global only

For a specified hook type, thread hooks are called first, then global hooks.

The global hooks are a shared resource, and installing one affects all applications in the same desktop as the calling thread. All global hook functions must be in libraries. Global hooks should be restricted to special-purpose applications or to use as a development aid during application debugging. Libraries that no longer need a hook should remove its hook procedure.

Windows 95/98/Me: SetWindowsHookEx is supported by the Microsoft Layer for Unicode (MSLU). However, it does not make conversions. To see Unicode messages, notifications, and so forth, you must subclass the window. To use this version of the API, you must add certain files to your application, as outlined in Installing and Releasing Hook Procedures.

Example

For an example, see Installing and Releasing Hook Procedures.

Function Information

Minimum DLL Version	user32.dll
Header	Declared in Winuser.h, include Windows.h
Import library	User32.lib
Minimum operating systems	Windows 95, Windows NT 3.1
Unicode	Implemented as ANSI and Unicode versions.

See Also

Hooks Overview, CallNextHookEx, CallWndProc, CallWndRetProc, CBTProc, DebugProc, ForegroundIdleProc, GetMsgProc, JournalPlaybackProc, JournalRecordProc, LowLevelKeyboardProc, LowLevelMouseProc, KeyboardProc, MouseProc, MessageProc, ShellProc, SysMsgProc, UnhookWindowsHookEx