The SetWindowsHookEx function installs an application-defined hook procedure into a hook chain. An application installs a hook procedure to monitor the system for certain types of events. A hook procedure can monitor events associated either with a specific thread or with all threads in the system. This function supersedes the SetWindowsHook function.

HHOOK SetWindowsHookEx(

    int idHook, // type of hook to install
    HOOKPROC lpfn, // address of hook procedure
    HINSTANCE hMod, // handle of application instance
    DWORD dwThreadId // identity of thread to install hook for
   );

Parameters

idHook

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

Value Description
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_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_MOUSE Installs a hook procedure that monitors mouse messages. For more information, see the MouseProc 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 system. For more information, see the SysMsgProc hook procedure.

lpfn

Points 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 dynamic-link library (DLL). Otherwise, lpfn can point to a hook procedure in the code associated with the current process.

hMod

Identifies 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

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.

Return Values

If the function succeeds, the return value is the handle of the hook procedure.
If the function fails, the return value is NULL.

Remarks

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.
Chaining to the next hook procedure (that is, calling the CallNextHookEx function) is optional. An application or library can call the next hook procedure either before or after any processing in its own hook procedure. Although chaining to the next hook is optional, it is highly recommended; otherwise, the other applications that have installed hooks will not receive hook notifications and may behave incorrectly as a result.

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 system scope; others can also be set for only a specific thread, as shown in the following list:

Hook Scope
WH_CALLWNDPROC Thread or system
WH_CBT Thread or system
WH_DEBUG Thread or system
WH_GETMESSAGE Thread or system
WH_JOURNALPLAYBACK System only
WH_JOURNALRECORD System only
WH_KEYBOARD Thread or system
WH_MOUSE Thread or system
WH_MSGFILTER Thread or system
WH_SHELL Thread or system
WH_SYSMSGFILTER System only

For a specified hook type, thread hooks are called first, then system hooks.
The system hooks are a shared resource, and installing one affects all applications. All system hook functions must be in libraries. System 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 the hook procedure.