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 system.
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 |
); |
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_KEYBOARD_LL |
Windows NT only: 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 only: 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 system. For more information, see the SysMsgProc hook procedure. |
If the function succeeds, the return value is the handle of the hook procedure.
If the function fails, the return value is NULL. To get extended error information, call GetLastError.
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 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_CALLWNDPROCRET |
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_KEYBOARD_LL |
Thread or system |
WH_MOUSE |
Thread or system |
WH_MOUSE_LL |
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.
CallNextHookEx, CallWndProc, CallWndRetProc, CBTProc, DebugProc, GetMsgProc, JournalPlaybackProc, JournalRecordProc, LowLevelKeyboardProc, LowLevelMouseProc KeyboardProc, MouseProc, MessageProc, ShellProc, SysMsgProc, UnhookWindowsHookEx
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