Specifies a function or function object to call automatically when the script receives the specified message.

OnMessage MsgNumber , Function, MaxThreads



Type: Integer

The number of the message to monitor or query, which should be between 0 and 4294967295 (0xFFFFFFFF). If you do not wish to monitor a system message (that is, one below 0x400), it is best to choose a number greater than 4096 (0x1000) to the extent you have a choice. This reduces the chance of interfering with messages used internally by current and future versions of AutoHotkey.


Type: String or Object

A function's name or a function object. To pass a literal function name, it must be enclosed in quotes.


Type: Integer

This integer is normally omitted, in which case the monitor function is limited to one thread at a time. This is usually best because otherwise, the script would process messages out of chronological order whenever the monitor function interrupts itself. Therefore, as an alternative to MaxThreads, consider using Critical as described below.

Specify 0 to unregister the previously registered function identified by Function.

By default, when multiple functions are registered for a single MsgNumber, they are called in the order that they were registered. To register a function to be called before any previously registered functions, specify a negative value for MaxThreads. For example, OnMessage Msg, Fn, -2 registers Fn to be called before any other functions previously registered for Msg, and allows Fn a maximum of 2 threads. However, if the function is already registered, the order will not change unless it is unregistered and then re-registered.


Any number of functions or function objects can monitor a given MsgNumber.

Either of these two lines registers a function object to be called after any previously registered functions:

OnMessage MsgNumber, Function     ; Option 1 - omit MaxThreads
OnMessage MsgNumber, Function, 1  ; Option 2 - specify MaxThreads 1

This registers a function object to be called before any previously registered functions:

OnMessage MsgNumber, Function, -1

To unregister a function object, specify 0 for MaxThreads:

OnMessage MsgNumber, Function, 0


An exception is thrown if Function:

  1. is not an object or the name of a user-defined function; or
  2. is known to require more than four parameters.

The Function's Parameters

A function assigned to monitor one or more messages can accept up to four parameters:

MyMessageMonitor(wParam, lParam, msg, hwnd)
    ... body of function...

Although the names you give the parameters do not matter, the following information is sequentially assigned to them:

Parameter #1: The message's WPARAM value.
Parameter #2: The message's LPARAM value.
Parameter #3: The message number, which is useful in cases where a function monitors more than one message.
Parameter #4: The HWND (unique ID) of the window or control to which the message was sent. The HWND can be used directly in a WinTitle parameter.

WPARAM and LPARAM are unsigned 32-bit integers (from 0 to 232-1) or signed 64-bit integers (from -263 to 263-1) depending on whether the exe running the script is 32-bit or 64-bit. For 32-bit scripts, if an incoming parameter is intended to be a signed integer, any negative numbers can be revealed by following this example:

if (A_PtrSize = 4 && wParam > 0x7FFFFFFF)  ; Checking A_PtrSize ensures the script is 32-bit.
    wParam := -(~wParam) - 1

You can omit one or more parameters from the end of the list if the corresponding information is not needed, but in this case an asterisk must be specified as the final parameter. For example, a function defined as MyMsgMonitor(wParam, lParam, *) would receive only the first two parameters, and one defined as MyMsgMonitor(*) would receive none of them.

Additional Information Available to the Function

In addition to the parameters received above, the function may also consult the built-in variable A_EventInfo, which contains 0 if the message was sent via SendMessage. If sent via PostMessage, it contains the tick-count time the message was posted.

A monitor function's last found window starts off as the parent window to which the message was sent (even if it was sent to a control). If the window is hidden but not a GUI window (such as the script's main window), turn on DetectHiddenWindows before using it. For example:

DetectHiddenWindows True
MsgParentWindow := WinExist()  ; This stores the unique ID of the window to which the message was sent.

What the Function Should Return

If a monitor function uses Return without any parameters, or it specifies a blank value such as "" (or it never uses Return at all), the incoming message goes on to be processed normally when the function finishes. The same thing happens if the function Exits or causes a runtime error such as running a nonexistent file. By contrast, returning an integer causes it to be sent immediately as a reply; that is, the program does not process the message any further. For example, a function monitoring WM_LBUTTONDOWN (0x201) may return an integer to prevent the target window from being notified that a mouse click has occurred. In many cases (such as a message arriving via PostMessage), it does not matter which integer is returned; but if in doubt, 0 is usually safest.

The range of valid return values depends on whether the exe running the script is 32-bit or 64-bit. Non-empty return values must be between -231 and 232-1 for 32-bit scripts (A_PtrSize = 4) and between -263 and 263-1 for 64-bit scripts (A_PtrSize = 8).

If there are multiple functions monitoring a given message number, they are called one by one until one returns a non-empty value.

General Remarks

Unlike a normal function-call, the arrival of a monitored message calls the function as a new thread. Because of this, the function starts off fresh with the default values for settings such as SendMode and DetectHiddenWindows. These defaults can be changed during script startup.

Messages sent to a control (rather than being posted) are not monitored because the system routes them directly to the control behind the scenes. This is seldom an issue for system-generated messages because most of them are posted.

Any script with active message monitors is automatically persistent, which means that it will not exit until ExitApp is used.

If a message arrives while its function is still running due to a previous arrival of the same message, the function will not be called again (except if MaxThreads is greater than 1); instead, the message will be treated as unmonitored. If this is undesirable, a message greater than or equal to 0x312 can be buffered until its function completes by specifying Critical as the first line of the function. Alternatively, Thread Interrupt can achieve the same thing as long as it lasts long enough for the function to finish. By contrast, a message less than 0x312 cannot be buffered by Critical or Thread Interrupt (however, Critical may help because it checks messages less often, which gives the function more time to finish). The only way to guarantee that no such messages are missed is to ensure the function finishes in under 6 milliseconds (though this limit can be raised via Critical 30). One way to do this is to have it queue up a future thread by posting to its own script a monitored message number higher than 0x312. That message's function should use Critical as its first line to ensure that its messages are buffered.

If a monitored message that is numerically less than 0x312 arrives while the script is absolutely uninterruptible -- such as while a menu is displayed, a KeyDelay/MouseDelay is in progress, or the clipboard is being opened -- the message's function will not be called and the message will be treated as unmonitored. By contrast, a monitored message of 0x312 or higher will be buffered during these uninterruptible periods; that is, its function will be called when the script becomes interruptible.

If a monitored message numerically less than 0x312 arrives while the script is uninterruptible merely due to the settings of Thread Interrupt or Critical, the current thread will be interrupted so that the function can be called. By contrast, a monitored message of 0x312 or higher will be buffered until the thread finishes or becomes interruptible.

The priority of OnMessage threads is always 0. Consequently, no messages are monitored or buffered when the current thread's priority is higher than 0.

Caution should be used when monitoring system messages (those below 0x400). For example, if a monitor function does not finish quickly, the response to the message might take longer than the system expects, which might cause side-effects. Unwanted behavior may also occur if a monitor function returns an integer to suppress further processing of a message, but the system expected different processing or a different response.

When the script is displaying a system dialog such as MsgBox, any message posted to a control is not monitored. For example, if the script is displaying a MsgBox and the user clicks a button in a GUI window, the WM_LBUTTONDOWN message is sent directly to the button without calling the monitor function.

Although an external program may post messages directly to a script's thread via PostThreadMessage() or other API call, this is not recommended because the messages would be lost if the script is displaying a system window such as a MsgBox. Instead, it is usually best to post or send the messages to the script's main window or one of its GUI windows.


CallbackCreate, OnExit, OnClipboardChange, PostMessage, SendMessage, Functions, List of Windows Messages, Threads, Critical, DllCall


#1: The following is a working script that monitors mouse clicks in a GUI window. Related topic: ContextMenu event

MyGui := Gui.New(, "Example Window")
MyGui.Add("Text",, "Click anywhere in this window.")
MyGui.Add("Edit", "w200")
MyGui.OnEvent("Close", (*) => ExitApp())
OnMessage 0x201, "WM_LBUTTONDOWN"

WM_LBUTTONDOWN(wParam, lParam, msg, hwnd)
    X := lParam & 0xFFFF
    Y := lParam >> 16
    Control := ""
    thisGui := GuiFromHwnd(hwnd)
    thisGuiControl := GuiCtrlFromHwnd(hwnd)
    if thisGuiControl
        thisGui := thisGuiControl.Gui
        Control := "`n(in control " . thisGuiControl.ClassNN . ")"
    ToolTip "You left-clicked in Gui window '" thisGui.Title "' at client coordinates " X "x" Y "." Control

#2: The following script detects system shutdown/logoff and allows the user to abort it. On Windows Vista and later, the system displays a user interface showing which program is blocking shutdown/logoff and allowing the user to force shutdown/logoff. On older OSes, the script displays a confirmation prompt. Related topic: OnExit

; The following DllCall is optional: it tells the OS to shut down this script first (prior to all other applications).
DllCall("kernel32.dll\SetProcessShutdownParameters", "UInt", 0x4FF, "UInt", 0)

    ENDSESSION_LOGOFF := 0x80000000
    if (lParam & ENDSESSION_LOGOFF)  ; User is logging off.
        EventType := "Logoff"
    else  ; System is either shutting down or restarting.
        EventType := "Shutdown"
        ; Set a prompt for the OS shutdown UI to display.  We do not display
        ; our own confirmation prompt because we have only 5 seconds before
        ; the OS displays the shutdown UI anyway.  Also, a program without
        ; a visible window cannot block shutdown without providing a reason.
        BlockShutdown("Example script attempting to prevent " EventType ".")
        return false
        ; ShutdownBlockReasonCreate is not available, so this is probably
        ; Windows XP, 2003 or 2000, where we can actually prevent shutdown.
        Result := MsgBox(EventType " in progress. Allow it?",, "YN")
        if (Result = "Yes")
            return true  ; Tell the OS to allow the shutdown/logoff to continue.
            return false  ; Tell the OS to abort the shutdown/logoff.

    ; If your script has a visible GUI, use it instead of A_ScriptHwnd.
    DllCall("ShutdownBlockReasonCreate", "ptr", A_ScriptHwnd, "wstr", Reason)

    OnExit(A_ThisFunc, 0)
    DllCall("ShutdownBlockReasonDestroy", "ptr", A_ScriptHwnd)

#3: Have a script receive a custom message and up to two numbers from some other script or program (to send strings rather than numbers, see the example after this one).

OnMessage 0x5555, "MsgMonitor"
OnMessage 0x5556, "MsgMonitor"

MsgMonitor(wParam, lParam, msg, *)
    ; Since returning quickly is often important, it is better to use a ToolTip than
    ; something like MsgBox that would prevent the function from finishing:
    ToolTip "Message " msg " arrived:`nWPARAM: " wParam "`nLPARAM: " lParam

; The following could be used inside some other script to run the function inside the above script:
SetTitleMatchMode 2
DetectHiddenWindows True
if WinExist("Name of Receiving Script.ahk ahk_class AutoHotkey")
    PostMessage 0x5555, 11, 22  ; The message is sent  to the "last found window" due to WinExist above.
DetectHiddenWindows False  ; Must not be turned off until after PostMessage.

#4: Send a string of any length from one script to another. Both scripts must use the same native encoding. This is a working example. To use it, save and run both of the following scripts then press Win+Space to show an InputBox that will prompt you to type in a string.

Save the following script as Receiver.ahk then launch it:

OnMessage 0x4a, "Receive_WM_COPYDATA"  ; 0x4a is WM_COPYDATA

Receive_WM_COPYDATA(wParam, lParam, msg, hwnd)
    StringAddress := NumGet(lParam, 2*A_PtrSize, "Ptr")  ; Retrieves the CopyDataStruct's lpData member.
    CopyOfData := StrGet(StringAddress)  ; Copy the string out of the structure.
    ; Show it with ToolTip vs. MsgBox so we can return in a timely fashion:
    ToolTip A_ScriptName "`nReceived the following string:`n" CopyOfData
    return true  ; Returning 1 (true) is the traditional way to acknowledge this message.

Save the following script as Sender.ahk then launch it. After that, press the Win+Space hotkey.

TargetScriptTitle := "Receiver.ahk ahk_class AutoHotkey"

#space::  ; Win+Space hotkey. Press it to show an InputBox for entry of a message string.
    global TargetScriptTitle
    ib := InputBox("Enter some text to Send:", "Send text via WM_COPYDATA")
    if ib.Result = "Cancel"  ; User pressed the Cancel button.
    result := Send_WM_COPYDATA(ib.Value, TargetScriptTitle)
    if result = ""
        MsgBox "SendMessage failed or timed out. Does the following WinTitle exist?:`n" TargetScriptTitle
    else if (result = 0)
        MsgBox "Message sent but the target window responded with 0, which may mean it ignored it."

Send_WM_COPYDATA(ByRef StringToSend, ByRef TargetScriptTitle)  ; ByRef saves a little memory in this case.
; This function sends the specified string to the specified window and returns the reply.
; The reply is 1 if the target window processed the message, or 0 if it ignored it.
    CopyDataStruct := BufferAlloc(3*A_PtrSize)  ; Set up the structure's memory area.
    ; First set the structure's cbData member to the size of the string, including its zero terminator:
    SizeInBytes := (StrLen(StringToSend) + 1) * 2
    NumPut( "Ptr", SizeInBytes  ; OS requires that this be done.
          , "Ptr", StrPtr(StringToSend)  ; Set lpData to point to the string itself.
          , CopyDataStruct, A_PtrSize)
    Prev_DetectHiddenWindows := A_DetectHiddenWindows
    Prev_TitleMatchMode := A_TitleMatchMode
    DetectHiddenWindows True
    SetTitleMatchMode 2
    TimeOutTime := 4000  ; Optional. Milliseconds to wait for response from receiver.ahk. Default is 5000
    ; Must use SendMessage not PostMessage.
    RetValue := SendMessage(0x4a, 0, CopyDataStruct,, TargetScriptTitle,,,, TimeOutTime) ; 0x4a is WM_COPYDATA.
    DetectHiddenWindows Prev_DetectHiddenWindows  ; Restore original setting for the caller.
    SetTitleMatchMode Prev_TitleMatchMode         ; Same.
    return RetValue  ; Return SendMessage's reply back to our caller.

See the WinLIRC client script for a demonstration of how to use OnMessage to receive a notification when data has arrived on a network connection.