GetMessageA function (winuser.h)

Article
02/09/2023

Retrieves a message from the calling thread's message queue. The function dispatches incoming sent messages until a posted message is available for retrieval.

Unlike GetMessage, the PeekMessage function does not wait for a message to be posted before returning.

Syntax

BOOL GetMessageA(
  [out]          LPMSG lpMsg,
  [in, optional] HWND  hWnd,
  [in]           UINT  wMsgFilterMin,
  [in]           UINT  wMsgFilterMax
);

Parameters

[out] lpMsg

Type: LPMSG

A pointer to an MSG structure that receives message information from the thread's message queue.

[in, optional] hWnd

Type: HWND

A handle to the window whose messages are to be retrieved. The window must belong to the current thread.

If hWnd is NULL, GetMessage retrieves messages for any window that belongs to the current thread, and any messages on the current thread's message queue whose hwnd value is NULL (see the MSG structure). Therefore if hWnd is NULL, both window messages and thread messages are processed.

If hWnd is -1, GetMessage retrieves only messages on the current thread's message queue whose hwnd value is NULL, that is, thread messages as posted by PostMessage (when the hWnd parameter is NULL) or PostThreadMessage.

[in] wMsgFilterMin

Type: UINT

The integer value of the lowest message value to be retrieved. Use WM_KEYFIRST (0x0100) to specify the first keyboard message or WM_MOUSEFIRST (0x0200) to specify the first mouse message.

Use WM_INPUT here and in wMsgFilterMax to specify only the WM_INPUT messages.

If wMsgFilterMin and wMsgFilterMax are both zero, GetMessage returns all available messages (that is, no range filtering is performed).

[in] wMsgFilterMax

Type: UINT

The integer value of the highest message value to be retrieved. Use WM_KEYLAST to specify the last keyboard message or WM_MOUSELAST to specify the last mouse message.

Use WM_INPUT here and in wMsgFilterMin to specify only the WM_INPUT messages.

If wMsgFilterMin and wMsgFilterMax are both zero, GetMessage returns all available messages (that is, no range filtering is performed).

Return value

Type: BOOL

If the function retrieves a message other than WM_QUIT, the return value is nonzero.

If the function retrieves the WM_QUIT message, the return value is zero.

If there is an error, the return value is -1. For example, the function fails if hWnd is an invalid window handle or lpMsg is an invalid pointer. To get extended error information, call GetLastError.

Because the return value can be nonzero, zero, or -1, avoid code like this:

while (GetMessage( lpMsg, hWnd, 0, 0)) ...

The possibility of a -1 return value in the case that hWnd is an invalid parameter (such as referring to a window that has already been destroyed) means that such code can lead to fatal application errors. Instead, use code like this:

BOOL bRet;

while( (bRet = GetMessage( &msg, hWnd, 0, 0 )) != 0)
{ 
    if (bRet == -1)
    {
        // handle the error and possibly exit
    }
    else
    {
        TranslateMessage(&msg); 
        DispatchMessage(&msg); 
    }
}

Remarks

An application typically uses the return value to determine whether to end the main message loop and exit the program.

The GetMessage function retrieves messages associated with the window identified by the hWnd parameter or any of its children, as specified by the IsChild function, and within the range of message values given by the wMsgFilterMin and wMsgFilterMax parameters. Note that an application can only use the low word in the wMsgFilterMin and wMsgFilterMax parameters; the high word is reserved for the system.

Note that GetMessage always retrieves WM_QUIT messages, no matter which values you specify for wMsgFilterMin and wMsgFilterMax.

During this call, the system delivers pending, nonqueued messages, that is, messages sent to windows owned by the calling thread using the SendMessage, SendMessageCallback, SendMessageTimeout, or SendNotifyMessage function. Then the first queued message that matches the specified filter is retrieved. The system may also process internal events. If no filter is specified, messages are processed in the following order:

Sent messages
Posted messages
Input (hardware) messages and system internal events
Sent messages (again)
WM_PAINT messages
WM_TIMER messages

To retrieve input messages before posted messages, use the wMsgFilterMin and wMsgFilterMax parameters.

GetMessage does not remove WM_PAINT messages from the queue. The messages remain in the queue until processed.

If a top-level window stops responding to messages for more than several seconds, the system considers the window to be not responding and replaces it with a ghost window that has the same z-order, location, size, and visual attributes. This allows the user to move it, resize it, or even close the application. However, these are the only actions available because the application is actually not responding. When in the debugger mode, the system does not generate a ghost window.

DPI Virtualization

This API does not participate in DPI virtualization. The output is in the mode of the window that the message is targeting. The calling thread is not taken into consideration.

Examples

For an example, see Creating a Message Loop.

Note

The winuser.h header defines GetMessage as an alias that automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that is not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.

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)
Library	User32.lib
DLL	User32.dll
API set	ext-ms-win-ntuser-message-l1-1-0 (introduced in Windows 8)

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