PeekMessageA function (winuser.h)
Dispatches incoming nonqueued messages, checks the thread message queue for a posted message, and retrieves the message (if any exist).
Syntax
BOOL PeekMessageA(
[out] LPMSG lpMsg,
[in, optional] HWND hWnd,
[in] UINT wMsgFilterMin,
[in] UINT wMsgFilterMax,
[in] UINT wRemoveMsg
);
Parameters
[out] lpMsg
Type: LPMSG
A pointer to an MSG structure that receives message information.
[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, PeekMessage 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, PeekMessage 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 value of the first message in the range of messages to be examined. Use WM_KEYFIRST (0x0100) to specify the first keyboard message or WM_MOUSEFIRST (0x0200) to specify the first mouse message.
If wMsgFilterMin and wMsgFilterMax are both zero, PeekMessage returns all available messages (that is, no range filtering is performed).
[in] wMsgFilterMax
Type: UINT
The value of the last message in the range of messages to be examined. Use WM_KEYLAST to specify the last keyboard message or WM_MOUSELAST to specify the last mouse message.
If wMsgFilterMin and wMsgFilterMax are both zero, PeekMessage returns all available messages (that is, no range filtering is performed).
[in] wRemoveMsg
Type: UINT
Specifies how messages are to be handled. This parameter can be one or more of the following values.
| Value | Meaning |
|---|---|
|
Messages are not removed from the queue after processing by PeekMessage. |
|
Messages are removed from the queue after processing by PeekMessage. |
|
Prevents the system from releasing any thread that is waiting for the caller to go idle (see WaitForInputIdle).
Combine this value with either PM_NOREMOVE or PM_REMOVE. |
By default, all message types are processed. To specify that only certain message should be processed, specify one or more of the following values.
Return value
Type: BOOL
If a message is available, the return value is nonzero.
If no messages are available, the return value is zero.
Remarks
PeekMessage 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 PeekMessage always retrieves WM_QUIT messages, no matter which values you specify for wMsgFilterMin and wMsgFilterMax.
During this call, the system dispatches (DispatchMessage) 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
The PeekMessage function normally does not remove WM_PAINT messages from the queue. WM_PAINT messages remain in the queue until they are processed. However, if a WM_PAINT message has a NULL update region, PeekMessage does remove it from the queue.
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 an application is being debugged, 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 Examining a Message Queue.
Note
The winuser.h header defines PeekMessage 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) |
See also
Conceptual
Other Resources
Reference