PeekMessage

The PeekMessage function checks a thread message queue for a message and places the message (if any) in the specified structure.

BOOL PeekMessage(

LPMSG lpMsg,	// pointer to structure for message
HWND hWnd,	// handle to window
UINT wMsgFilterMin,	// first message
UINT wMsgFilterMax,	// last message
UINT wRemoveMsg	// removal flags
);

Parameters

lpMsg

Points to an MSG structure that contains message information from the Windows-based application queue.

hWnd

Identifies the window whose messages are to be examined.

wMsgFilterMin

Specifies the value of the first message in the range of messages to be examined.

wMsgFilterMax

Specifies the value of the last message in the range of messages to be examined.

wRemoveMsg

Specifies how messages are handled. This parameter can be one of the following values:

Value	Meaning
PM_NOREMOVE	Messages are not removed from the queue after processing by PeekMessage.
PM_REMOVE	Messages are removed from the queue after processing by PeekMessage.

You can optionally combine the value PM_NOYIELD with either PM_NOREMOVE or PM_REMOVE. However, PM_NOYIELD has no effect on 32-bit Windows applications. It is defined in Win32 solely to provide compatibility with applications written for previous versions of Windows, where it was used to prevent the current task from halting and yielding system resources to another task. 32-bit Windows applications always run simultaneously.

Return Values

If a message is available, the return value is nonzero.

If no messages are available, the return value is zero.

Remarks

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

PeekMessage retrieves only 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. If hWnd is NULL, PeekMessage retrieves messages for any window that belongs to the current thread making the call. (PeekMessage does not retrieve messages for windows that belong to other threads.) If hWnd is 1, PeekMessage only returns messages with a hWnd value of NULL, as posted by the PostAppMessage function. If wMsgFilterMin and wMsgFilterMax are both zero, PeekMessage returns all available messages (that is, no range filtering is performed).

The WM_KEYFIRST and WM_KEYLAST constants can be used as filter values to retrieve all keyboard messages; the WM_MOUSEFIRST and WM_MOUSELAST constants can be used to retrieve all mouse 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.

See Also

GetMessage, IsChild, MSG, PostAppMessage, WaitMessage

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