Examining a Message Queue
Occasionally, an application needs to examine the contents of a thread's
message queue from outside the thread's message loop. For example, if an
application's window procedure performs a lengthy drawing operation, you may want the user
to be able to interrupt the operation. Unless your application periodically
examines the message queue during the operation for mouse and keyboard messages,
it will not respond to user input until after the operation has completed. The
reason for this is that the DispatchMessage
function in the thread's message loop does not return until the window
procedure finishes processing a message.
You can use the PeekMessage
function to examine a message queue during a lengthy operation. PeekMessage
is similar to the GetMessage
function; both check a message queue for a message that matches the filter
criteria and then copy the message to an MSG
structure. The main difference between the two functions is that GetMessage
does not return until a message matching the filter criteria is placed in the
queue, whereas PeekMessage
returns immediately regardless of whether a message is in the queue.
The following example shows how to use PeekMessage
to examine a message queue for mouse clicks and keyboard input during a
// Begin the operation and continue until it is complete
// or until the user clicks the mouse or presses a key.
fDone = FALSE;
fDone = DoLengthyOperation(); // application-defined function
// Remove any messages that may be in the queue. If the
// queue contains any mouse or keyboard
// messages, end the operation.
while (PeekMessage(&msg, hwnd, 0, 0, PM_REMOVE))
// Perform any required cleanup.
fDone = TRUE;
Other functions, including GetQueueStatus
, also allow you to examine the contents of a thread's message queue. GetQueueStatus
returns an array of flags that indicates the types of messages in the queue;
using it is the fastest way to discover whether the queue contains any
returns TRUE if the queue contains mouse or keyboard messages. Both of these
functions can be used to determine whether the queue contains messages that
need to be processed.
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