Posting and Sending Messages
Any application can post and send messages. Like the system, an application
posts a message by copying it to a message queue and sends a message by passing
the message data as arguments to a window procedure. To post messages, an
application uses the PostMessage
function. An application can send a message by calling the SendMessage
, or SendDlgItemMessage
An application typically posts a message to notify a specific window to
perform a task. PostMessage
creates an MSG
structure for the message and copies the message to the message queue. The
application's message loop eventually retrieves the message and dispatches it to
the appropriate window procedure.
An application typically sends a message to notify a window procedure to
perform a task immediately. The SendMessage
function sends the message to the window procedure corresponding to the given
window. The function waits until the window procedure completes processing and
then returns the message result. Parent and child windows often communicate by
sending messages to each other. For example, a parent window that has an edit
control as its child window can set the text of the control by sending a
message to it. The control can notify the parent window of changes to the text that
are carried out by the user by sending messages back to the parent.
function also sends a message to the window procedure corresponding to the
given window. However, this function returns immediately. After the window
procedure processes the message, the system calls the specified callback function.
For more information about the callback function, see the SendAsyncProc
Occasionally, you may want to send or post a message to all top-level windows
in the system. For example, if the application changes the system time, it must
notify all top-level windows about the change by sending a WM_TIMECHANGE
message. An application can send or post a message to all top-level windows
by calling the SendMessage
function and specifying HWND_TOPMOST in the hwnd
parameter. You can also broadcast a message to all applications by calling
function and specifying BSM_APPLICATIONS in the lpdwRecipients
An application can post a message without specifying a window. If the
application supplies a NULL window handle when calling PostMessage
, the message is posted to the queue associated with the current thread.
Because no window handle is specified, the application must process the message in
the message loop. This is one way to create a message that applies to the entire
application, instead of to a specific window.
By using the InSendMessage
function, a window procedure can determine whether it is processing a message
sent by another thread. This capability is useful when message processing
depends on the origin of the message.
A common programming error is to assume that the PostMessage
function always posts a message. This is not true when the message queue is
full. An application should check the return value of the PostMessage
function to determine whether the message has been posted and, if it has not
been, repost it.
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