The InSendMessage function determines whether the current window procedure is processing a message that was sent from another thread (in the same process or a different process) by a call to the SendMessage function.
BOOL InSendMessage(VOID);
This function has no parameters.
If the window procedure is processing a message sent to it from another thread using the SendMessage function, the return value is nonzero.
If the window procedure is not processing a message sent to it from another thread using the SendMessage function, the return value is zero.
Windows NT: Requires version 3.1 or later.
Windows: Requires Windows 95 or later.
Windows CE: Unsupported.
Header: Declared in winuser.h.
Import Library: Use user32.lib.
Messages and Message Queues Overview, Message and Message Queue Functions, SendMessage