WM_GETTEXT

  This message is sent to a window when the application wants to receive a
copy of its text. For edit controls this text is the contents of the edit
buffer. For button controls, this text is the button name. For all other
windows, this text is the window's caption.

Parameters
  wParam specifies the maximum number of bytes to copy, including the null
   terminator.
  lParam is a far pointer to a buffer that will receive the text.

Returns
  The number of bytes copied.