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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 440
SEC. 5.6
USER INTERFACES: KEYBOARD, MOUSE, MONITOR
409
also a 32-bit signed integer), s (string), sz (string terminated by a zero byte), p
(pointer), fn (function), and h (handle).
Thus
szCmd
is a zero-terminated string
and
iCmdShow
is an integer, for example. Many programmers believe that en-
coding the type in variable names this way has little value and makes Windows
code hard to read. Nothing analogous to this convention is present in UNIX.
Every window must have an associated class object that defines its properties.
In Fig. 5-36, that class object is
wndclass
.
An object of type
WNDCLASS
has 10
fields, four of which are initialized in Fig. 5-36. In an actual program, the other six
would be initialized as well. The most important field is
lpfnWndProc
, which is a
long (i.e., 32-bit) pointer to the function that handles the messages directed to this
window. The other fields initialized here tell which name and icon to use in the
title bar, and which symbol to use for the mouse cursor.
After
wndclass
has been initialized,
RegisterClass
is called to pass it to Win-
dows. In particular, after this call Windows knows which procedure to call when
various events occur that do not go through the message queue. The next call,
Cre-
ateWindow
, allocates memory for the window’s data structure and returns a handle
for referencing it later. The program then makes two more calls in a row, to put the
window’s outline on the screen, and finally fill it in completely.
At this point we come to the program’s main loop, which consists of getting a
message, having certain translations done to it, and then passing it back to Win-
dows to have Windows invoke
WndProc
to process it.
To answer the question of
whether this whole mechanism could have been made simpler, the answer is yes,
but it was done this way for historical reasons and we are now stuck with it.
Following the main program is the procedure
WndProc
, which handles the
various messages that can be sent to the window. The use of
CALLBACK
here, like
WINAPI
above, specifies the calling sequence to use for parameters. The first pa-
rameter is the handle of the window to use. The second parameter is the message
type. The third and fourth parameters can be used to provide additional infor-
mation when needed.
Message types
WM
CREATE
and
WM
DESTROY
are sent at the start and end
of the program, respectively. They give the program the opportunity, for example,
to allocate memory for data structures and then return it.
The third message type,
WM
PAINT
, is an instruction to the program to fill in
the window. It is called not only when the window is first drawn, but often during
program execution as well.
In contrast to text-based systems, in Windows a pro-
gram cannot assume that whatever it draws on the screen will stay there until it re-
moves it. Other windows can be dragged on top of this one, menus can be pulled
down over it, dialog boxes and tool tips can cover part of it, and so on. When these
items are removed, the window has to be redrawn. The way Windows tells a pro-
gram to redraw a window is to send it a
WM
PAINT
message. As a friendly ges-
ture, it also provides information about what part of the window has been overwrit-
ten, in case it is easier or faster to regenerate that part of the window instead of
redrawing the whole thing from scratch.
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