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KERNEL MODULE
=============
...
GDI MODULE
==========
1. X Windows System interface
-----------------------------
The X libraries used to implement X clients (such as Wine) do not work
properly if multiple threads access the same display concurrently. It is
possible to compile the X libraries to perform their own synchronization
(initiated by calling XInitThreads()). However, Wine does not use this
approach. Instead Wine performs its own synchronization py putting a
wrapper around every X call that is used. This wrapper protects library
access with a critical section, and also arranges things so that X
libraries compiled without -D_REENTRANT (eg. with global errno variable)
will work with Wine.
To make this scheme work, all calls to X must use the proper wrapper
functions (or do their own synchronization that is compatible with the
wrappers). The wrapper for a function X...() is calles TSX...() (for
"Thread Safe X ..."). So for example, instead of calling XOpenDisplay()
in the code, TSXOpenDisplay() must be used. Likewise, X include files
that contain function prototypes are wrapped, so that eg. "TSXutil.h" must
be included rather than <X11/Xutil.h>. It is important that this scheme
is used everywhere to avoid the introduction of nondeterministic and
hard-to-find errors in Wine.
The code for the thread safe X wrappers is contained in the tsx11/
directory and in include/TS*.h. To use a new (ie. not previously used) X
function in Wine, a new wrapper must be created. The wrappers are
generated (semi-)automatically from the X11R6 includes using the
tools/make_X11wrappers perl script. In simple cases it should be enough
to add the name of the new function to the list in tsx11/X11_calls; if
this does not work the wrapper must be added manually to the
make_X11wrappers script. See comments in tsx11/X11_calls and
tools/make_X11wrappers for further details.
USER MODULE
===========
USER implements windowing and messaging subsystems. It also
contains code for common controls and for other miscellaneous
stuff (rectangles, clipboard, WNet, etc). Wine USER code is
located in windows/, controls/, and misc/ directories.
1. Windowing subsystem
----------------------
windows/win.c
windows/winpos.c
Windows are arranged into parent/child hierarchy with one
common ancestor for all windows (desktop window). Each window
structure contains a pointer to the immediate ancestor (parent
window if WS_CHILD style bit is set), a pointer to the sibling
(returned by GetWindow(..., GW_NEXT)), a pointer to the owner
window (set only for popup window if it was created with valid
hwndParent parameter), and a pointer to the first child
window (GetWindow(.., GW_CHILD)). All popup and non-child windows
are therefore placed in the first level of this hierarchy and their
ancestor link (wnd->parent) points to the desktop window.
Desktop window - root window
| \ `-.
| \ `-.
popup -> wnd1 -> wnd2 - top level windows
| \ `-. `-.
| \ `-. `-.
child1 child2 -> child3 child4 - child windows
Horizontal arrows denote sibling relationship, vertical lines
- ancestor/child. To summarize, all windows with the same immediate
ancestor are sibling windows, all windows which do not have desktop
as their immediate ancestor are child windows. Popup windows behave
as topmost top-level windows unless they are owned. In this case the
only requirement is that they must precede their owners in the top-level
sibling list (they are not topmost). Child windows are confined to the
client area of their parent windows (client area is where window gets
to do its own drawing, non-client area consists of caption, menu, borders,
intrinsic scrollbars, and minimize/maximize/close/help buttons).
Another fairly important concept is "z-order". It is derived from
the ancestor/child hierarchy and is used to determine "above/below"
relationship. For instance, in the example above, z-order is
child1->popup->child2->child3->wnd1->child4->wnd2->desktop. Current
active window ("foreground window" in Win32) is moved to the front
of z-order unless its top-level ancestor owns popup windows.
All these issues are dealt with (or supposed to be) in windows/winpos.c
with SetWindowPos() being the primary interface to the window manager.
Wine specifics: in default and managed mode each top-level window
gets its own X counterpart with desktop window being basically a
fake stub. In desktop mode, however, only desktop window has an X
window associated with it. Also, SetWindowPos() should eventually be
implemented via Begin/End/DeferWindowPos() calls and not the other way
around.
1.1 Visible region, clipping region and update region
windows/dce.c
windows/winpos.c
windows/painting.c
________________________
|_________ | A and B are child windows of C
| A |______ |
| | | |
|---------' | |
| | B | |
| | | |
| `------------' |
| C |
`------------------------'
Visible region determines which part of the window is not obscured
by other windows. If a window has the WS_CLIPCHILDREN style then all
areas below its children are considered invisible. Similarily, if
the WS_CLIPSIBLINGS bit is in effect then all areas obscured by its
siblings are invisible. Child windows are always clipped by the
boundaries of their parent windows.
B has a WS_CLIPSIBLINGS style:
. ______
: | |
| ,-----' |
| | B | - visible region of B
| | |
: `------------'
When the program requests a display context (DC) for a window it
can specify an optional clipping region that further restricts the
area where the graphics output can appear. This area is calculated
as an intersection of the visible region and a clipping region.
Program asked for a DC with a clipping region:
______
,--|--. | . ,--.
,--+--' | | : _: |
| | B | | => | | | - DC region where the painting will
| | | | | | | be visible
`--|-----|---' : `----'
`-----'
When the window manager detects that some part of the window
became visible it adds this area to the update region of this
window and then generates WM_ERASEBKGND and WM_PAINT messages.
In addition, WM_NCPAINT message is sent when the uncovered area
intersects a nonclient part of the window. Application must reply
to the WM_PAINT message by calling BeginPaint()/EndPaint() pair of
functions. BeginPaint() returns a DC that uses accumulated update
region as a clipping region. This operation cleans up invalidated
area and the window will not receive another WM_PAINT until the
window manager creates a new update region.
A was moved to the left:
________________________ ... / C update region
|______ | : .___ /
| A |_________ | => | ...|___|..
| | | | | : | |
|------' | | | : '---'
| | B | | | : \
| | | | : \
| `------------' | B update region
| C |
`------------------------'
Windows maintains a display context cache consisting of entries that
include DC itself, window to which it belongs, and an optional clipping
region (visible region is stored in the DC itself). When an API call
changes the state of the window tree, window manager has to go through
the DC cache to recalculate visible regions for entries whose windows
were involved in the operation. DC entries (DCE) can be either private
to the window, or private to the window class, or shared between all
windows (Windows 3.1 limits the number of shared DCEs to 5).
1.2
2. Messaging subsystem
----------------------
windows/queue.c
windows/message.c
Each Windows task/thread has its own message queue - this is where
it gets messages from. Messages can be generated on the fly
(WM_PAINT, WM_NCPAINT, WM_TIMER), they can be created by the system
(hardware messages), they can be posted by other tasks/threads
(PostMessage), or they can be sent by other tasks/threads (SendMessage).
Message priority:
First the system looks for sent messages, then for posted messages,
then for hardware messages, then it checks if the queue has the
"dirty window" bit set, and, finally, it checks for expired
timers. See windows/message.c.
From all these different types of messages, only posted messages go
directly into the private message queue. System messages (even in
Win95) are first collected in the system message queue and then
they either sit there until Get/PeekMessage gets to process them
or, as in Win95, if system queue is getting clobbered, a special
thread ("raw input thread") assigns them to the private
queues. Sent messages are queued separately and the sender sleeps
until it gets a reply. Special messages are generated on the fly
depending on the window/queue state. If the window update region is
not empty, the system sets the QS_PAINT bit in the owning queue and
eventually this window receives a WM_PAINT message (WM_NCPAINT too
if the update region intersects with the non-client area). A timer
event is raised when one of the queue timers expire. Depending on
the timer parameters DispatchMessage either calls the callback
function or the window procedure. If there are no messages pending
the task/thread sleeps until messages appear.
There are several tricky moments (open for discussion) -
a) System message order has to be honored and messages should be
processed within correct task/thread context. Therefore when
Get/PeekMessage encounters unassigned system message and this
message appears not to be for the current task/thread it should
either skip it (or get rid of it by moving it into the private
message queue of the target task/thread - Win95, AFAIK) and
look further or roll back and then yield until this message
gets processed when system switches to the correct context
(Win16). In the first case we lose correct message ordering, in
the second case we have the infamous synchronous system message
queue. Here is a post to one of the OS/2 newsgroup I found to
be relevant:
" Here's the problem in a nutshell, and there is no good solution.
Every possible solution creates a different problem.
With a windowing system, events can go to many different windows.
Most are sent by applications or by the OS when things relating to
that window happen (like repainting, timers, etc.)
Mouse input events go to the window you click on (unless some window
captures the mouse).
So far, no problem. Whenever an event happens, you put a message on
the target window's message queue. Every process has a message
queue. If the process queue fills up, the messages back up onto the
system queue.
This is the first cause of apps hanging the GUI. If an app doesn't
handle messages and they back up into the system queue, other apps
can't get any more messages. The reason is that the next message in
line can't go anywhere, and the system won't skip over it.
This can be fixed by making apps have bigger private message queues.
The SIQ fix does this. PMQSIZE does this for systems without the SIQ
fix. Applications can also request large queues on their own.
Another source of the problem, however, happens when you include
keyboard events. When you press a key, there's no easy way to know
what window the keystroke message should be delivered to.
Most windowing systems use a concept known as "focus". The window
with focus gets all incoming keyboard messages. Focus can be changed
from window to window by apps or by users clicking on winodws.
This is the second source of the problem. Suppose window A has focus.
You click on window B and start typing before the window gets focus.
Where should the keystrokes go? On the one hand, they should go to A
until the focus actually changes to B. On the other hand, you
probably want the keystrokes to go to B, since you clicked there
first.
OS/2's solution is that when a focus-changing event happens (like
clicking on a window), OS/2 holds all messages in the system queue
until the focus change actually happens. This way, subsequent
keystrokes go to the window you clicked on, even if it takes a while
for that window to get focus.
The downside is that if the window takes a real long time to get focus
(maybe it's not handling events, or maybe the window losing focus
isn't handling events), everything backs up in the system queue and
the system appears hung.
There are a few solutions to this problem.
One is to make focus policy asynchronous. That is, focus changing has
absolutely nothing to do with the keyboard. If you click on a window
and start typing before the focus actually changes, the keystrokes go
to the first window until focus changes, then they go to the second.
This is what X-windows does.
Another is what NT does. When focus changes, keyboard events are held
in the system message queue, but other events are allowed through.
This is "asynchronous" because the messages in the system queue are
delivered to the application queues in a different order from that
with which they were posted. If a bad app won't handle the "lose
focus" message, it's of no consequence - the app receiving focus will
get its "gain focus" message, and the keystrokes will go to it.
The NT solution also takes care of the application queue filling up
problem. Since the system delivers messages asynchronously, messages
waiting in the system queue will just sit there and the rest of the
messages will be delivered to their apps.
The OS/2 SIQ solution is this: When a focus-changing event happens,
in addition to blocking further messages from the application queues,
a timer is started. When the timer goes off, if the focus change has
not yet happened, the bad app has its focus taken away and all
messages targetted at that window are skipped. When the bad app
finally handles the focus change message, OS/2 will detect this and
stop skipping its messages.
As for the pros and cons:
The X-windows solution is probably the easiest. The problem is that
users generally don't like having to wait for the focus to change
before they start typing. On many occasions, you can type and the
characters end up in the wrong window because something (usually heavy
system load) is preventing the focus change from happening in a timely
manner.
The NT solution seems pretty nice, but making the system message queue
asynchronous can cause similar problems to the X-windows problem.
Since messages can be delivered out of order, programs must not assume
that two messages posted in a particular order will be delivered in
that same order. This can break legacy apps, but since Win32 always
had an asynchronous queue, it is fair to simply tell app designers
"don't do that". It's harder to tell app designers something like
that on OS/2 - they'll complain "you changed the rules and our apps
are breaking."
The OS/2 solution's problem is that nothing happens until you try to
change window focus, and then wait for the timeout. Until then, the
bad app is not detected and nothing is done." (by David Charlap)
b) Intertask/interthread SendMessage. The system has to inform the
target queue about the forthcoming message, then it has to carry
out the context switch and wait until the result is available.
Win16 stores necessary parameters in the queue structure and then
calls DirectedYield() function. However, in Win32 there could be
several messages pending sent by preemptively executing threads,
and in this case SendMessage has to build some sort of message
queue for sent messages. Another issue is what to do with messages
sent to the sender when it is blocked inside its own SendMessage.