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wine/documentation/architecture.sgml
Francois Gouget fbb3343547 Assorted spelling fixes.
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<chapter id="architecture">
<title>Overview</title>
<para>Brief overview of Wine's architecture...</para>
<sect1 id="basic-overview">
<title>Wine Overview</title>
<para>
With the fundamental architecture of Wine stabilizing, and
people starting to think that we might soon be ready to
actually release this thing, it may be time to take a look at
how Wine actually works and operates.
</para>
<sect2>
<title>Foreword</title>
<para>
Wine is often used as a recursive acronym, standing for
"Wine Is Not an Emulator". Sometimes it is also known to be
used for "Windows Emulator". In a way, both meanings are
correct, only seen from different perspectives. The first
meaning says that Wine is not a virtual machine, it does not
emulate a CPU, and you are not supposed to install
Windows nor any Windows device drivers on top of it; rather,
Wine is an implementation of the Windows API, and can be
used as a library to port Windows applications to Unix. The
second meaning, obviously, is that to Windows binaries
(<filename>.exe</filename> files), Wine does look like
Windows, and emulates its behaviour and quirks rather
closely.
</para>
<note>
<title>"Emulator"</title>
<para>
The "Emulator" perspective should not be thought of as if
Wine is a typical inefficient emulation layer that means
Wine can't be anything but slow - the faithfulness to the
badly designed Windows API may of course impose a minor
overhead in some cases, but this is both balanced out by
the higher efficiency of the Unix platforms Wine runs on,
and that other possible abstraction libraries (like Motif,
GTK+, CORBA, etc) has a runtime overhead typically
comparable to Wine's.
</para>
</note>
</sect2>
<sect2>
<title>Executables</title>
<para>
Wine's main task is to run Windows executables under non
Windows operating systems. It supports different types of
executables:
<itemizedlist>
<listitem>
<para>
DOS executable. Those are even older programs, using
the DOS format (either <filename>.com</filename> or
<filename>.exe</filename> (the later being also called
MZ)).
</para>
</listitem>
<listitem>
<para>
Windows NE executable, also called 16 bit. They were
the native processes run by Windows 2.x and 3.x. NE
stands for New Executable &lt;g&gt;.
</para>
</listitem>
<listitem>
<para>
Windows PE executable. These are programs were
introduced in Windows 95 (and became the native
formats for all later Windows version), even if 16 bit
applications were still supported. PE stands for
Portable Executable, in a sense where the format of
the executable (as a file) is independent of the CPU
(even if the content of the file - the code - is CPU
dependent).
</para>
</listitem>
<listitem>
<para>
WineLib executable. These are applications, written
using the Windows API, but compiled as a Unix
executable. Wine provides the tools to create such
executables.
</para>
</listitem>
</itemizedlist>
</para>
<para>
Let's quickly review the main differences for the supported
executables:
<table>
<title>Wine executables</title>
<tgroup cols="5" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry></entry>
<entry>DOS (.COM or .EXE)</entry>
<entry>Win16 (NE)</entry>
<entry>Win32 (PE)</entry>
<entry>WineLib</entry>
</row>
</thead>
<tbody>
<row>
<entry>Multitasking</entry>
<entry>Only one application at a time (except for TSR)</entry>
<entry>Cooperative</entry>
<entry>Preemptive</entry>
<entry>Preemptive</entry>
</row>
<row>
<entry>Address space</entry>
<entry>
One MB of memory, where each application is loaded
and unloaded.
</entry>
<entry>
All 16 bit applications share a single address
space, protected mode.
</entry>
<entry>
Each application has it's own address
space. Requires MMU support from CPU.
</entry>
<entry>
Each application has it's own address
space. Requires MMU support from CPU.
</entry>
</row>
<row>
<entry>Windows API</entry>
<entry>
No Windows API but the DOS API (like <function>Int
21h</function> traps).
</entry>
<entry>
Will call the 16 bit Windows API.
</entry>
<entry>
Will call the 32 bit Windows API.
</entry>
<entry>
Will call the 32 bit Windows API, and possibly
also the Unix APIs.
</entry>
</row>
<row>
<entry>Code (CPU level)</entry>
<entry>
Only available on x86 in real mode. Code and data
are in segmented forms, with 16 bit
offsets. Processor is in real mode.
</entry>
<entry>
Only available on IA-32 architectures, code and
data are in segmented forms, with 16 bit offsets
(hence the 16 bit name). Processor is in protected
mode.
</entry>
<entry>
Available (with NT) on several CPUs, including
IA-32. On this CPU, uses a flat memory model with
32 bit offsets (hence the 32 bit name).
</entry>
<entry>
Flat model, with 32 bit addresses.
</entry>
</row>
<row>
<entry>Multi-threading</entry>
<entry>Not available.</entry>
<entry>Not available.</entry>
<entry>
Available.
</entry>
<entry>
Available, but must use the Win32 APIs for
threading and synchronization, not the Unix ones.
</entry>
</row>
</tbody>
</tgroup>
</table>
</para>
<para>
Wine deals with this issue by launching a separate Wine
process (which is in fact a Unix process) for each Win32
process, but not for Win16 tasks. Win16 tasks (as well as
DOS programs) are run as different intersynchronized
Unix-threads in the same dedicated Wine process; this Wine
process is commonly known as a <firstterm>WOW</firstterm>
process (Windows on Windows), referring to a similar
mechanism used by Windows NT.
</para>
<para>
Synchronization between the Win16 tasks running in the WOW
process is normally done through the Win16 mutex - whenever
one of them is running, it holds the Win16 mutex, keeping
the others from running. When the task wishes to let the
other tasks run, the thread releases the Win16 mutex, and
one of the waiting threads will then acquire it and let its
task run.
</para>
</sect2>
</sect1>
<sect1>
<title>Standard Windows Architectures</title>
<sect2>
<title>Windows 9x architecture</title>
<para>
The windows architecture (Win 9x way) looks like this:
<screen>
+---------------------+ \
| Windows EXE | } application
+---------------------+ /
+---------+ +---------+ \
| Windows | | Windows | \ application & system DLLs
| DLL | | DLL | /
+---------+ +---------+ /
+---------+ +---------+ \
| GDI32 | | USER32 | \
| DLL | | DLL | \
+---------+ +---------+ } core system DLLs
+---------------------+ /
| Kernel32 DLL | /
+---------------------+ /
+---------------------+ \
| Win9x kernel | } kernel space
+---------------------+ /
+---------------------+ \
| Windows low-level | \ drivers (kernel space)
| drivers | /
+---------------------+ /
</screen>
</para>
</sect2>
<sect2>
<title>Windows NT architecture</title>
<para>
The windows architecture (Windows NT way) looks like the
following drawing. Note the new DLL (NTDLL) which allows
implementing different subsystems (as win32); kernel32 in NT
architecture implements the Win32 subsystem on top of NTDLL.
<screen>
+---------------------+ \
| Windows EXE | } application
+---------------------+ /
+---------+ +---------+ \
| Windows | | Windows | \ application & system DLLs
| DLL | | DLL | /
+---------+ +---------+ /
+---------+ +---------+ +-----------+ \
| GDI32 | | USER32 | | | \
| DLL | | DLL | | | \
+---------+ +---------+ | | \ core system DLLs
+---------------------+ | | / (on the left side)
| Kernel32 DLL | | Subsystem | /
| (Win32 subsystem) | |Posix, OS/2| /
+---------------------+ +-----------+ /
+---------------------------------------+
| NTDLL.DLL |
+---------------------------------------+
+---------------------------------------+ \
| NT kernel | } NT kernel (kernel space)
+---------------------------------------+ /
+---------------------------------------+ \
| Windows low-level drivers | } drivers (kernel space)
+---------------------------------------+ /
</screen>
</para>
<para>
Note also (not depicted in schema above) that the 16 bit
applications are supported in a specific subsystem.
Some basic differences between the Win9x and the NT
architectures include:
<itemizedlist>
<listitem>
<para>
Several subsystems (Win32, Posix...) can be run on NT,
while not on Win 9x
</para>
</listitem>
<listitem>
<para>
Win 9x roots its architecture in 16 bit systems, while
NT is truly a 32 bit system.
</para>
</listitem>
<listitem>
<para>
The drivers model and interfaces in Win 9x and NT are
different (even if Microsoft tried to bridge the gap
with some support of WDM drivers in Win 98 and above).
</para>
</listitem>
</itemizedlist>
</para>
</sect2>
</sect1>
<sect1>
<title>Wine architecture</title>
<sect2>
<title>Global picture</title>
<para>
Wine implementation is closer to the Windows NT
architecture, even if several subsystems are not implemented
yet (remind also that 16bit support is implemented in a 32-bit
Windows EXE, not as a subsystem). Here's the overall picture:
<screen>
+---------------------+ \
| Windows EXE | } application
+---------------------+ /
+---------+ +---------+ \
| Windows | | Windows | \ application & system DLLs
| DLL | | DLL | /
+---------+ +---------+ /
+---------+ +---------+ +-----------+ +--------+ \
| GDI32 | | USER32 | | | | | \
| DLL | | DLL | | | | Wine | \
+---------+ +---------+ | | | Server | \ core system DLLs
+---------------------+ | | | | / (on the left side)
| Kernel32 DLL | | Subsystem | | NT-like| /
| (Win32 subsystem) | |Posix, OS/2| | Kernel | /
+---------------------+ +-----------+ | | /
| |
+---------------------------------------+ | |
| NTDLL | | |
+---------------------------------------+ +--------+
+---------------------------------------+ \
| Wine executable (wine-?thread) | } unix executable
+---------------------------------------+ /
+---------------------------------------------------+ \
| Wine drivers | } Wine specific DLLs
+---------------------------------------------------+ /
+------------+ +------------+ +--------------+ \
| libc | | libX11 | | other libs | } unix shared libraries
+------------+ +------------+ +--------------+ / (user space)
+---------------------------------------------------+ \
| Unix kernel (Linux,*BSD,Solaris,OS/X) | } (Unix) kernel space
+---------------------------------------------------+ /
+---------------------------------------------------+ \
| Unix device drivers | } Unix drivers (kernel space)
+---------------------------------------------------+ /
</screen>
</para>
<para>
Wine must at least completely replace the "Big Three" DLLs
(KERNEL/KERNEL32, GDI/GDI32, and USER/USER32), which all
other DLLs are layered on top of. But since Wine is (for
various reasons) leaning towards the NT way of implementing
things, the NTDLL is another core DLL to be implemented in
Wine, and many KERNEL32 and ADVAPI32 features will be
implemented through the NTDLL.
</para>
<para>
As of today, no real subsystem (apart the Win32 one) has
been implemented in Wine.
</para>
<para>
The Wine server provides the backbone for the implementation
of the core DLLs. It mainly implementents inter-process
synchronization and object sharing. It can be seen, from a
functional point of view, as a NT kernel (even if the APIs
and protocols used between Wine's DLL and the Wine server
are Wine specific).
</para>
<para>
Wine uses the Unix drivers to access the various hardware
pieces on the box. However, in some cases, Wine will
provide a driver (in Windows sense) to a physical hardware
device. This driver will be a proxy to the Unix driver
(this is the case, for example, for the graphical part
with X11 or SDL drivers, audio with OSS or ALSA drivers...).
</para>
<para>
All DLLs provided by Wine try to stick as much as possible
to the exported APIs from the Windows platforms. There are
rare cases where this is not the case, and have been
propertly documented (Wine DLLs export some Wine specific
APIs). Usually, those are prefixed with
<function>__wine</function>.
</para>
<para>
Let's now review in greater details all of those components.
</para>
</sect2>
<sect2>
<title>The Wine server</title>
<para>
The Wine server is among the most confusing concepts in
Wine. What is its function in Wine? Well, to be brief, it
provides Inter-Process Communication (IPC),
synchronization, and process/thread management. When the
Wine server launches, it creates a Unix socket for the
current host based on (see below) your home directory's
<filename>.wine</filename> subdirectory (or wherever the
<constant>WINEPREFIX</constant> environment variable
points to) - all Wine processes launched later connects to
the Wine server using this socket. (If a Wine server was
not already running, the first Wine process will start up
the Wine server in auto-terminate mode (i.e. the Wine
server will then terminate itself once the last Wine
process has terminated).)
</para>
<para>
In earlier versions of Wine the master socket mentioned
above was actually created in the configuration directory;
either your home directory's <filename>/wine</filename>
subdirectory or wherever the
<constant>WINEPREFIX</constant> environment variable
points>. Since that might not be possible the socket is
actually created within the <filename>/tmp</filename>
directory with a name that reflects the configuration
directory. This means that there can actually be several
separate copies of the Wine server running; one per
combination of user and configuration directory. Note that
you should not have several users using the same
configuration directory at the same time; they will have
different copies of the Wine server running and this could
well lead to problems with the registry information that
they are sharing.
</para>
<para>
Every thread in each Wine process has its own request
buffer, which is shared with the Wine server. When a
thread needs to synchronize or communicate with any other
thread or process, it fills out its request buffer, then
writes a command code through the socket. The Wine server
handles the command as appropriate, while the client
thread waits for a reply. In some cases, like with the
various <function>WaitFor???</function> synchronization
primitives, the server handles it by marking the client
thread as waiting and does not send it a reply before the
wait condition has been satisfied.
</para>
<para>
The Wine server itself is a single and separate Unix
process and does not have its own threading - instead, it
is built on top of a large <function>poll()</function>
loop that alerts the Wine server whenever anything
happens, such as a client having sent a command, or a wait
condition having been satisfied. There is thus no danger
of race conditions inside the Wine server itself - it is
often called upon to do operations that look completely
atomic to its clients.
</para>
<para>
Because the Wine server needs to manage processes,
threads, shared handles, synchronization, and any related
issues, all the clients' Win32 objects are also managed by
the Wine server, and the clients must send requests to the
Wine server whenever they need to know any Win32 object
handle's associated Unix file descriptor (in which case
the Wine server duplicates the file descriptor, transmits
it back to the client, and leaves it to the client to
close the duplicate when the client has finished with
it).
</para>
</sect2>
<sect2>
<title>
Wine builtin DLLs: about Relays, Thunks, and DLL
descriptors
</title>
<para>
This section mainly applies to builtin DLLs (DLLs provided
by Wine). See section <xref linkend="arch-dlls"> for the
details on native vs. builtin DLL handling.
</para>
<para>
Loading a Windows binary into memory isn't that hard by
itself, the hard part is all those various DLLs and entry
points it imports and expects to be there and function as
expected; this is, obviously, what the entire Wine
implementation is all about. Wine contains a range of DLL
implementations. You can find the DLLs implementation in the
<filename>dlls/</filename> directory.
</para>
<para>
Each DLL (at least, the 32 bit version, see below) is
implemented in a Unix shared library. The file name of this
shared library is the module name of the DLL with a
<filename>.dll.so</filename> suffix (or
<filename>.drv.so</filename> or any other relevant extension
depending on the DLL type). This shared library contains the
code itself for the DLL, as well as some more information,
as the DLL resources and a Wine specific DLL descriptor.
</para>
<para>
The DLL descriptor, when the DLL is instanciated, is used to
create an in-memory PE header, which will provide access to
various information about the DLL, including but not limited
to its entry point, its resources, its sections, its debug
information...
</para>
<para>
The DLL descriptor and entry point table is generated by
the <command>winebuild</command> tool (previously just
named <command>build</command>), taking DLL specification
files with the extension <filename>.spec</filename> as
input. Resources (after compilation by
<command>wrc</command>) or message tables (after
compilation by <command>wmc</command>) are also added to
the descriptor by <command>winebuild</command>.
</para>
<para>
Once an application module wants to import a DLL, Wine
will look at:
<itemizedlist>
<listitem>
<para>
through its list of registered DLLs (in fact, both
the already loaded DLLs, and the already loaded
shared libraries which has registered a DLL
descriptor). Since, the DLL descriptor is
automatically registered when the shared library is
loaded - remember, registration call is put inside a
shared library constructor - using the
<constant>PRELOAD</constant> environment variable
when running a Wine process can force the
registration of some DLL descriptors.
</para>
</listitem>
<listitem>
<para>
If it's not registered, Wine will look for it on
disk, building the shared library name from the DLL
module name. Directory searched for are specified by
the <constant>WINEDLLPATH</constant> environment
variable.
</para>
</listitem>
<listitem>
<para>
Failing that, it will look for a real Windows
<filename>.DLL</filename> file to use, and look
through its imports, etc) and use the loading of
native DLLs.
</para>
</listitem>
</itemizedlist>
</para>
<para>
After the DLL has been identified (assuming it's still a
native one), it's mapped into memory using a
<function>dlopen()</function> call. Note, that Wine doesn't
use the shared library mechanisms for resolving and/or
importing functions between two shared libraries (for two
DLLs). The shared library is only used for providing a way
to load a piece of code on demand. This piece of code,
thanks the DLL descriptor, will provide the same type of
information a native DLL would. Wine can then use the same
code for native and builtin DLL to handle imports/exports.
</para>
<para>
Wine also relies on the dynamic loading features of the Unix
shared libraries to relocate the DLLs if needed (the same
DLL can be loaded at different address in two different
processes, and even in two consecutive run of the same
executable if the order of loading the DLLs differ).
</para>
<para>
The DLL descriptor is registered in the Wine realm using
some tricks. The <command>winebuild</command> tool, while
creating the code for DLL descriptor, also creates a
constructor, that will be called when the shared library is
loaded into memory. This constructor will actually register
the descriptor to the Wine DLL loader. Hence, before the
<function>dlopen</function> call returns, the DLL descriptor
will be known and registered. This also helps to deal with
the cases where there's still dependencies (at the ELF
shared lib level, not at the embedded DLL level) between
different shared libraries: the embedded DLLs will be
properly registered, and even loaded (from a Windows point
of view).
</para>
<para>
Since Wine is 32-bit code itself, and if the compiler
supports Windows' calling convention, <type>stdcall</type>
(<command>gcc</command> does), Wine can resolve imports
into Win32 code by substituting the addresses of the Wine
handlers directly without any thunking layer in
between. This eliminates the overhead most people
associate with "emulation", and is what the applications
expect anyway.
</para>
<para>
However, if the user specified <parameter>WINEDEBUG=+relay
</parameter>, a thunk layer is inserted between the
application imports and the Wine handlers (actually the
export table of the DLL is modified, and a thunk is
inserted in the table); this layer is known as "relay"
because all it does is print out the arguments/return
values (by using the argument lists in the DLL
descriptor's entry point table), then pass the call on,
but it's invaluable for debugging misbehaving calls into
Wine code. A similar mechanism also exists between Windows
DLLs - Wine can optionally insert thunk layers between
them, by using <parameter>WINEDEBUG=+snoop</parameter>,
but since no DLL descriptor information exists for
non-Wine DLLs, this is less reliable and may lead to
crashes.
</para>
<para>
For Win16 code, there is no way around thunking - Wine
needs to relay between 16-bit and 32-bit code. These
thunks switch between the app's 16-bit stack and Wine's
32-bit stack, copies and converts arguments as appropriate
(an int is 16 bit 16-bit and 32 bits in 32-bit, pointers
are segmented in 16 bit (and also near or far) but are 32
bit linear values in 32 bit), and handles the Win16
mutex. Some finer control can be obtained on the
conversion, see <command>winebuild</command> reference
manual for the details. Suffice to say that the kind of
intricate stack content juggling this results in, is not
exactly suitable study material for beginners.
</para>
<para>
A DLL descriptor is also created for every 16 bit
DLL. However, this DLL normally paired with a 32 bit
DLL. Either, it's the 16 bit counterpart of the 16 bit DLL
(KRNL386.EXE for KERNEL32, USER for USER32...), or a 16
bit DLL directly linked to a 32 bit DLL (like SYSTEM for
KERNEL32, or DDEML for USER32). In those cases, the 16 bit
descriptor(s) is (are) inserted in the same shared library
as the the corresponding 32 bit DLL. Wine will also create
symbolic links between kernel32.dll.so and system.dll.so
so that loading of either
<filename>kernel32.dll</filename> or
<filename>system.dll</filename> will end up on the same
shared library.
</para>
</sect2>
<sect2 id="arch-dlls">
<title>Wine/Windows DLLs</title>
<para>
This document mainly deals with the status of current DLL
support by Wine. The Wine ini file currently supports
settings to change the load order of DLLs. The load order
depends on several issues, which results in different settings
for various DLLs.
</para>
<sect3>
<title>Pros of Native DLLs</title>
<para>
Native DLLs of course guarantee 100% compatibility for
routines they implement. For example, using the native USER
DLL would maintain a virtually perfect and Windows 95-like
look for window borders, dialog controls, and so on. Using
the built-in Wine version of this library, on the other
hand, would produce a display that does not precisely mimic
that of Windows 95. Such subtle differences can be
engendered in other important DLLs, such as the common
controls library COMMCTRL or the common dialogs library
COMMDLG, when built-in Wine DLLs outrank other types in load
order.
</para>
<para>
More significant, less aesthetically-oriented problems can
result if the built-in Wine version of the SHELL DLL is
loaded before the native version of this library. SHELL
contains routines such as those used by installer utilities
to create desktop shortcuts. Some installers might fail when
using Wine's built-in SHELL.
</para>
</sect3>
<sect3>
<title>Cons of Native DLLs</title>
<para>
Not every application performs better under native DLLs. If
a library tries to access features of the rest of the system
that are not fully implemented in Wine, the native DLL might
work much worse than the corresponding built-in one, if at
all. For example, the native Windows GDI library must be
paired with a Windows display driver, which of course is not
present under Intel Unix and Wine.
</para>
<para>
Finally, occasionally built-in Wine DLLs implement more
features than the corresponding native Windows DLLs.
Probably the most important example of such behavior is the
integration of Wine with X provided by Wine's built-in USER
DLL. Should the native Windows USER library take load-order
precedence, such features as the ability to use the
clipboard or drag-and-drop between Wine windows and X
windows will be lost.
</para>
</sect3>
<sect3>
<title>Deciding Between Native and Built-In DLLs</title>
<para>
Clearly, there is no one rule-of-thumb regarding which
load-order to use. So, you must become familiar with
what specific DLLs do and which other DLLs or features
a given library interacts with, and use this information
to make a case-by-case decision.
</para>
</sect3>
<sect3>
<title>Load Order for DLLs</title>
<para>
Using the DLL sections from the wine configuration file, the
load order can be tweaked to a high degree. In general it is
advised not to change the settings of the configuration
file. The default configuration specifies the right load
order for the most important DLLs.
</para>
<para>
The default load order follows this algorithm: for all DLLs
which have a fully-functional Wine implementation, or where
the native DLL is known not to work, the built-in library
will be loaded first. In all other cases, the native DLL
takes load-order precedence.
</para>
<para>
The <varname>DefaultLoadOrder</varname> from the
[DllDefaults] section specifies for all DLLs which version
to try first. See manpage for explanation of the arguments.
</para>
<para>
The [DllOverrides] section deals with DLLs, which need a
different-from-default treatment.
</para>
<para>
The [DllPairs] section is for DLLs, which must be loaded in
pairs. In general, these are DLLs for either 16-bit or
32-bit applications. In most cases in Windows, the 32-bit
version cannot be used without its 16-bit counterpart. For
Wine, it is customary that the 16-bit implementations rely
on the 32-bit implementations and cast the results back to
16-bit arguments. Changing anything in this section is bound
to result in errors.
</para>
<para>
For the future, the Wine implementation of Windows DLL seems
to head towards unifying the 16 and 32 bit DLLs wherever
possible, resulting in larger DLLs. They are stored in the
<filename>dlls/</filename> subdirectory using the 32-bit
name.
</para>
</sect3>
</sect2>
<sect2 id="arch-mem">
<title>Memory management</title>
<para>
Every Win32 process in Wine has its own dedicated native
process on the host system, and therefore its own address
space. This section explores the layout of the Windows
address space and how it is emulated.
</para>
<para>
Firstly, a quick recap of how virtual memory works. Physical
memory in RAM chips is split into
<emphasis>frames</emphasis>, and the memory that each
process sees is split into <emphasis>pages</emphasis>. Each
process has its own 4 gigabytes of address space (4gig being
the maximum space addressable with a 32 bit pointer). Pages
can be mapped or unmapped: attempts to access an unmapped
page cause an
<constant>EXCEPTION_ACCESS_VIOLATION</constant> which has
the easily recognizable code of
<constant>0xC0000005</constant>. Any page can be mapped to
any frame, therefore you can have multiple addresses which
actually "contain" the same memory. Pages can also be mapped
to things like files or swap space, in which case accessing
that page will cause a disk access to read the contents into
a free frame.
</para>
<sect3>
<title>Initial layout (in Windows)</title>
<para>
When a Win32 process starts, it does not have a clear
address space to use as it pleases. Many pages are already
mapped by the operating system. In particular, the EXE
file itself and any DLLs it needs are mapped into memory,
and space has been reserved for the stack and a couple of
heaps (zones used to allocate memory to the app
from). Some of these things need to be at a fixed address,
and others can be placed anywhere.
</para>
<para>
The EXE file itself is usually mapped at address 0x400000
and up: indeed, most EXEs have their relocation records
stripped which means they must be loaded at their base
address and cannot be loaded at any other address.
</para>
<para>
DLLs are internally much the same as EXE files but they
have relocation records, which means that they can be
mapped at any address in the address space. Remember we
are not dealing with physical memory here, but rather
virtual memory which is different for each
process. Therefore <filename>OLEAUT32.DLL</filename> may
be loaded at one address in one process, and a totally
different one in another. Ensuring all the functions
loaded into memory can find each other is the job of the
Windows dynamic linker, which is a part of NTDLL.
</para>
<para>
So, we have the EXE and its DLLs mapped into memory. Two
other very important regions also exist: the stack and the
process heap. The process heap is simply the equivalent of
the libc <function>malloc</function> arena on UNIX: it's a
region of memory managed by the OS which
<function>malloc</function>/<function>HeapAlloc</function>
partitions and hands out to the application. Windows
applications can create several heaps but the process heap
always exists.
</para>
<para>
Windows 9x also implements another kind of heap: the
shared heap. The shared heap is unusual in that
anything allocated from it will be visible in every other
process.
</para>
</sect3>
<sect3>
<title>Comparison</title>
<para>
So far we've assumed the entire 4 gigs of address space is
available for the application. In fact that's not so: only
the lower 2 gigs are available, the upper 2 gigs are on
Windows NT used by the operating system and hold the
kernel (from 0x80000000). Why is the kernel mapped into
every address space? Mostly for performance: while it's
possible to give the kernel its own address space too -
this is what Ingo Molnars 4G/4G VM split patch does for
Linux - it requires that every system call into the kernel
switches address space. As that is a fairly expensive
operation (requires flushing the translation lookaside
buffers etc) and syscalls are made frequently it's best
avoided by keeping the kernel mapped at a constant
position in every processes address space.
</para>
<para>
Basically, the comparison of memory mappings looks as
follows:
<table>
<title>Memory layout (Windows and Wine)</title>
<tgroup cols="4" align="left" colsep="1" rowsep="1">
<thead>
<row>
<entry>Address</entry>
<entry>Windows 9x</entry>
<entry>Windows NT</entry>
<entry>Linux</entry>
</row>
</thead>
<tbody>
<row>
<entry>00000000-7fffffff</entry>
<entry>User</entry>
<entry>User</entry>
<entry>User</entry>
</row>
<row>
<entry>80000000-bfffffff</entry>
<entry>Shared</entry>
<entry>User</entry>
<entry>User</entry>
</row>
<row>
<entry>c0000000-ffffffff</entry>
<entry>Kernel</entry>
<entry>Kernel</entry>
<entry>Kernel</entry>
</row>
</tbody>
</tgroup>
</table>
</para>
<para>
On Windows 9x, in fact only the upper gigabyte
(<constant>0xC0000000</constant> and up) is used by the
kernel, the region from 2 to 3 gigs is a shared area used
for loading system DLLs and for file mappings. The bottom
2 gigs on both NT and 9x are available for the programs
memory allocation and stack.
</para>
</sect3>
<sect3>
<title>Implementation</title>
<para>
Wine (with a bit of black magic) is able to map all items
at the correct locations as depicted above.
</para>
<para>
Wine also implements the shared heap so native win9x DLLs
can be used. This heap is always created at the
<constant>SYSTEM_HEAP_BASE</constant> address or
<constant>0x80000000</constant> and defaults to 16
megabytes in size.
</para>
<para>
There are a few other magic locations. The bottom 64k of
memory is deliberately left unmapped to catch null pointer
dereferences. The region from 64k to 1mb+64k are reserved
for DOS compatibility and contain various DOS data
structures. Finally, the address space also contains
mappings for the Wine binary itself, any native libaries
Wine is using, the glibc malloc arena and so on.
</para>
</sect3>
<sect3 id="address-space">
<title>Laying out the address space</title>
<para>
Up until about the start of 2004, the Linux address space
very much resembled the Windows 9x layout: the kernel sat
in the top gigabyte, the bottom pages were unmapped to
catch null pointer dereferences, and the rest was
free. The kernels mmap algorithm was predictable: it would
start by mapping files at low addresses and work up from
there.
</para>
<para>
The development of a series of new low level patches
violated many of these assumptions, and resulted in Wine
needing to force the Win32 address space layout upon the
system. This section looks at why and how this is done.
</para>
<para>
The exec-shield patch increases security by randomizing
the kernels mmap algorithms. Rather than consistently
choosing the same addresses given the same sequence of
requests, the kernel will now choose randomized
addresses. Because the Linux dynamic linker
(ld-linux.so.2) loads DSOs into memory by using mmap, this
means that DSOs are no longer loaded at predictable
addresses, so making it harder to attack software by using
buffer overflows. It also attempts to relocate certain
binaries into a special low area of memory known as the
ASCII armor so making it harder to jump into them when
using string based attacks.
</para>
<para>
Prelink is a technology that enhances startup times by
precalculating ELF global offset tables then saving the
results inside the native binaries themselves. By grid
fitting each DSO into the address space, the dynamic
linker does not have to perform as many relocations so
allowing applications that heavily rely on dynamic linkage
to be loaded into memory much quicker. Complex C++
applications such as Mozilla, OpenOffice and KDE can
especially benefit from this technique.
</para>
<para>
The 4G VM split patch was developed by Ingo Molnar. It
gives the Linux kernel its own address space, thereby
allowing processes to access the maximum addressable
amount of memory on a 32-bit machine: 4 gigabytes. It
allows people with lots of RAM to fully utilise that in
any given process at the cost of performance: the reason
behind giving the kernel a part of each processes address
space was to avoid the overhead of switching on each
syscall.
</para>
<para>
Each of these changes alter the address space in a way
incompatible with Windows. Prelink and exec-shield mean
that the libraries Wine uses can be placed at any point in
the address space: typically this meant that a library was
sitting in the region that the EXE you wanted to run had
to be loaded (remember that unlike DLLs, EXE files cannot
be moved around in memory). The 4G VM split means that
programs could receive pointers to the top gigabyte of
address space which some are not prepared for (they may
store extra information in the high bits of a pointer, for
instance). In particular, in combination with exec-shield
this one is especially deadly as it's possible the process
heap could be allocated beyond ADDRESS_SPACE_LIMIT which
causes Wine initialization to fail.
</para>
<para>
The solution to these problems is for Wine to reserve
particular parts of the address space so that areas that
we don't want the system to use will be avoided. We later
on (re/de)allocate those areas as needed. One problem is
that some of these mappings are put in place automatically
by the dynamic linker: for instance any libraries that
Wine is linked to (like libc, libwine, libpthread etc)
will be mapped into memory before Wine even gets
control. In order to solve that, Wine overrides the
default ELF initialization sequence at a low level and
reserves the needed areas by using direct syscalls into
the kernel (ie without linking against any other code to
do it) before restarting the standard initialization and
letting the dynamic linker continue. This is referred to
as the preloader and is found in loader/preloader.c.
</para>
<para>
Once the usual ELF boot sequence has been completed, some
native libraries may well have been mapped above the 3gig
limit: however, this doesn't matter as 3G is a Windows
limit, not a Linux limit. We still have to prevent the
system from allocating anything else above there (like the
heap or other DLLs) though so Wine performs a binary
search over the upper gig of address space in order to
iteratively fill in the holes with MAP_NORESERVE mappings
so the address space is allocated but the memory to
actually back it is not. This code can be found in libs/wine/mmap.c:reserve_area.
</para>
</sect3>
</sect2>
<sect2>
<title>Processes</title>
<para>
Let's take a closer look at the way Wine loads and run
processes in memory.
</para>
<sect3>
<title>Starting a process from command line</title>
<para>
When starting a Wine process from command line (we'll get
later on to the differences between NE, PE and Winelib
executables), there are a couple of things Wine need to do
first. A first executable is run to check the threading
model of the underlying OS (see <xref linkend="threading">
for the details) and will start the real Wine loader
corresponding to the choosen threading model.
</para>
<para>
Then Wine graps a few elements from the Unix world: the
environment, the program arguments. Then the
<filename>ntdll.dll.so</filename> is loaded into memory
using the standard shared library dynamic loader. When
loaded, NTDLL will mainly first create a decent Windows
environment:
<itemizedlist>
<listitem>
<para>create a PEB and a TEB</para>
</listitem>
<listitem>
<para>
set up the connection to the Wine server - and
eventually launching the Wine server if none runs
</para>
</listitem>
<listitem>
<para>create the process heap</para>
</listitem>
</itemizedlist>
</para>
<para>
Then <filename>Kernel32</filename> is loaded (but now
using the Windows dynamic loading capabilities) and a Wine
specific entry point is called
<function>__wine_kernel_init</function>. This function
will actually handle all the logic of the process loading
and execution, and will never return from it's call.
</para>
<para>
<function>__wine_kernel_init</function> will undergo the
following tasks:
<itemizedlist>
<listitem>
<para>
initialization of program arguments from Unix
program arguments
</para>
</listitem>
<listitem>
<para>
lookup of executable in the file system
</para>
</listitem>
<listitem>
<para>
If the file is not found, then an error is printed
and the Wine loader stops.
</para>
</listitem>
<listitem>
<para>
We'll cover the non-PE file type later on, so assume
for now it's a PE file. The PE module is loaded in
memory using the Windows shared library
mechanism. Note that the dependencies on the module
are not resolved at this point.
</para>
</listitem>
<listitem>
<para>
A new stack is created, which size is given in the
PE header, and this stack is made the one of the
running thread (which is still the only one in the
process). The stack used at startup will no longer
be used.
</para>
</listitem>
<listitem>
<para>
Which this new stack,
<function>ntdll.LdrInitializeThunk</function> is
called which performs the remaining initialization
parts, including resolving all the DLL imports on
the PE module, and doing the init of the TLS slots.
</para>
</listitem>
<listitem>
<para>
Control can now be passed to the
<function>EntryPoint</function> of the PE module,
which will let the executable run.
</para>
</listitem>
</itemizedlist>
</para>
</sect3>
<sect3>
<title>Creating a child process from a running process</title>
<para>
The steps used are closely link to what is done in the
previous case.
</para>
<para>
There are however a few points to look at a bit more
closely. The inner implementation creates the child
process using the <function>fork()</function> and
<function>exec()</function> calls. This means that we
don't need to check again for the threading model, we can
use what the parent (or the grand-parent process...)
started from command line has found.
</para>
<para>
The Win32 process creation allows to pass a lot of
information between the parent and the child. This
includes object handles, windows title, console
parameters, environment strings... Wine makes use of both
the standard Unix inheritance mechanisms (for environment
for example) and the Wine server (to pass from parent to
child a chunk of data containing the relevant information).
</para>
<para>
The previously described loading mechanism will check in
the Wine server if such a chunk exists, and, if so, will
perform the relevant initialization.
</para>
<para>
Some further synchronization is also put in place: a
parent will wait until the child has started, or has
failed. The Wine server is also used to perform those
tasks.
</para>
</sect3>
<sect3>
<title>Starting a Winelib process</title>
<para>
Before going into the gory details, let's first go back to
what a Winelib application is. It can be either a regular
Unix executable, or a more specific Wine beast. This later
form in fact creates two files for a given executable (say
<filename>foo.exe</filename>). The first one, named
<filename>foo</filename> will be a symbolic link to the
Wine loader (<filename>wine</filename>). The second one,
named <filename>foo.exe.so</filename>, is the equivalent
of the <filename>.dll.so</filename> files we've already
described for DLLs. As in Windows, an executable is, among
other things, a module with its import and export
information, as any DLL, it makes sense Wine uses the same
mechanisms for loading native executables and DLLs.
</para>
<para>
When starting a Winelib application from the command line
(say with <command>foo arg1 arg2</command>), the Unix
shell will execute <command>foo</command> as a Unix
executable. Since this is in fact the Wine loader, Wine
will fire up. However, will notice that it hasn't been
started as <command>wine</command> but as
<command>foo</command>, and hence, will try to load (using
Unix shared library mechanism) the second file
<filename>foo.exe.so</filename>. Wine will recognize a 32
bit module (with its descriptor) embedded in the shared
library, and once the shared library loaded, it will
proceed the same path as when loading a standard native PE
executable.
</para>
<para>
Wine needs to implement this second form of executable in
order to maintain the order of initialization of some
elements in the executable. One particular issue is when
dealing with global C++ objects. In standard Unix
executable, the call of the constructor to such objects is
stored in the specific section of the executable
(<function>.init</function> not to name it). All
constructors in this section are called before the
<function>main</function> function is called. Creating a
Wine executable using the first form mentionned above will
let those constructors being called before Wine gets a
chance to initialize itself. So, any constructor using a
Windows API will fail, because Wine infrastructure isn't
in place. The use of the second form for Winelib
executables ensures that we do the initialization using
the following steps:
<itemizedlist>
<listitem>
<para>
initialize the Wine infrastructure
</para>
</listitem>
<listitem>
<para>
load the executable into memory
</para>
</listitem>
<listitem>
<para>
handle the import sections for the executable
</para>
</listitem>
<listitem>
<para>
call the global object constructors (if any). They
now can properly call the Windows APIs
</para>
</listitem>
<listitem>
<para>
call the executable entry point
</para>
</listitem>
</itemizedlist>
</para>
<para>
The attentive reader would have noted that the resolution
of imports for the executable is done, as for a DLL, when
the executable/DLL descriptor is registered. However, this
is done also by adding a specific constructor in the
<function>.init</function> section. For the above describe
scheme to function properly, this constructor must be the
first constructor to be called, before all the other
constructors, generated by the executable itself. The Wine
build chain takes care of that, and also generating the
executable/DLL descriptor for the Winelib executable.
</para>
</sect3>
<sect3>
<title>Starting a NE (Win16) process</title>
<para>
When Wine is requested to run a NE (Win 16 process), it
will in fact hand over the execution of it to a specific
executable <filename>winevdm</filename>. VDM stands for
Virtual DOS Machine. This <filename>winevdm</filename>
will in fact set up the correct 16 bit environment to run
the executable. Any new 16 bit process created by this
executable (or its children) will run into the same
<filename>winevdm</filename> instance. Among one instance,
several functionalities will be provided to those 16 bit
processes, including the cooperative multitasking, sharing
the same address space, managing the selectors for the 16
bit segments needed for code, data and stack.
</para>
<para>
Note that several <filename>winevdm</filename> instances
can run in the same Wine session, but the functionalities
described above are only shared among a given instance,
not among all the instances. <filename>winevdm</filename>
is built as Winelib application, and hence has access to
any facility a 32 bit application has.
</para>
<para>
The behaviour we just described also applies to DOS
executables, which are handled the same way by
<filename>winevdm</filename>.
</para>
</sect3>
</sect2>
<sect2>
<title>Wine drivers</title>
<para>
Wine will not allow running native Windows drivers under
Unix. This comes mainly because (look at the generic
architecture schemas) Wine doesn't implement the kernel
features of Windows (kernel here really means the kernel,
not the KERNEL32 DLL), but rather sets up a proxy layer on
top of the Unix kernel to provide the NTDLL and KERNEL32
features. This means that Wine doesn't provide the inner
infrastructure to run native drivers, either from the Win9x
family or from the NT family.
</para>
<para>
In other words, Wine will only be able to provide access to
a specific device, if and only if, 1/ this device is
supported in Unix (there is Unix-driver to talk to it), 2/
Wine has implemented the proxy code to make the glue between
the API of a Windows driver, and the Unix interface of the
Unix driver.
</para>
<para>
Wine, however, tries to implement in the various DLLs
needing to access devices to do it through the standard
Windows APIs for device drivers in user space. This is for
example the case for the multimedia drivers, where Wine
loads Wine builtin DLLs to talk to the OSS interface, or the
ALSA interface. Those DLLs implement the same interface as
any user space audio driver in Windows.
</para>
</sect2>
</sect1>
</chapter>
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