[ros-kernel] How to get started writing ReactOS drivers

[Vizzini]

I just got an e-mail from someone off-list about how to get started
writing drivers, and I thought I'd copy my response to this list in case
anyone else out there in TV land has the same question.

 -Vizzini

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> I was wondering, how do you plan to do (or already do) device driver 
> development for ReactOS.
> I guess using MS DDK is not a very "free" option...

We have people who use both the DDK and the ReactOS build environment. 
In general, the code is almost completely compatible (with a couple of
exceptions, noted below), and the binaries are 100% compatible.  The
build systems are not compatible, however, so you do have to either pick
one or go out of your way to support both.

The DDK method is exactly as it is for normal Windows driver
development.  You create a SOURCES file and copy in a Makefile.  Do your
coding, run build, and get a .sys file in the end.  This sys file can be
copied onto your reactos disk, and once you edit the registry to create
the SCM entry (just like windows, too), the driver will load and run.  

The other method is using the ReactOS source tree.  The easiest way is
to start with a Makefile and .rc file from another driver in the
reactos/drivers/ directory.  The whole system runs off of standard GNU
make, rather than  using build and/or nmake.  You must use a ReactOS
Makefile, though, which collides with the Makefile that build expects
from the DDK.  To get around this, you can name your makefile
GNUmakefile, so you can have both files in the same directory if you
choose to support both build environments.  MinGW make will only look at
GNUmakefile, if it exists.

In practice, I usually support both, as it's critical to develop and
test drivers under Windows 2000/XP/2003, including running under
Verifier and running Prefast.  This ensures that if there are any
compatibilities between the ReactOS kernel and the Windows kernel, they
will be found.

One minor legal point - I don't think it's legal to distribute a binary
driver built from the Windows DDK for use with ReactOS.  You can
distribute the SOURCES and Makefile files, though, and you can build and
test with the DDK yourself, so this isn't really an issue.

As far as source code compatibility goes, Microsoft's CL.EXE supports
some extensions that our compiler (MinGW GCC) doesn't.  In particular,
there is no support for SEH constructs, including __try, __except, and
__finally.  If you call ProbeForRead(), ProbeForWrite(),
MmProbeAndLockPages(), ZwClose(), or anything else that can throw an
exception, you have to surround the SEH stuff with 

#if MSC_VER
#endif

Inline ASM syntax is different (MS uses Intel syntax, whereas GCC
expects AT&T), and include paths are parsed differently (backslash vs
forward slash).  File names are case-sensitive on UNIX, of course, as
well.  Also, declarations like __stdcall are a little different.  See an
example driver for details.  Finally, many (all?) DDK headers use a
compiler switch to force the default calling convention to stdcall.  We
do not, however, so you have to be careful to actually define any
callbacks that need to be stdcall in the proper wya.

I have ported a few of my own drivers to build in the native ReactOS
environment, and in no case has it taken me more than a few hours.  

Our kernel is fairly complete, but generally windows drivers are missing
a few imports on ReactOS.  This is where you go from being a driver
developer to a kernel developer. ;-)  If you find that some kernel
functionality is missing, I would encourage you to dive in and take a
shot at coding it up.

There is one major caveat to the above:  our kernel does not support PnP
or Power Management just yet.  It is being actively worked on, but so
far, WDM drivers that expect PnP notifications won't work without some
special work on your part.  If PnP is a project you'd like to chip in
on, let me know, and I'll set you up with the guys who are working on it
now.  I'm sure they'd appreciate the help.

What sorts of drivers are you interested in working on?  I like to try
to keep a general idea of what is going on in the kernel, so if you do
decide to venture out on a project, drop me a note.

Best of luck, and don't hesitate to let me know if you have questions or
need help.  Also, come by #reactos on irc.freenode.net - there are
usually people there (myself included) that can help.