Holy cow, I wrote a book!
You may have noticed that there's a copy of Notepad
and another in
Compatibility, of course.
Windows 3.0 put Notepad in the Windows directory.
Windows NT put it in the System32 directory.
Notepad is perhaps the most commonly hardcoded program in Windows.
many Setup programs use it to view the Readme file,
you can use your imagination to come up with other places
where a program or batch file or
printed instructions will hard-code the path to Notepad.
In order to be compatible with programs designed for
Windows 95, there needs to be a copy of Notepad
in the Windows directory.
And in order to be compatible with programs designed for
Windows NT, there also needs to be a copy in the
And now that Notepad exists in both places,
new programs have a choice of Notepads,
and since there is no clear winner,
half of them will choose the one in the Windows directory
and half will choose the one in the System32 directory,
thereby ensuring the continued existence of two copies
of Notepad for years to come.
I'm satisfied with the MSDN documentation for
the WM_COMMAND message,
but for the sake of mind-numbing completeness,
I'm going to state the obvious in the hope that you,
dear readers, can use this technique to
fill in the obvious in other parts of MSDN.
The one-line summary of the WM_COMMAND message
"The WM_COMMAND message is sent when the user selects
a command item from a menu,
when a control sends a notification message to its parent window,
or when an accelerator keystroke is translated."
In a nutshell, there are three scenarios that generate a
WM_COMMAND message, namely the three listed above.
You want to think of the menu and accelerator scenarios of the
WM_COMMAND message as special cases of the control scenario.
The high-order word of the wParam parameter
"specifies the notification code if the message is from a control".
What does "control" mean here?
Remember that you have to take things in context.
The WM_COMMAND message is being presented in the
context of Win32 in general, and in the context of the window manager
Windows such as edit boxes, push buttons, and list boxes
are commonly called "controls", as are all the window classes
in the "common controls library".
In the world of the window manager, a "control" is a window
whose purpose is to provide some degree of interactivity
(which, in the case of the static control, might be no interactivity at all)
in the service of its parent window.
The fact that the WM_COMMAND is used primarily
in the context of dialog boxes further emphasizes the point
that the term "control" here is just a synonym for "child window".
What does "notification code" mean here?
Control notification codes are arbitrary 16-bit values defined
by the control itself.
By convention, they are named xxN_xxxx, where the "N"
stands for "notification".
Be careful, however, not to confuse this with notification codes
associated with the WM_NOTIFY message.
Fortunately, every notification code specifies in its documentation
whether it arrives as a WM_COMMAND notification or
a WM_NOTIFY notification.
A modern control designer is more likely to use
WM_NOTIFY notifications since they allow
additional information to be passed with the notification.
The WM_COMMAND message, by comparison, passes
only the notification itself; the other parameters to
the WM_COMMAND message are forced, as we'll see below.
If WM_NOTIFY is superior to WM_COMMAND,
why do some controls use WM_COMMAND?
Because WM_NOTIFY wasn't available until
Controls that were written prior to Windows 95 had to
content themselves with the WM_COMMAND message.
"If the message is from an accelerator, this value [the high-order
word of the wParam parameter] is 1."
Remember, we're still in the context of the window manager,
and particular in the context of the WM_COMMAND
The accelerator here refers to messages generated by the call to
TranslateAccelerator in the message loop.
"If the message is from a menu, this value is zero."
If the WM_COMMAND mesage was triggered by
the user selecting an item from a menu, then the high-order
word of the wParam is zero.
The low-order word of the wParam parameter
"specifies the identifier of the menu item, control, or
The identifier of a menu item or accelerator
is the command code you associated
with it in your menu or accelerator template or (in the case of a menu item)
when you manually created the menu item with a function like
(You probably named your menu item identifiers and accelerator
The identifier of a control is determined by the creator of the
control; recall that the
hMenu parameter to the CreateWindow
and CreateWindowEx functions is treated as a child
window identifier if you're creating a child window.
It is that identifier that the control identifier.
(You can retrieve the identifier for a control by calling the
Finally, the lParam parameter is the
"handle to the control sending the message if the message is from a control.
Otherwise, this parameter is NULL."
If the notification is generated by a child window
(with a notification code appropriate for that child window, obviously),
then that child window handle is passed as the lParam.
If the notification is generated by an accelerator or a menu,
then the lParam is zero.
Notice that nearly all of the parameters to the
WM_COMMAND message are forced, once you've decided
what notification you're generating.
If you are generating a notification from a control,
you must pass the notification code in the high word of the
wParam, the control identifier in the low word
of the wParam, and the control handle as the
In other words, once you've decided that the
hwndC window wants to send a
CN_READY notification, you have no choice but
In other words, all control notifications take the form
where hwndC is the control generating the notification
and notificationCode is the notification code.
Of course, you can use PostMessage instead of
SendMessage if you would rather post the notification
rather than sending it.
The other two cases (accelerators and menus) are not cases you
would normally code up, since you typically let the
TranslateAccelerator function deal with accelerators
and let the menu system deal with menu identifiers.
But if for some reason, you wanted to pretend that the user
had typed an accelerator or selected a menu item, you can generate
the notification manually by following the rules set out in the
// simulate the accelerator IDM_WHATEVER
Here, hwnd is the window that you want to pretend was
the window passed to the TranslateAccelerator function,
and IDM_WHATEVER is the accelerator identifier.
Simulating a menu selection is exactly the same, except that
(according to the rules above), you set the high-order word of
the wParam to zero.
// simulate the menu item IDM_WHATEVER
Here, hwnd is the window associated with the menu.
A window can be associated with a menu either by being created
with the menu (having passed the menu handle to the
CreateWindow or CreateWindowEx function
explicitly, or having it done implicitly by including it with the
class registration) or by having been passed explicitly as
the window parameter to a function like
One significant difference between the accelerator/menu case
and the control notification case is that accelerator and menu
identifiers are defined by the calling application,
whereas control notifications are defined by the control.
You may have noticed the opportunity to "pun"
the control notification codes.
If a control defines a notification code as zero, then it will
"look like" a menu item selection, since the high-order word
of the wParam in the case of a menu item selection
The button control takes advantage of this pun:
#define BN_CLICKED 0
This means that when the user clicks a button control,
the WM_COMMAND message that is generated
"smells like" a menu selection notification.
You probably take advantage of this in your dialog procedure
without even realizing it.
(The static control also takes advantage of this pun:
#define STN_CLICKED 0
but in order for the static control to generate the
you have to set the SS_NOTIFY style.)
I stated at the start that the accelerator and menu scenarios
are just special cases of the control scenario.
If you take the pieces of the WM_COMMAND message
apart, you'll see that they fall into two categories:
In the case of a menu or an accelerator, the "What happened?"
is "The user clicked on the menu (0)" or
"The user typed the accelerator (1)".
The "Whom did it happen to?" is "This menu ID" or "This accelerator ID".
Since the notification is not coming from a control, the control
handle is NULL.
I apologize to all you Win32 programmers for whom
this is just stating the obvious.
Now that you're an expert on the WM_COMMAND message,
perhaps you can solve
this person's problem.
There are some basic ground rules that apply to all system programming,
so obvious that most documentation does not bother explaining them
because these rules should have been internalized by practitioners
of the art to the point where they need not be expressed.
In the same way that when plotting driving directions
you wouldn't even consider taking a shortcut through somebody's
backyard or going the wrong way down a one-way street,
and in the same way that an experienced chess player doesn't even consider
illegal moves when deciding what to do next,
an experienced programmer doesn't even consider violating the following
basic rules without explicit permission in the documentation to the contrary:
(Remember, every statement here is a basic ground rule, not an
absolute inescapable fact. Assume every sentence here is
prefaced with "In the absence of indications to the contrary".
If the caller and callee have agreed
on an exception to the rule, then that exception applies.
a pointer is prototyped as volatile
is explicitly marked as "This value can change from another thread,"
so the rule against modifying function parameters does not apply to
such a pointer.)
Coming up with this was hard,
in the same way it's hard to come up with a list of
illegal chess moves.
The rules are so automatic that they aren't really rules
so much as things that simply are and it would be crazy
even to consider otherwise.
As a result, I'm sure there are other "rules so obvious they need
not be said" that are missing.
(For example, "You cannot terminate a thread while it is inside
somebody else's function.")
One handy rule of thumb for what you can do to a function call
is to ask, "How would I like it if somebody did that to me?"
(This is a special case of the "Imagine if this were possible" test.)
there were an awful lot of comments yesterday
and it will take me a while to work through them all.
But I'll start with some more background on the problem
and clarifying some issues that people had misinterpreted.
As a few people surmised, the network file server software
in question is Samba,
a version of which comes with most Linux distributions.
(I'll have to do a better job next time of disguising the
identities of the parties involved.)
Samba is also very popular as the network file server for
embedded devices such as network-attached storage.
The bug in question is fixed in the latest version of Samba,
but none of the major distributions have picked up the fix yet.
Not that that helps the network-attached storage scenario any.
It appears that a lot of people though the buggy driver
was running on the Windows Vista machine,
since they started talking about
blocking its installation.
The problem is not on the Windows Vista machine;
the problem is on the file server, which is running Linux.
WHQL does not certify Linux drivers,
it can't stop you from installing a driver
on some other Linux machine,
and it certainly can't
download an updated driver and somehow upgrade your Linux machine
Remember, the bug is on the server,
which is another computer running some other operating system.
Asking Windows to update the driver on the remote server makes about
as much sense as asking Internet Explorer to upgrade the version
of Apache running on slashdot.org.
You're the client; you have no power over the server.
Some people lost sight of the network-attached storage scenario,
probably because they weren't familiar with the term.
A network-attached storage device is a self-contained device
consisting of a large hard drive, a tiny computer, and a place
to plug in a network cable.
The computer has an operating system burned into its ROMs
(often a cut-down version of Linux with Samba),
and when you turn it on, the device boots the computer,
loads the operating system, and acts as a file server on your network.
Since everything is burned into ROM,
the driver will get upgraded and the problem will eventually be long
is wishful thinking.
It's not like you can download a new Samba driver and install
it into your network-attached storage device.
You'll have to wait for the manufacturer to release a new ROM.
As for detecting a buggy driver, the CIFS protocol doesn't
really give the client much information about what's running
on the server, aside from a "family" field that identifies
the general category of the server (OS/2, Samba, Windows NT, etc.)
All that a client can tell, therefore, is "Well, the server
is running some version of Samba."
It can't tell whether it's a buggy version or a fixed version.
The only way to tell that you are talking to a buggy server
is to wait for the bug to happen.
(Which means that people who said, "Windows Vista should just default
to the slow version," are saying that they want Windows Vista
to run slow against Samba servers and fast against Windows NT servers.
This plays right into the hands of the conspiracy theorists.)
My final remark for today is explaining how a web site can
"bloat the cache" of known good/bad servers and create a denial
of service if the cache did not have a size cap:
First, set up a DNS server that directs all requests for *.hackersite.com
to your Linux machine.
On this Linux machine, install one of the buggy versions of Samba.
Now serve up this web page:
<IFRAME SRC="\\a1.hackersite.com\b" HEIGHT=1 WIDTH=1></IFRAME>
<IFRAME SRC="\\a2.hackersite.com\b" HEIGHT=1 WIDTH=1></IFRAME>
<IFRAME SRC="\\a3.hackersite.com\b" HEIGHT=1 WIDTH=1></IFRAME>
<IFRAME SRC="\\a4.hackersite.com\b" HEIGHT=1 WIDTH=1></IFRAME>
<IFRAME SRC="\\a10000.hackersite.com" HEIGHT=1 WIDTH=1></IFRAME>
Each of those IFRAMEs displays an Explorer window
with the contents of the directory \\a1.hackersite.com\b.
(Since all the names resolve to the same machine,
all the \\*.hackersite.com machines are really the same.)
In that directory, put 200 files, so as to trigger the
"more than 100 files" bug and force Windows Vista to cache the server
as a "bad" server.
In this way, you forced Windows Vista to create ten thousand records
for the ten thousand bad servers you asked to be displayed.
Throw in a little more script and you can turn this into a loop that
accesses millions of "different" servers (all really the same server).
If the "bad server" cache did not have a cap, you just allowed a
bad server to consume megabytes of memory that will never be freed
until the computer is rebooted.
Pretty neat trick.
Even worse, if you proposed preserving this cache across reboots,
then you're going to have to come up with a place to save this information.
Whether you decide that it goes in a file or in the registry,
the point is that an attacker can use this "bloat attack" and cause
the poor victim's disk space/registry usage to grow without bound
until they run out of quota.
And once they hit quota, be it disk quota or registry quota,
not only do bad things start happening, but they don't even know
what file or registry key they have to delete to get back under quota.
Next time, I'll start addressing some of the proposals that people came
up with, pointing out disadvantages that they may have missed
in their analysis.
Okay, everybody, here's your chance to solve a compatibility problem.
There is no answer yet;
I'm looking to see how you folks would attack it.
This is a real bug in the Windows Vista database.
A beta tester reported that Explorer fails to show more
than about a hundred files per directory from file servers running
a particular brand of the file server software.
The shell and networking teams investigated the problem together
and tracked it down to the server incorrectly handling
certain types of directory queries.
Although the server claims to support both slow and fast queries,
if you try a fast query,
it returns only the first hundred or so files and then gives up with
a strange error code.
On the other hand, if Explorer switches to the slow query,
then everything works fine.
(Windows XP always used the slow query.)
An update to the server software was released earlier this year
which claims to fix the bug.
However (as of this writing),
all of the vendor's distributors continue to ship the buggy version
of the driver.
What should we do?
Here are some options.
Choose of of the below or make up your own!
Make no accomodation for this particular buggy protocol implementation.
People who are running that particular implementation will get
incomplete directory listings.
Publish a Knowledge Base article describing the problem and directing
customers to contact the vendor for an updated driver.
Explorer should recognize the strange error code
and display an error message to the user saying,
"The server \\servername appears to be running an old version
of the XYZ driver that does not report the contents of large
Not all items in the directory are shown here.
Please contact the administrator of the machine \\servername to have the
(Possibly with a "Don't show this dialog again" check-box.)
Explorer should recognize the strange error code
and say, "Oh, this server must have the buggy driver.
It's too late to do anything about the current directory information,
but I'll remember that I should do things the slow way in the future
when talking to this server."
To avoid denial-of-service attacks, remember only the last 16 (say)
servers that exhibit the problem.
(If the list of "known bad" servers were unbounded, then an attacker
could consume all the memory on your computer by creating a server
that responded to a billion different names and
using HTTP redirects to get you to visit all of those servers in turn.)
Add a configuration setting to the Windows network client
to tell it "If somebody asks whether a server supports fast queries,
always say No, even if the server says Yes."
In this manner, no program will attempt to use fast queries;
they will all use slow queries.
Directory queries will run slower, but at least they will work.
Add a configuration setting to Explorer
to tell it "Always issue slow queries; never issue fast queries."
Directory queries will run slower, but at least they will work.
But this affects only Explorer;
other programs which ask the server "Do you support fast queries?"
will receive an affirmative response and attempt to use fast queries,
only to rediscover the problem that Explorer worked around.
Stop supporting "fast mode" in the network client
since it is unreliable;
there are some servers that don't handle "fast mode" correctly.
This forces all programs to use "slow mode".
Optionally, have a configuration setting to re-enable "fast mode".
Make sure to list both advantages and disadvantages of your proposal.
Often, people will not even realize that their solution to a problem
merely replaces it with another problem.
The quip attributed to Jamie Zawinski captures the sentiment:
Some people, when confronted with a problem, think "I know, I'll use
regular expressions." Now they have two problems.
For example, in response to
"How do I write a batch file that..."
some people will say,
"First, install <perl|bash|monad|...>".
This doesn't actually solve the problem;
it merely replaces it with a different problem.
In particular, if the solution begins with
you've pretty much lost out of the gate.
Solving a five-minute problem by taking a half hour to download and
install a program is a net loss.
In a corporate environment, adding a program to a deployment
is extraordinarily expensive.
You have to work with your company's legal team to make sure
the licensing terms for the new program are acceptable
and do not create undue risk from a legal standpoint.
What is your plan of action if
the new program stops working, and your company starts losing
tens of thousands of dollars a day?
You have to do interoperability testing to make sure the new
program doesn't conflict with the other programs in the deployment.
(In the non-corporate case, you still run the risk that the
new program will conflict with one of your existing programs.)
Second, many of these "solutions" require that you
abandon your partial solution so far and rewrite it in the
If you've invested years in tweaking a batch file and you just need
one more thing to get that new feature working,
and somebody says,
"Oh, what you need to do is throw away you batch file and
start over in this new language,"
you're unlikely to take up that suggestion.
So be careful when you suggest a solution that has a high
Sure, something could be taken care of by a one-line perl script,
but getting perl onto the machine is hardly a one-line endeavor.
The folks on the logon team wish me to remind you that
the ForceAutoLogon setting
does more than just log on an account automatically.
They've had to deal with large numbers of people who set the key
without really understanding what it does, and then getting into
trouble because what they get is not what they expected.
In addition to logging on an account automatically,
the ForceAutoLogon setting also
logs you back on after you log off.
It is designed for machines running as kiosks
or other publically-accessible scenarios
where you want the kiosk account to be the only account
Even if the user manages to fiddle with the machine and
log off the kiosk user,
the logon system will just log the kiosk user back on.
As a result, setting the ForceAutoLogon setting effectively
locks out all users aside from the one you are forcing.
If you do this to one of your machines,
you'd better have some other way of administering the machine.
(Typically, this is done via remote administration.)
Windows File Protection works by replacing files after they have
Why didn't Windows just apply ACLs to deny write permission to the
We tried that.
It didn't work.
Programs expect to be able to overwrite the files.
A program's setup would run and it decided that it needed to "update"
some system file and attempt to overwrite it.
If the system tried to stop the file from being overwritten,
the setup program would halt and report that it was unable
to install the file.
Even if the operating system detected that somebody was trying
to overwrite a system file and instead gave them a handle to
those programs would nevertheless notice that they had been
hoodwinked because as a "verification" step,
they would open the file they had just copied
and compare it against the "master copy"
on the installation CD.
The solution was to let the program think it had won,
and then, when it wasn't looking,
put the original back.
Now that Windows File Protection has been around for a few years,
software installers have learned that it's not okay to overwrite
system files (and trying to do it won't work anyway),
so starting in Windows Vista,
the Windows File Protection folks have started taking stronger
steps to protect system files,
and this includes using ACLs to make the files harder to replace.
Presumably, they will have compatibility plans in place to
accomodate programs whose setup really wants to overwrite a file.
A common obstacle when trying to help people solve their problems
is that what people ask for and what they actually want are not
always the same thing.
For technical problems, you often get a question that
makes you shake your head in disbelief,
but upon closer questioning, you find that the person really
doesn't want what they're asking for.
What they really want is something else, but they've
already "solved" half of the problem and only need your help
with the other half—the half that doesn't make any sense.
For example, the literal answer to
"How do I write a regular expression that matches
everything except XYZ" is often a horrible mess,
but if you dig deeper, you'll find that they really don't need
a regular expression that matches everything except XYZ;
they just simplified their problem to
"I know, I'll use regular expressions"
ended up creating a bigger problem.
(The best solution is often a mix of regular expressions and
simple program logic.)
This problem also exists in user interface design.
Rick Schaut describes one case where a user asked for a feature,
when what they really wanted was an entirely different feature.
Understanding the customer's problem is the first step towards solving it.
On occasion, you might notice that every window on the desktop
flickers and repaints itself.
One of the causes for this is a simple null handle bug.
The InvalidateRect function is one you're
probably well-familiar with.
It is used to indicate to the window manager that the pixels
of a particular window are no longer current and should be
(You can optionally pass a rectangle that specifies a subset
of the window's client area that you wish to mark invalid.)
This is typically done when the state of the data underlying
the window has changed and you want the window to repaint with
the new data.
If however you end up passing NULL as the window
handle to the InvalidateRect function,
this is treated as a special case for compatibility with
early versions of Windows: It invalidates
all the windows on the desktop
and repaints them.
Consequently, if you, say, try to invalidate a window
but get your error checking or timing wrong and end up
passing NULL by mistake,
the result will be that the entire screen flickers.
Even more strangely, passing NULL as the
first parameter ValidateRect
has the same behavior of invalidating
all the windows.
(Yes, it's the "Validate" function, yet it invalidates.)
This wacko behavior exists for the same compatibility reason.
Yet another example of how programs rely on bugs or undocumented
behavior, in this case, the peculiar way a NULL parameter
was treated by very early versions of Windows due to lax
Changing nearly anything in the window manager
raises a strong probability that there will be many programs
that were relying on the old behavior, perhaps entirely by accident,
and breaking those programs means an angry phone call from a
major corporation because their factory control software stopped