REVERSE ENGINEERING EXERCISES FOR THE MASSES (lesson 2)
HOW TO DISASSEMBLE A WINDOWS PROGRAM
WINDOWS 3.1. - DISASSEMBLING TASKMAN
by Fravia
After we've found and analyzed WinMain() (-> lesson 1), the
next places to inspect when you crack a program are the windows
procedures and dialog procedures (this is true only for Windows
*programs*; for DLL, on the countrary, the cracking procedures
are different and the relvant techniques will be discussed in
another lesson).
These WndProcs and DialogProcs are "callback" procedures: they
are *exported* from Windows executables, almost as the program
were a DLL, so that Windows can call them.
And -hear, hear!- beacuse they are exported these crucial
procedures have *names* (almost always useful) that are
accessible to any decent Windows disassembler. In Taskman.lst,
for example, WCB clearly identifies TASKMANDLGPROC:
Exported names by location:
1:007B 1 TASKMANDLGPROC <- It's a DialogProc !
It works out well that the WndProcs and DialogProcs show up so
nicely in the disassembled listings, because, as we know from
Windows programming, these subroutines are "where the action is"
in event driven Windows applications... or at least where the
action begins.
Furthermore we know that these subroutines will be most likely
little more than (possibly very large) message handling
switch/case statements. These usually look something like this:
long FAR PASCAL _export WndProc(HWND hWnd, WORD message, WORD
wParam, LONG lPAram)
{ ...
switch (message)
{
case WM_CREATE:
//... handle WM_CREATE message
break;
case WM_COMMAND:
//... handle WM_COMMAND message
break;
default:
return DefWindowProc(hwnd, message, wParam, lParam);
}
}
Wow! Yes! As you already guessed this means that... that we get
immediately 4 parameters for EACH exported WndProc or DlgProc!
Actually there's no rule that states that a Windows WndProc or
DlgProc has to look like this... it's just that they almost
always do!
Here is how the parameters to the WndProc or DialogProc will
appear in the assembly language listing (after the function
prologue):
long FAR PASCAL _export WndOrDialogProc(HWND hwnd, WORD
message, WORD wParam, LONG lParam);
lParam = dword ptr [bp+6]
wParam = word ptr [bp+0Ah]
message = word ptr [bp+0Ch]
hWnd or hWndDlg = word ptr [bp+0Eh]
With this knowledge, we can replace an otherwise meaningless
[bp+0Ch] with a label such as "message", a [bp+0Eh] with a "hwnd"
or "hwndDlg", and so on in *ANY* DialogProc and WndProc in *ANY*
Windows program.
The boilerplate nature of Windows programming greatly simplifies
cracking. For example, here is part of our Taskman exported
DialogProc: TASKMANDLGPROC:
1.007B ; TASKMANDLGPROC
... (function prologue)
1.008A 8B760E mov si, hWndDlg ;[bp+0E]
1.008D 56 push si
1.008E 6A64 push 0064
1.0090 9AFFFF0000 call USER.GETDLGITEM
1.0095 8BF8 mov di, ax
1.0097 8B460C mov ax, message ;[bp+0C]
1.009A 2D1C00 sub ax, 001C
1.009D 7416 je 00B5
1.009F 2DF400 sub ax, 00F4
1.00A2 7436 je 00DA
1.00A4 48 dec ax
1.00A5 7503 jne 00AA
1.00A7 E98301 jmp 022D
1.00AA >2D5303 sub ax, 0353
1.00AD 7503 jne 00B2
1.00AF E9D602 jmp 0388
1.00B2 >E9C801 jmp 027D
1.00B5 >837E0A00 cmp word ptr wParam, 0 ;[bp+0A]
1.00B9 7403 je 00BE
1.00BB E9BF01 jmp 027D
...
The problem here, of course, is what to make of all these magic
numbers: 0064, OO1C, 00F4 and so on... how are we going to figure
out what these mean?
When examined via disassembled listings, Windows programs tend
to contain a lot of "magic numbers". Of course the actual source
code would
* #include and
* #define numeric constants for the various resources (menus,
strings, dialog controls, etc.) that it uses.
Given a disassembled listing, it should be possible to turn a
lot of these seemingly senseless numbers back into something
understandable.
Let's start with the number 001C in TaskManDlgProc():
1.0097 8B460C mov ax, message ;[bp+0C]
1.009A 2D1C00 sub ax, 001C
1.009D 7416 je 00B5
If AX holds the *message* parameter to TaskManDlgProc() (line
1.0097)... then the value 001C must be a Windows WM_ message
number (one of those you can breakpoint to with WINICE's BMSG
command, by the way). Looking in WINDOWS.H, we find that 0x1C is
WM_ACTIVATEAPP.
TaskManDlgProc() is subtracting this value from AX and then
jumping somewhere (let's call it ON_ACTIVATEAPP) if the result
is zero... i.e. if it is WM_ACTIVATEAPP.
This is an odd way to test whether (message == WM_ACTIVATEAPP):
if the test fails, and we do not take the jump to ON_ACTIVATEAPP,
the message number has 1C subtracted from it... and this value
must be taken account of by the next switch statement:
1.009F 2DF400 sub ax, 00F4 ;(+1C=110=WM_INITDIALOG)
1.00A2 7436 je 00DA ;jump to ON_INITDIALOG
1.00A4 48 dec ax ;(110+1=111=WM_COMMAND)
1.00A5 7503 jne 00AA ;no, go elsewhere
1.00A7 E98301 jmp 022D ;yes, jump to ON_COMMAND
Other WndProcs & DialogProcs will contain straightforward tests,
rather than testing via subtraction, is a matter of compiler
choice. In any case, a WndProc or DialogProc generally contains
a collection of handlers for different messages.
In the case of TaskManDlgProc(), we can see that's handling
WM_ACTIVATEAPP, WM_INITDIALOG and WM_COMMAND. By itself, this
information is rather boring... however, it tells us what is
happening *elsewhere* in the function: 1.00B5 must be handling
WM_ACTIVATEAPP messages (therefore let's call it ON_ACTIVATEAPP),
1.00DA must be handling WM_INITDIALOG, and 1.022D must be
handling WM_COMMAND messages.
Write it down! This same basic technique -find where the
[bp+0Ch] "message" parameter to the WndProc or DialogProc is
being rested, and from that identify the locations that handle
various messages- can be used in *ANY* Windows program.
Because handling messages is mostly what Windows applications
do, once we know where the message handling is, we pretty much
can have our way with teh disassembled listing.
Let's look now at TaskManDlgProc():
TASKMANDLGPROC proc far
...
DISPATCH_ON_MSG:
1.0097 8B460C mov ax, message ;[bp+0C]
1.009A 2D1C00 sub ax, WM_ACTIVATEAPP ;001C
1.009D 7416 je ON_ACTIVATEAPP
1.009F 2DF400 sub ax, 00F4 ;(+1C=110=WM_INITDIALOG)
1.00A2 7436 je ON_INITDIALOG
1.00A4 48 dec ax ;(110+1=111=WM_COMMAND)
1.00A5 7503 jne DEFAULT
1.00A7 E98301 jmp ON_COMMAND
DEFAULT:
1.00AA >2D5303 sub ax, 0353 ;(111+353=464=WM_USER+64
1.00AD 7503 jne ON_PRIVATEMSG ;00B2= some private msg
1.00AF E9D602 jmp 0388
ON_PRIVATEMSG:
1.00B2 >E9C801 jmp 027D
ON_ACTIVATEAPP:
1.00B5 >837E0A00 cmp word ptr wParam, 0 ;[bp+0A]
... ; code to handle WM_ACTIVATEAPP
ON_INITDIALOG:
... ; code to handle WM_INITDIALOG
ON_COMMAND:
... ; code to handle WM_COMMAND
1.022D >8B460A mov ax, wParam ;[bp+0A]
1.0230 3D6800 cmp ax, 0068 ; ? What's this ?
1.0233 7503 jne 0238
1.0235 E93301 jmp 036B
...
This is starting to look pretty reasonable. In particular, once
we know where WM_COMMAND is being handled, we are well on the way
to understand what the application does.
WM_COMMANd is *very* important for understanding an application
behavior because the handler for WM_COMMAND is where it dals with
user commands such as Menu selections and dialog push button
clicks... a lot of what an application unique.
If you click on "Cascade" in Task manager, for instance, it
comes as a WM_COMMAND, the same occurs if you click on "Tile" or
"Switch To" or "End Task".
An application can tell which command a user has given it by
looking in the wParam parameter to the WM_COMMAND message.
This is what we started to see at the ned of the
TaskManDlgProc() exerpt:
ON_COMMAND:
; We are handling WM_COMMAND, therefore wParam is here idItem,
; i.e. a control or menu item identifier
1.022D >8B460A mov ax, wParam ;[bp+0A]
1.0230 3D6800 cmp ax, 0068 ;ID number for a dialog control
1.0233 7503 jne 0238
1.0235 E93301 jmp 036B
1.0238 >7603 jbe 023D
1.023A E96001 jmp 039D
1.023D >FEC8 dec al ;1
1.023F 7420 je 0261 ;if wParam==1 goto 1.0261
1.0241 FEC8 dec al ;1+1=2
1.0243 7503 jne 0248
1.0245 E94701 jmp 038F ;if wParam==2 goto 1.038F
1.0248 >2C62 sub al, 62 ;2+62=64
1.024A 742A je 0276
1.024C FEC8 dec al ;64+1=65
1.024E 7432 je 0282
1.0250 2C01 sub al, 01 ;65+1=66
1.0252 7303 jnb 0257
1.0254 E94601 jmp 039D
1.0257 >2C01 sub al, 01 ;66+1=67
1.0259 7703 ja 025E
1.025B E9D200 jmp 0330
It's clear that wParam is being compared (in an odd subtraction
way) to valus 1,2,65,66 and 67. What's going on?
The values 1 and 2 are standard dialog button IDs:
#define IDOK 1
#define IDCANCEL 2
Therefore we have here the two "classical" push buttons:
1.023D >FEC8 dec al ;1 = OK
1.023F 7420 je ON_OK ;If 1 goto 1.0261= ON_OK
1.0241 FEC8 dec al ;1+1=2= CANCEL
1.0243 7503 jne NOPE ;goto neither OK nor CANCEL
1.0245 E94701 jmp ON_CANCEL ;if 2 goto 1.038F= ON_CANCEL
The numbers 65, 66 etc are specific to TaskManager however, we
will not find them inside WINDOWS.H... so there is no home to
find the names of the commands to which these magic number
correspond, unless we happen to have a debug version of the
program true? NO! FALSE!
One of the notable things about Windows is that remarkably
little information is lost or thrown away compiling the source
code. These magic numbers seem to correspond in some way to the
different Task Manager push buttons... it's pretty obvious that
there must be a way of having applications tell Windows what
wParam they want sent when one of their buttons is clicked or
when one of their menu items is selected.
Applications almost always provide Windows with this
information in their resources (they could actually define menus
and controls dynamycally, on the fly, but few applications take
advantage of this). These resources are part of the NE executable
and are available for ur snooping around.
This inspections of the resources in an EXE file is carried out
by means of special utilities, like RESDUMP, included with
Windows source (-> see my tool page). For example (I am using
"-verbose" mode):
DIALOG 10 (0Ah), "Task List" [30,22,160,107]
FONT "Helv"
LISTBOX 100 (64h), "" [3,3,154,63]
DEFPUSHBUTTON 1 (01h), "&Switch To" [1,70,45,14]
PUSHBUTTON 101 (65h), "&End Task" [52,70,45,14]
PUSHBUTTON 2 (02h), "Cancel" [103,70,55,14]
STATIC 99 (63h), "" [0,87,160,1]
PUSHBUTTON 102 (66h), "&Cascade" [1,90,45,14]
PUSHBUTTON 103 (67h), "&Tile" [52,90,45,14]
PUSHBUTTON 104 (68h), "&Arrange Icons" [103,90,55,14]
YEAH! It's now apparent what the numbers 64h, 65h etc. mean.
Imagine you would write Taskmanager yourself... you would write
something on these lines:
#define IDD_SWITCHTO IDOK
#define IDD_TASKLIST 0x64
#define IDD_ENDTASK 0x65
#define IDD_CASCADE 0x66
#define IDD_TILE 0x67
#define IDD_ARRANGEICONS 0x68
Let's look back at the last block of code... it makes now a lot
more sense:
ON_COMMAND:
; We are handling WM_COMMAND, therefore wParam is here idItem,
; i.e. a control or menu item identifier
1.022D >8B460A mov ax, wParam ;[bp+0A]
1.0230 3D6800 cmp ax, 0068 ;is it the ID 68h?
...
1.023D >FEC8 dec al ;1=IDOK=IDD_SWITCHTO
1.023F 7420 je ON_SWITCHTO ;0261
1.0241 FEC8 dec al ;1+1=2=ID_CANCEL
1.0243 7503 jne neither_OK_nor_CANCEL ;0248
1.0245 E94701 jmp ON_CANCEL ;038F
neither_OK_nor_CANCEL:
1.0248 >2C62 sub al, 62 ;2+62=64= IDD_TASKLIST
1.024A 742A je ON_TASKLIST ;0276
1.024C FEC8 dec al ;64+1=65= IDD_ENDTASK
1.024E 7432 je ON_ENDTASK ;0282
1.0250 2C01 sub al, 01 ;65+1=66= IDD_CASCADE
1.0252 7303 jnb check_for_TILE ;0257
1.0254 E94601 jmp 039D ;something different
check_for_TILE:
1.0257 >2C01 sub al, 01 ;66+1=67= IDD_TILE
1.0259 7703 ja 025E ;it's something else
1.025B E9D200 jmp ON_TILE_or_CASCADE ;0330
In this way we have identified location 0330 as the place where
Taskman's "Cascade" and "Tile" buttons are handled... we have
renaimed it ON_TILe_or_CASCADE... let's examine its code and
ensure it makes sense:
ON_TILE_or_CASCADE:
1.0330 >56 push hwndDlg ;si
1.0331 6A00 push 0000
1.0333 9A6F030000 call USER.SHOWWINDOW
1.0338 9A74030000 call USER.GETDESKTOPWINDOW
1.033D 8BF8 mov di, ax ;hDesktopWnd
1.033F 837E0A66 cmp word ptr wParam, 0066 ;IDD_CASCADE
1.0343 750A jne ON_TILE ;034F
1.0345 57 push di ;hDesktopWnd
1.0346 6A00 push 0000
1.0348 9AFFFF0000 call USER.CASCADECHILDWINDOWS
1.034D EB2F jmp 037E
ON_TILE:
1.034F >57 push di
1.0350 6A10 push 0010
1.0352 9AFFFF0000 call USER.GETKEYSTATE
1.0357 3D0080 cmp ax, 8000
1.035A 7205 jb 0361
1.035C B80100 mov ax, 0001 ;1= MDITILE_HORIZONTAL
1.035F EB02 jmp 0363
1.0361 >2BC0 sub ax, ax ;0= MDITILE_VERTICAL
1.0363 >50 push ax
1.0364 9AFFFF0000 call USER.TILECHILDWINDOWS
1.0369 EB13 jmp 037E
Yes, it makes a lot of sense: We have found that the "Cascade"
option in Tile manager, after switching through the usual bunch
of switch/case loops, finally ends up calling an undocumented
Windows API function: CascadeChildWindows()... similarly, the
"Tile" routine ends up calling TileChildWindow().
One thing screams for attention in the disassembled listing of
ON_TILE: the call to GetKeyState().
As an example of the kind of information you should be able to
gather for each of these functions, if you are serious about
cracking, I'll give you now here, in extenso, the definition from
H. Schildt's "General purpose API functions", Osborne's Windows
Programming Series, Vol. 2, 1994 edition (I found both this
valuable book and its companion: volume 3: "Special purpose API
functions", in a second hand shop, in february 1996, costing the
equivalent of a pizza and a beer!). Besides this function is also
at times important for our cracking purposes, and represents
therefore a good choice. Here the description from pag.385:
---------------------------------------------------------
void GetKeyState(int iVirKey)
Use GetKeyState() to determine the up, down or toggled status of
the specified virtual key. iVirKey identifies the virtual key.
To return the status of a standard alphanumeric character in the
range A-Z, a-z or 0-9, iVirKey must be set equal to its ANSI
ASCII value. All other key must use their related virtual key
codes. The function returns a value indicating the status of the
selected key. If the high-order bit of the byte entry is 1, the
virtual key is pressed (down); otherwise it is up. If you examine
a byte emlement's low-order bit and find it to be 1, the virtual
key has been toggled. A low-order bit of 0 indicates that the key
is untoggled.
Under Windows NT/Win32, this function returns type SHORT.
USAGE
If your application needs to distinguish wich ALT, CTRL, or
SHIFT key (left or right) has been pressed, iVirKey can be set
equal to one of the following:
VK_LMENU VK_RMENU
VK_LCONTROL VK_RCO?TROL
VK_LSHIFT VK_RSHIFT
Setting iVirKey equal to VK_MENU, VK_CONTROL or VK_SHIFT
instructs GetKeyState() to ignore left and right, and only to
report back the status of teh virtual key category. This ability
to distinguish among virtual-key states is only available with
GetKeyState() and the related functions listed below.
The following fragment obtains the state of the SHIFT key:
if(GetKeyState(VK_SHIFT) {
...
}
RELATED FUNCTIONS
GetAsyncKeyState()
GetKeyboardState()
MapVirtualKey()
SetKeyboardState()
---------------------------------------------------------
Ok, let's go on... so we have in our code a "funny" call to
GetKeyState(). Becaus ethe Windows USer's Guide says nothing
about holding down a "state" (shift/ctrl/alt) key while selecting
a button, this sounds like another undocumented "goodie" hidden
inside TASKMAN.
Indeed, if you try it out on the 3.1 Taskman, you'll see that
clicking on the Tile button arranges all the windows on the
desktop side by side, but if you hold down the SHIFT key while
clicking on the Tile button, the windows are arranged in a
stacked formation.
To summarize, when the 3.1. Taskman Tile button is selected, the
code that runs in response looks like this:
Tile:
ShowWindow(hWndDlg, SW_HIDE); // hide TASKMAN
hDesktopWnd = GetDesktopWindow();
if (GetKeyState(VK_SHIFT) == 0x8000)
TileChildWindows(hDesktopWnd, MDITILE_HORIZONTAL);
else
TileChildWindows(hDesktopWnd, MDITILE_VERTICAL);
Similarly, the CASCADE option in 3.1. TASKMAN runs the following
code:
Cascade:
ShowWindow(hWndDlg, SW_HIDE); // hide TASKMAN
CAscadeChildWindows(GetDesktopWindow(), 0);
We can then proceed through each TASKMAN option like this,
rendering the assembly language listing into more concise C.
The first field to examine in TASKMAN is the Task List itself:
how is the "Task List" Listbox filled with the names of each
running application?
What the List box clearly shows is a title bar for each visible
top level window, and the title bar is undoubtedly supplied with
a call to GetWindowText()... a function that obtains a copy of
the specified window handle's title.
But how does TASKMAN enumerate all the top-level Windows?
TAskman exports TASKMANDLGPROC, but does not export any
enumeration procedure.
Most of the time Windows programs iterate through all existing
windows by calling EnumWindows(). Usually they pass to this
function a pointer to an application-supplied enumeration
function, which therefore MUST be exported. This callback
function must have following prototype:
BOOL CALLBACK EnumThreadCB(HWND hWnd, LPARAM lParam)
Of course, the name a programmer chooses for such an exported
function is arbitrary. hWnd will receive the handle of each
thread-associated window.lParam receives lAppData, a 32-bit user-
defined value. This exported function must return non-zero to
receive the next enumerated thread-based window, or zero to stop
the process.
But here we DO NOT have something like TASKMANENUMPROC in the
list of exported functions... what's going on? Well... for a
start TASKMAN IS NOT calling EnumWindows()... Taskman uses a
GetWindow() loop to fill the "Task List" list box, study
following C muster, sipping a good cocktail and comparing it with
the disassembled code you have printed:
Task List:
listbox = GetDlgItem(hWndDlg, IDD_TASKLIST);
hwnd = GetWindow(hwndDlg, GW_HWNDFIRST);
while (hwnd)
{ if ((hwnd != hwndDlg) && //excludes self from list
IsWindowVisible(hwnd) &&
GetWindow(hwnd, GW_OWNER))
{ char buf[0x50];
GetWindowText(hwnd, buf, 0x50); // get titlebar
SendMessage(listbox, LB_SETITEMDATA,
SendMessage(listbox, LB_ADDSTRING, 0, buf),
hwnd); // store hwnd as data to go
} // with the titlebar string
hwnd = GetWindow(hwnd, GW_HWNDNEXT);
}
SendMessage(lb, LB_SETCURSEL, 0, 0); // select first item
The "End Task" opton in Taskman just sends a WM_CLOSE message
to the selected window, but only if it's not a DOS box. TASKMAN
uses the undocumented IsWinOldApTask() function, in combination
with the documented GetWindowTask() function, to determine if a
given HWND corresponds to a DOS box:
End Task:
... // boring details omitted
if(IsWinOldApTask(GetWindowTask(hwndTarget)))
MaybeSwitchToSelecetedWindow(hwndTarget);
if(IsWindow(hwndTarget) &&
(! (GetWindowLong(hwndTarget, GWL 5STYLE) & WS_DISABLED))
{
PostMessage(hwndTarget, WM_CLOSE, 0, 0);
}
The "Arrange Icons" option simply runs the documented
ARrangeIconicWindows() function:
Arrange Icons:
Showwindow(hWndDlg, SW_HIDE);
ArrangeIconiCWindows(GetDesktopWindow());
The "Switch To" option in TASKMAN is also interesting. Like
"Tile" and "Cascade", this too it's just a user-interface
covering an undocupented Windows API function, in this case
SwitchToThisWindow().
Let's walk through the process of deciphering a COMPLETELY
unlabelled Windows disassembly listing, that will be most of the
time your starting situation when you crack, and let's turn it
into a labelled C code.
By the way, there does exist an interesting school of research,
that attempts to produce an "EXE_TO_C" automatical converter. The
only cracked version of this program I am aware of is called
E2C.EXE, is 198500 bytes long, has been developed in 1991 by "The
Austin Code Works and Polyglot International" in Jerusalem (Scott
Guthery: guthery@acw.com), and has been boldly brought to the
cracking world by Mithrandir/AlPhA/MeRCeNarY. Try to get a copy
of this tool... it can be rather interesting for our purposes ;-)
Here is the raw WCB disassembled code for a subroutine within
TASKMAN, called from the IDD_SWITCHTO handling code in
TaskManDlgProc():
1.0010 >55 push bp
1.0011 8BEC mov bp, sp
1.0013 57 push di
1.0014 56 push si
1.0015 FF7604 push word ptr [bp+04]
1.0018 681A04 push 041A
1.001B FF7604 push word ptr [bp+04]
1.001E 680904 push 0409
1.0021 6A00 push 0000
1.0023 6A00 push 0000
1.0025 6A00 push 0000
1.0027 9A32000000 call USER.SENDMESSAGE
1.002C 50 push ax
1.002D 6A00 push 0000
1.002F 6A00 push 0000
1.0031 9AEF010000 call USER.SENDMESSAGE
1.0036 8BF8 mov di, ax
1.0038 57 push di
1.0039 9A4C000000 call USER.ISWINDOW
1.003E 0BC0 or ax, ax
1.0040 742A je 006C
1.0042 57 push di
1.0043 9AFFFF0000 call USER.GETLASTACTIVEPOPUP
1.0048 8BF0 mov si, ax
1.004A 56 push si
1.004B 9AA4020000 call USER.ISWINDOW
1.0050 0BC0 or ax, ax
1.0052 7418 je 006C
1.0054 56 push si
1.0055 6AF0 push FFF0
1.0057 9ACD020000 call USER.GETWINDOWLONG
1.005C F7C20008 test dx, 0800
1.0060 750A jne 006C
1.0062 56 push si
1.0063 6A01 push 0001
1.0065 9AFFFF0000 call USER.SWITCHTOTHISWINDOW
1.006A EB07 jmp 0073
1.006C >6A00 push 0000
1.006E 9ABC020000 call USER.MESSAGEBEEP
1.0073 >5E pop si
1.0074 5F pop di
1.0075 8BE5 mov sp, bp
1.0077 5D pop bp
1.0078 C20200 ret 0002
The RET 0002 at the end tells us that this is a near Pascal
function thatexpects one WORd parameter, which appears as [bp+4]
at the top of the code.
Because [bp+4] is being used as the first parameter to
SendMessage(), it must be an HWND of some sort.
Here is the muster for SendMessage(): LRESULT SendMessage(HWND
hWnd, UINT uMsg, WPARAM wMsgParam1, LPARAM lMsgParam2), where
hWnd identifies the Window receiving the message, uMsg identifies
the message being sent, wMsgParam1 & lMsgParam2 contain 16 bits
and 32 bits of message-specific information.
Finally, we don't see anything being moved into AX or DX near
the end of the function, so it looks as if this function has no
return value:
void near pascal some_func(HWND hwnd)
Let's look once more at it... the function starts off with two
nested calls to SendMessage(à, using the message numbers 41Ah and
409h. These numbers are greater than 400h, they must therefore
be WM_USER+XX values. Windows controls such as edit, list and
combo boxes all use WM_USER+XX notification codes.
The only appropriate control in TASKMAN is the list box, so we
can just look at the list of LB_XXX codes in WINDOWS.H. 1Ah is
26 decimal, therefore 41Ah is WM_USER+26, or LB_GETITEMDATA.
Let's see what Osborne's "Special Purpose API functions" says
about it (pag.752):
------------------------------------------------------
LB_GETITEMDATA
When sent: To return the value associated with a list-box item.
wParam: Contains the index to the item in question
lParam: Not used, must be 0
Returns: The 32-bit value associated with the item
------------------------------------------------------
Similarly,, 409h is WM_USER+9, which in the case of a list box
means LB_GETCURSEL. We saw earlier that TASKMAN uses
LB_SETITEMDATA to store each window title's associated HWND.
LB_GETITEMDATA will now retrive this hwnd:
hwnd = SendMessage(listbox, LB_GETITEMDATA,
SendMessage(listbox, LB_GETCURSEL, 0, 0), 0);
Notice that now we are caling the parameter to some_func() a
listbox, and that the return value from LB_GETITEMDATA is an
HWND.
How would we know it's an hwnd without our references? We can
see the LB_GETITEMDATA return value (in DI) immediatly being
passed to IsWindow() at line 1.0039:
; IsWindow(hwnd = SendMessage(...));
1.0031 9AEF010000 call far ptr SENDMESSAGE
1.0036 8BF8 mov di, ax
1.0038 57 push di
1.0039 9A4C000000 call far ptr ISWINDOW
Next, the hwnd is passed to GetLastActivePopup(), and the HWND
that GetLastActivePopup() returns is then checked with
IsWindow()... IsWindow() returns non-zero if the specified hWnd
is valid, and zero if it is invalid:
; IsWindow(hwndPopup = GetLastActivePopup(hwnd));
1.0042 57 push di
1.0043 9AFFFF0000 call USER.GETLASTACTIVEPOPUP
1.0048 8BF0 mov si, ax ; save hwndPopup in SI
1.004A 56 push si
1.004B 9AA4020000 call USER.ISWINDOW
Next, hwndPopup (in SI) is passed to GetWindowLong(), to get
informations about this window. Here is time to lok at WINDOWS.H
to figure out what 0FFF0h at line 1.055 and 800h at line 1.005C
are supposed to mean:
; GetWindowLong(hwndPopup, GWL_STYLE) & WS_DISABLED
1.0054 56 push si ;hwndPopup
1.0055 6AF0 push GWL 5STYLE ;0FFF0h = -16
1.0057 9ACD020000 call USER.GETWINDOWLONG
1.005C F7C20008 test dx, 0800 ;DX:AX= 800:0= WS_DISABLED
Finally, as the whole point of this exercise, asuming this
checked window passes all its tests, its last active popup is
switched to:
; SwitchToRhisWindow(hwndPopup, TRUE)
1.0062 56 push si :hwndPopup
1.0063 6A01 push 0001
1.0065 9AFFFF0000 call USER.SWITCHTOTHISWINDOW
It's here that all possible questions START: SwitchToThisWindow
is not documented... therefore we do not know the purpose of its
second parameter, apparently a BOOL. We cannot even tell why
SwitchToThisWindow() is being used... when SetActiveWindow(),
SetFocus() or BringWindowToTop() might do the trick. And why is
the last active popup and not the window switched to?
But let's resume for now our unearthed mysterious function, taht
will switch to the window selected in the Task List if the window
meets all the function's many preconditions:
void MaybeSwitchToSelecetedWindow(HWND listbox)
{
HWND hwnd, hwndPopup;
// first figure out wich window was selected in the Task List
if (IsWindow(hwnd = SendMessage(listbox, LB_GETITEMDATA,
SendMessage(listbox, LB_GETCURSEL, 0, 0), 0)))
{
if (IsWindow(hwndPopup = GetLastActivePopup(hwnd)))
{
if (! (GetWindowLong(hwndPopup, GWL 5STYLE) & WS_DISABLED))
{
SwitchToThisWindow(hwndPopup, TRUE);
return;
}
}
MessageBeep(0); //Still here... error!
}
Now we have a good idea of what TASKMAN does (it sure took a long
time to understand thos 3K bytes of code!). In the next lessons
we'll use what we have learned to crack together some common
Windows programs.
(->lesson 3)