Macro trap

[jj2007]

Just ran into a noob problem: .elseif len(offset Str7)==7 will create bugs that are very difficult to chase. The reason is that macros are being expanded right before the line that invokes them.

Code Select Expand

include \masm32\include\masm32rt.inc

.code
AppName db "MacTest:", 0
Str8 db "12345678", 0
Str7 db "1234567", 0

start:
.if len(offset Str8)==9
MsgBox 0, "9 bytes, wrong", offset AppName, MB_OK
.elseif len(offset Str7)==7
MsgBox 0, "7 bytes, correct", offset AppName, MB_OK
.else
MsgBox 0, "Surprise, nothing is correct", addr AppName, MB_OK
.endif

.if len(offset Str8)==9
MsgBox 0, "9 bytes, wrong", offset AppName, MB_OK
.elseif len(offset Str8)==8 ; this looks like we are testing the string again, but...
nop
MsgBox 0, "8 bytes, yeah - but check that in Olly!", offset AppName, MB_OK
nop
.else
MsgBox 0, "Surprise, nothing is correct", addr AppName, MB_OK
.endif

exit

end start

Excerpt from Olly - check the green part...

00401076    |. 68 09104000        push 00401009               ;  ASCII "12345678"
0040107B    |. E8 90000000        call szLen
00401080    |. 83F8 09            cmp eax, 9
00401083    |. 75 1F              jnz short 004010A4
00401085    |. 6A 00              push 0                      ; /Style = MB_OK|MB_APPLMODAL
00401087    |. 68 00104000        push 00401000               ; |Title = "MacTest:"
0040108C    |. 68 3D404000        push 0040403D               ; |Text = "9 bytes, wrong"
00401091    |. 6A 00              push 0                      ; |hOwner = NULL
00401093    |. E8 BE000000        call MessageBoxA            ; \MessageBoxA
00401098    |. 68 09104000        push 00401009               ;  ASCII "12345678"
0040109D    |. E8 6E000000        call szLen
004010A2    |. EB 2F              jmp short 004010D3
004010A4    |> 83F8 08            cmp eax, 8
004010A7    |. 75 17              jnz short 004010C0
004010A9    |. 90                 nop
004010AA    |. 6A 00              push 0                      ; /Style = MB_OK|MB_APPLMODAL
004010AC    |. 68 00104000        push 00401000               ; |Title = "MacTest:"
004010B1    |. 68 4C404000        push 0040404C               ; |Text = "8 bytes, yeah - but check that in Olly!"
004010B6    |. 6A 00              push 0                      ; |hOwner = NULL
004010B8    |. E8 99000000        call MessageBoxA            ; \MessageBoxA
004010BD    |. 90                 nop