scanf troubles

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DStevens5833

Joined: 20 May 2011
Posts: 2

DStevens5833

Hello everyone! To be honest, I'm completely new to assembly programming; I've spend the last few days reading documentation and source code, and it has been flowing pretty well. So keep in mind that I'm new, and try not to make too much fun! Razz

Anyways, I'm having issues with the following code:

Code:

format PE console
entry start

include 'win32a.inc'

proc Addition stdcall uses ecx edx, a:DWORD, b:DWORD
     mov ecx, [a]
     mov edx, [b]
     add ecx, edx
     mov eax, ecx

     ret
endp

section '.data' data readable writeable

        _RequestInput   db      "Enter two numbers to add: ", 0
        _Result         db      "%d + %d = %d", 0
        _Input1         dd      "%d", 0
        _Input2         dd      "%d", 0

        _Number1        dd      ?
        _Number2        dd      ?

section '.text' code readable executable

        start:
                push _RequestInput
                call [printf]

                push _Number1
                push _Input1
                call [scanf]

                push _Number2
                push _Input2
                call [scanf]

                push _Number1
                push _Number2
                call Addition

                push eax
                push _Number1
                push _Number2
                push _Result
                call [printf]

                push 0
                call [ExitProcess]

section '.idata' import data readable

        library msvcrt, 'MSVCRT.DLL', \
                kernel, 'KERNEL32.DLL'

        import msvcrt, \
                       printf, 'printf', \
                       scanf, 'scanf'

        import kernel, \
                       ExitProcess, 'ExitProcess'

Results:

Code:

Enter two numbers to add: 3
4
4202556 + 4202552 = 8405108

Obviously 3 + 4 != 8405108. Please, someone shed some light. Also, advice regarding a learning path is welcome and appreciated! Thank you!

24 May 2011, 18:53

ctl3d32

Joined: 30 Dec 2009
Posts: 204
Location: Brazil

ctl3d32

Hi! These should be changed:

Code:

push [_Number1]
push [_Number2]
call Addition

push eax
push [_Number1]
push [_Number2]
push _Result
call [printf]

24 May 2011, 19:42

mindcooler

Joined: 01 Dec 2009
Posts: 423
Location: Västerås, Sweden

mindcooler

Code:

                push _Number1 
                push _Number2

You're pushing the addresses of your variables, not their values.

Code:

                push [_Number1]
                push [_Number2]
                call Addition 

                push eax 
                push [_Number1]
                push [_Number2]
                push _Result 
                call [printf]

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24 May 2011, 19:44

DStevens5833

Joined: 20 May 2011
Posts: 2

DStevens5833

Such a simple mistake! It works now; thank you both for your help!

24 May 2011, 19:52

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4633
Location: Argentina

LocoDelAssembly

In this particular case nothing will blow up but you should use "add esp, total_size_of_arguments" after each call to msvcrt functions (and wsprintf), where total_size_of_arguments in all your cases is the number of PUSHes multiplied by 4. (Using cinvoke would do this automatically for you as long as all the parameters are specified in the cinvoke call and not some of them pre pushed.)

24 May 2011, 20:51

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