Problem with function

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Cthulhu

Joined: 12 May 2005
Posts: 29

Cthulhu 26 Feb 2008, 11:29

I'm having problems creating functions with fasm it seems it has to do with calling coventions but I can't figure out what is wrong Embarassed

Here is the code:

Code:

format PE GUI 4.0
entry start

include 'win32a.inc'

section '.code' code readable executable

;---------------------------------------------------------------
proc Dummy a1, a2, a3
   MOV EAX, [a1]
       ADD EAX, [a2]
       ADD EAX, [a3]
       RET
endp     

;---------------------------------------------------------------
start:

  push 1
      push 2
      push 3
      call Dummy
  
    PUSH 0                  ; <<====== Code never returns here
    CALL [ExitProcess]

;=====================================
section '.idata' import data readable writeable

  library kernel,'KERNEL32.DLL'
  import  kernel, ExitProcess,'ExitProcess'

After calling dummy the code never returns to PUSH 0.
What I did wrong?

26 Feb 2008, 11:29

shoorick

Joined: 25 Feb 2005
Posts: 1614
Location: Ukraine

shoorick 26 Feb 2008, 11:35

try "ret" instead "RET"

26 Feb 2008, 11:35

Cthulhu

Joined: 12 May 2005
Posts: 29

Cthulhu 26 Feb 2008, 11:38

shoorick wrote:

try "ret" instead "RET"

I could never imagine it was the case of the ret instruction Shocked

Thanks a lot shoorick!

26 Feb 2008, 11:38

dap

Joined: 01 Dec 2007
Posts: 61
Location: Belgium

dap 26 Feb 2008, 12:23

You should pop the arguments from the stack when the function returns. You can simply write "ret 12" instead of ret (because there are 12 bytes of arguments left on the stack). That's how the stdcall convention handles it. Or you can use C convention where you add manually the number of bytes to ESP after the function returns (add esp, 12). If you have only one argument you can also simply pop it from the stack (in ECX for example which is a register you can freely use), it will be shorter than adding 4 to ESP.

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26 Feb 2008, 12:23

bitRAKE

Joined: 21 Jul 2003
Posts: 4071
Location: vpcmpistri

bitRAKE 26 Feb 2008, 15:09

"ret" has been redefined by win32a.inc (actually proc32.inc) to work with "proc" macro. When you use "RET" the macro for ret isn't executed. This macro changes the ret into "ret 12" - based on proc arguments.

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26 Feb 2008, 15:09

Cthulhu

Joined: 12 May 2005
Posts: 29

Cthulhu 26 Feb 2008, 15:45

Thanks a lot guys!

26 Feb 2008, 15:45

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