18 byte dynamic retn

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r22

Joined: 27 Dec 2004
Posts: 805

r22 28 Jan 2005, 05:43

Did you ever want to make a function that can take a variable amount of arguments? Example wsprintf - but it doesn't clean the stack for you. This code puts ESP back where it belongs.

Code:

Start:
        push 1
        push 1
        push 1
        push 1
        push 1
        push 1
        push 7 ;# of arguments
        call dynamicreturn
        push 0
        call [ExitProcess]

dynamicreturn:
        push ebp
        mov ebp,esp
        mov esp,ebp
        pop ebp ;dynamic return starts
        shl dword[esp+4],2 ;#of args to #of dwords
        mov ecx,[esp]      ;save return addr
        mov edx,[esp+4]    ;adjust esp to
        lea esp,[esp+edx+4];preCall position
        jmp ecx            ;jmp return addr

You don't even need an arg for the number of arguments IF one of your args is say a format string you can have your function store how many args in a local from the stack frame.

I hope you all enjoy this as much as I enjoyed coding it.

28 Jan 2005, 05:43

f0dder

Joined: 19 Feb 2004
Posts: 3174
Location: Denmark

f0dder 28 Jan 2005, 09:46

You shouldn't use this approach for stuff where speed matters, as P4 architecture (perhaps other as well) has a CALL/RET optimization...

28 Jan 2005, 09:46

Frank

Joined: 17 Jun 2003
Posts: 100

Frank 28 Jan 2005, 13:03

Simpler:

Code:

Start:
        push 1
        push 1
        push 1
        push 1
        push 1
        push 1
        push 6 ;# of arguments, excluding this one
        call dynamicreturn
        push 0
        call [ExitProcess]

dynamicreturn:
        pop   edx
        pop   ecx
        ; code goes here, ESP points to parameters, ECX and EDX must not be changed
        lea   esp, [esp + ecx*4]
        push  edx
        ret

28 Jan 2005, 13:03

r22

Joined: 27 Dec 2004
Posts: 805

r22 28 Jan 2005, 20:53

its just an amusing peice of code, that's pretty much it

28 Jan 2005, 20:53

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