Calling DLL from C

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BombStrike

Joined: 03 Jan 2006
Posts: 1

BombStrike 04 Jan 2006, 00:48

ok i wrote a DLL in fASM
i pass a char* to a function in the DLL from my C apps
i'm searching how to mod the content of this pointer

here is an exemple

Application ( in C ):

Code:

extern int TextFunc(char *pString,int SomethingElse);
...
TextFunc(string,5); // where string is a char *

DLL Function ( in ASM ):

Code:

proc TestFunc stdcall pString,SomethingElse
  ; how to modify the value pointed in pString ?
  mov eax,3 ;returning 3
  ret
endp

sry if it's hard to understand, i'm not english Smile

04 Jan 2006, 00:48

r22

Joined: 27 Dec 2004
Posts: 805

r22 04 Jan 2006, 02:30

you pass a ptr to a string to your dll function
so
asm dll

Code:

TestFunc:
;;esp+4 = pString ;;esp+8 = SomethingElse 
  mov eax, [esp+4]
;; now the ptr in is eax
;; make the first character of the string '1'
  mov byte[eax],'1'
  xor eax,eax ;;0 return value for success
  ret 8 ;; return 8 because we passed 2 dword(4bytes each) params

hope that helps you
I don't like using the proc macro just a preference.

04 Jan 2006, 02:30

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