Local variables initial values

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typedef

Joined: 25 Jul 2010
Posts: 2909
Location: 0x77760000

typedef 29 Oct 2012, 14:02

So when we write this code

Code:

local strHey[5]:BYTE

Does the system allocate 5 null bytes on the stack so that the following would be ok without explicitly mov-ing a 0 at the 4th index of the string "hey!" to terminate it?

Code:

mov dword[strHey], "hey!" ; 4 bytes
;------------------------------ I want to skip mov byte[strHey+4],0
invoke MessageBoxA,0,addr strHey,0,0
...

I want to skip the extra step of having to manually put a null byte but at the same time I'm not sure if there'll be garbage or not.

29 Oct 2012, 14:02

AsmGuru62

Joined: 28 Jan 2004
Posts: 1618
Location: Toronto, Canada

AsmGuru62 29 Oct 2012, 14:41

There will be whatever is left on stack from previous code.

29 Oct 2012, 14:41

typedef

Joined: 25 Jul 2010
Posts: 2909
Location: 0x77760000

typedef 29 Oct 2012, 23:50

That makes sense. Thanks. I checked in OllyDbg and in my case since the proc was the first to execute, there was a huge cave of null's on the stack after ntdll and kernel used it.

Thanks for the answer though.

29 Oct 2012, 23:50

AsmGuru62

Joined: 28 Jan 2004
Posts: 1618
Location: Toronto, Canada

AsmGuru62 30 Oct 2012, 02:32

My code generator will have an option for this:

Code:

locals @8 =0      ; Align EBP to 8 byte boundary and set all local bytes to zero
{
    HANDLE  hFile
       INT32   value = -1
  PWCHAR  path = stkalloc (WCHAR, MAX_PATH)
   PVOID   pData
}

30 Oct 2012, 02:32

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