Qbasicnews.com
January 24, 2021, 09:42:44 AM
 Pages: [1]
 Author Topic: Initialised Array In Function ??? Problem ???  (Read 2363 times)
Erdemal
New Member

Posts: 7

 « on: April 23, 2005, 05:35:12 PM »

It seem that this is a bug ?

Code:

Dim myLong As Long
myLong = &HAABBCCDD
''
Function myFunc (Arg1 As Long) As Long
Dim myArray (0 To 3) As Long => {1, 2, 3, 4}
Dim myData As Long
myData = 33
Function = myData
End Function
''
Print Hex\$(myFunc (myLong))
Print "   *** FINISHED OK *** "

Translates in ASM (for the function part)

Code:
.globl _MYFUNC@4
_MYFUNC@4:
push ebp
mov ebp, esp
sub esp,52     ; (1) ??????????????
push ebx
push esi
push edi
lea edi, [ebp-52]
mov ecx,13
xor eax, eax
rep stosd
_t0004:
push 3
push 0
push 1
push 4
lea eax, [ebp-20]
push eax
lea eax, [ebp-48]
push eax
call _fb_ArraySetDesc
add esp, 24                          ; (2) ?????????????
mov dword ptr [ebp-20], 1
mov dword ptr [ebp-16], 2
mov dword ptr [ebp-12], 3
mov dword ptr [ebp-8], 4
mov dword ptr [ebp-52], 33  ; (3) ?????????????
mov eax, dword ptr [ebp-52] ; (4) ?????????????
mov dword ptr [ebp-4], eax
_t0003:
mov eax, dword ptr [ebp-4]
pop edi
pop esi
pop ebx
mov esp, ebp
pop ebp
ret 4

First 52 bytes of local datas are reserved  on the stack (sub esp, 52)(1)
...
Then 24 bytes of these 52 bytes are "Freed "
...
Then the "freed space" is read/Write !  (3) & (4)
mov dword ptr [ebp-52], 33
mov eax, dword ptr [ebp-52]

That [ebp+52] is a space below  esp, thus in free stack space and may be overwriten by any push, call ...

erdemal
 Logged
Antoni Gual
Na_th_an

Posts: 1434

 « Reply #1 on: May 02, 2005, 09:52:58 AM »

wrong forum?
 Logged

Antoni
 Pages: [1]