Question

I have the following code

void PacketEncrypt(Packet* packet, int sizeofpacket)
{
    int* pointer;
    pointer = ((int*)packet+sizeofpacket)-2;
    (int)*pointer = packet->PacketSize^0x1A3C;
    packet->Type += 0x0FFF7;
}

Problem is when i debug it compiler set it as:

0041585E   8B45 0C          MOV EAX,DWORD PTR SS:[EBP+C]
00415861   8B4D 08          MOV ECX,DWORD PTR SS:[EBP+8]
00415864   8D5481 F8        LEA EDX,DWORD PTR DS:[ECX+EAX*4-8]

But what i really want is:

0041585E   8B45 0C          MOV EAX,DWORD PTR SS:[EBP+C]
00415861   8B4D 08          MOV ECX,DWORD PTR SS:[EBP+8]
00415864   8D5481 F8        LEA EDX,DWORD PTR DS:[ECX+EAX-2]

I am kind of new into C++, so can you help me on what i am doing wrong, or why the compiler is adding scalar *4? thanks !

Answer 1

int pointer has size of 4 bytes on your machine and can be shifted only by 4*n bytes. If you need to move pointer by 2 bytes cast it to char, since sizeof(char) = 1

char *ptr = (char*)otherPtr;
ptr += 2; //shift by 2 bytes

int *ptr = (int*)otherPtr;
ptr += 2; //shift by 8 bytes. (2 * sizeof(int))

EDIT

(short*)pointer = something. – You can’t assign to rvalue. I think you meant:

pointer = (char*) something

Or if you need short pointer:

short *someOtherPointer = something