New/delete[] and VirtualAlloc

Question

#include <Windows.h>
#include <iostream>

using namespace std;

int main(void)
{
    unsigned char* pFoo = new unsigned char[1000];

    pFoo = (unsigned char*)VirtualAlloc(NULL, 1000, MEM_COMMIT, PAGE_EXECUTE_READWRITE);

    VirtualFree(pFoo, 0, MEM_RELEASE);

    delete[] pFoo;

    cin.ignore();
    cin.get();

    return 0;
}

This crashes for me at

delete[] pFoo;

I know this is crashing because of VirtualAlloc but I'm not sure how to fix this...

score 1 · Accepted Answer · answered Oct 10 '11 at 02:02

1

You're using the same variable. So your first allocation gets leaked.

After you free it with VirtualFree, the pointer is invalid. So delete on it is undefined.

Furthermore:

You can't mix VirtualAlloc and delete for the same reason you can't mix malloc with delete.

answered Oct 10 '11 at 02:02

Mysticial

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So it just VirtualFree(pFoo, 0, MEM_RELEASE); sufficient? – replicant Oct 10 '11 at 02:05
Well, the main problem is that you have two different allocations going to the same variable. The second allocation will overwrite the pointer from the first one. Then you try to free/deallocate it twice. – Mysticial Oct 10 '11 at 02:07
Could explain what you're trying to accomplish with two allocations? Or perhaps you are misunderstanding something? – Mysticial Oct 10 '11 at 02:09
Thanks! I understand, the new isn't needed now. – replicant Oct 10 '11 at 02:13

score 1 · Answer 2 · answered Oct 10 '11 at 02:05

unsigned char* pFoo = new unsigned char[1000];

Now pFoo holds a pointer to dynamic memory.

pFoo = (unsigned char*)VirtualAlloc(NULL, 1000, MEM_COMMIT, PAGE_EXECUTE_READWRITE);

This overwrites the old pointer, the 1000 char array is leaked.

Try:

unsigned char* pFoo = new unsigned char[1000];
unsigned char* pBar = (unsigned char*)VirtualAlloc(NULL, 1000, MEM_COMMIT, PAGE_EXECUTE_READWRITE);
VirtualFree(pBar, 0, MEM_RELEASE);
delete[] pFoo;

score 1 · Answer 3 · answered Oct 10 '11 at 02:05

Either use new/delete or VirtualAlloc/VirtualFree. You are allocating two separate memory blocks, using pFoo to refer to both (when of course it can only refer to one at a time) and then calling the two free functions with pFoo. One of those is going to fail :)

New/delete[] and VirtualAlloc

3 Answers3