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...

Answer 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.

Answer 2

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;

Answer 3

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 :)