C++ Pointers, Unable To Read Memory

Question

Why, when I initiate my pointer, I can read the assigned value:

DWORD *pBytesReturned = new DWORD[0];
_result =  EnumProcesses(pProcessIds, 1000, pBytesReturned);

But when I initialize it like this I can't read the assigned value:

DWORD *pBytesReturned = 0;
_result =  EnumProcesses(pProcessIds, 1000, pBytesReturned);

Here's the complete code if needed:

#include "stdafx.h"
#include <Psapi.h>

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR pCmdLine, int iCmdShow)
{
    bool _result;
    DWORD *pProcessIds = new DWORD[1000];
    DWORD cb;
    DWORD *pBytesReturned =0;
    _result =  EnumProcesses(pProcessIds, 1000, pBytesReturned);
}

Answer 1

The intent of the function

BOOL
WINAPI
EnumProcesses (
    _Out_writes_bytes_(cb) DWORD * lpidProcess,
    _In_ DWORD cb,
    _Out_ LPDWORD lpcbNeeded
    );

is to have a pointer as a third parameter, and this pointer must point to a valid memory location where the function will be storing a dword. The expected function call should look like this

DWORD *pProcessIds = new DWORD[1000];
DWORD bytesReturned = 0;
bool _result = EnumProcesses(pProcessIds, 1000, &bytesReturned);

or like this

DWORD *pProcessIds = new DWORD[1000];
DWORD *pBytesReturned = new DWORD[1];
bool _result = EnumProcesses(pProcessIds, 1000, pBytesReturned);

but you should not be using a NULL pointer and expect the debugger to dereference it.