CODEWITHSUNDEEP
c++windowsmemory-management
Gizmo
Gizmo

Reputation: 2210

Allocate data at specific address in windows?

I was browsing around for stuff to do and I got the idea to allocate memory at specific addresses in windows.

So I read a few questions here on stackoverflow, but none actually presenting a working example, so I had to come up with my own because I really wanted to try:

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

struct Variable {
    int var;
};

#define ACCESS() ((Variable*)0x50000000)

int main()
{
    DWORD ptr;
    VirtualAlloc((void*)0x50000000,sizeof(Variable),MEM_COMMIT | MEM_RESERVE | MEM_PHYSICAL,PAGE_READWRITE);                    
    VirtualProtect((void*)0x50000000,sizeof(Variable),PAGE_READWRITE,&ptr);
    ACCESS()->var = 5;
    while(!GetAsyncKeyState('Q')){}
}

But this alway causes an access voilation..

What is the correct way to allocate data at a specific address? Because, this way somehow won't work.. yet confusing at the same time with "Why not?".

Edit:

Second code after first answers not working too:

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

struct Variable {
    int var;
};

#define ACCESS() ((Variable*)0x50000000)
int main()
{   
    std::cout << VirtualAlloc((void*)0x50000000,sizeof(Variable),MEM_COMMIT,PAGE_READWRITE) << std::endl;
    std::cout << GetLastError() << std::endl;
    ACCESS()->var = 6;
    std::cout << ACCESS() << std::endl;
    while(!GetAsyncKeyState('Q')){}
}

the return value is 0, which indicates failure and the GetLastError() equals 487,

and 487 according to this page means:

ERROR_INVALID_ADDRESS; Attempt to access invalid address.

So, all I read over the internet and stack overflow is fake? You >Cannot< allocate data at a specified address? And if this is true, why do we need this function anyway?

Next edit:

Seems it's totally possible, working code:

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

struct Variable {
    int var;
};

#define ACCESS() ((Variable*)0x50000000)
int main()
{   
    std::cout << VirtualAlloc((void*)0x50000000,sizeof(Variable),MEM_COMMIT | MEM_RESERVE,PAGE_READWRITE) << std::endl;
    std::cout << GetLastError() << std::endl;
    std::cout << ACCESS() << std::endl;
    ACCESS()->var = 6;
    std::cout << ACCESS()->var << std::endl;
    while(!GetAsyncKeyState('Q')){}
}

Upvotes: 2

Views: 4705

Answers (1)

avakar
avakar

Reputation: 32635

The call to VirtualAlloc is failing. You would have figured that out if you checked its return value.

The reason it's failing comes directly from the documentation: MEM_PHYSICAL must be used with MEM_RESERVE and no other values. You likely don't want MEM_PHYSICAL anyway, it has to do with AWE. Removing the flag will make the allocation succeed.

The call to VirtualProtect is unnecessary.

The following should work:

#include <windows.h>
int main()
{
    VirtualAlloc((void*)0x50000000,0x1000,MEM_COMMIT|MEM_RESERVE,PAGE_READWRITE);
    *((int*)0x50000000) = 5;
}

Upvotes: 5

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