Access to specific memory points

Question

How do I access an exact address in memory on Windows?

unsigned char * mem = {??};

Answer 1

You cannot access a random address like this in most situations. Today there is protection for access to memory.

In some cases you will be able to access by doing:

#include <cstdint>

uintptr_t p = 0x0001FFFF;
int value = *reinterpret_cast<int *>(p);

Nothing guarantees that access will work as you expect. The result may be different depending on the situation.

I found this other reply from Guilherme Bernal that shows how to do something that works but again will not give consistent results:

#include <windows.h>
#include <stdio.h>

int main(void) {
    SYSTEM_INFO sysinfo;
    GetSystemInfo(&sysinfo);
    unsigned pageSize = sysinfo.dwPageSize;
    printf("page size: %d\n", pageSize);
    void* target = (void*)0x4e0f68;
    printf("trying to allocate exactly one page containing 0x%p...\n", target);
    void* ptr = VirtualAlloc(target, pageSize, MEM_COMMIT, PAGE_READWRITE);
    if (ptr) printf("got: 0x%p\n", ptr); //   ptr <= target < ptr+pageSize
    else printf("failed! OS wont let us use that address.\n");
}

I put in the Github for future reference.

Note that this situation is a little more controlled.

In C the void * means you’re using a pointer to anything.

Answer 2

Your operating system will not allow you to access memory that does not belong to your program.

#include <stdio.h>

int main(void) {
    unsigned char *mem = 0xdeadbeef; // ou = 3735928559;
    printf("O endereco %p tem %d\n", (void*)mem, *mem);
    return 0;
}