C++, how to access with a pointer to a specific address - c++

Question

I was curious, so I tried to do this:
First program:
#include <iostream>
using namespace std;
int a = 5;
int main()
{
cout << "Value of a: " << a << endl;
cout << "Address of a: " << &a << endl;
system("pause");
return 0;
}
Output:
Value of a: 5
Address of a: 0x472010
Second program:
#include <iostream>
using namespace std;
int* p = reinterpret_cast<int*>(0x472010);
int main()
{
cout << "Value of p: " << *p << endl;
cout << "The address that the pointer points to: " << p << endl;
cout << endl;
system("pause");
return 0;
}
So, I want to read the value of the variable 'a' with
the 'p' pointer belonging to the another program, and the 'p' pointer point to specific address.
everything is fine until i run the second pragram, as the second program doesn't give the desired results
Output second program:
Value of p: 4661264
The address that the pointer points to: 0x472010
The result does not change if I keep the window of the first program open.
I promise I’m a beginner and I’m trying new things
What am I doing wrong?

Answer 1

You can't directly access another process's memory in the manner you are attempting. Each process runs in its own address space. Your 2nd process is trying to access an (invalid) address within its own address space, not within the address space of the 1st process.
On Windows, to read another process's memory, your 2nd process must obtain a HANDLE to the 1st process, such as from OpenProcess(), and then must use ReadProcessMemory() to read memory from an address within the 1st process.
The alternative is for the 1st process to allocate a block of shared memory which the 2nd process can access directly. On Windows, you can use CreateFileMapping() and MapViewOfFile() for that purpose (see Creating Named Shared Memory on MSDN). On 'Nix systems, you can use shm_open() mmap().