Can I access random data with random memory Addresses outside of my C++ Program

Question

If 2 programs are running, and one program stores a number at a memory address, and if I know that memory address, and hard code it into the 2nd program and print out the va

Answer 1

No, you'd get a Segmentation Fault

If I try to run this code:

int main(int argc, char *argv[]) {
    int *ptr = (int*) 0x1234;
    *ptr = 10;
}

I'd get a segmentation fault (unless 0x1234 has been allocated by the process for some reason), which is the operating system's way of telling you that you're not allowed to do that. Usually they'll happen when you're doing tricky things with pointers, but they can also happen elsewhere.

By default, they'll terminate your program immediately unless you're running in a debugger or have registered a signal handler to continue your program

Edit: If you really want, there's ways to get the operating system to let you do that, used by debuggers and such.

Answer 2

On system with no virtual memory management and no address space protection this would work. It would be undefined behavior from the point of view of the C standard, but it would produce the behavior that you expect.

Bad news is that most computer systems in use these days have both virtual memory management and address space protection. What this means is that a memory address, the number that your program sees, is not unique in the system. Every process in the system may see the same address, but it would be mapped to a different physical address on your computer at any given moment in time. The operating system and the hardware will create illusion to each process that it has the control of that memory address, while in fact the memory spaces of the processes would not overlap.

Good news is that modern operating systems support some form of shared memory access, meaning that one process can share a segment of memory with other processes, and exchange data by reading and writing the data into that shared segment.