In virtual memory, can two different processes have the same address?

Question

This is an interview question I found in a website, the questions says: "In virtual memory, can two different processes have the same address? When you answer "No" which is correct, how one process can access another process' memory, for example the debugger can access the variables and change them while debugging?"

What I understand is :

1. 2 diff process can have same virtual memory address. This is because each process has its own page table. Each process thinks it as 4Gb memory on a 32-bit machine. So both P1 and P2 can access address 0xabcdef - but the physical memory location might be different. Isnt this right ?

2. The debugger works on the same principle - 2 processes can access the same address. So it can modify variables etc on the fly.

Answer 1

Yes, it's definitely possible for the same address to map to different physical memory depending on the process that's referencing it. This is in fact the case under Windows.

Answer 2

Sometimes I feel like the "elder" in the Minolta commercial... In the 1960's Multics was created using Virtual Memory. The last Multics system was shut down October 30, 2000 at 17:08Z.

In Multics, only one copy of any program was running, regardless of how many users were using it. So that means that it had both the same physical and virtual address.

When I look at the Windows Task Manager and see multiple copies of a program (e.g. svchost.exe) I wonder why / how the revolutionary concepts in Multics were lost.

Answer 3

1)

• Same physical memory address at the same time: NO
• Same virtual memory address at the same time: YES (each one maps to differnet physical address, or swap space)

2) I think the debuggers don't access directly the other process debugged but communicates with the runtime in the debugged process to do that changes.

That said, maybe the OS or processor instructions provide access/modify to other's memory access if you have the right. That doesn't mean it has the SAME address, it only says process 1 can say "access memory @address1 in Process2". Someone (processor / OS / runtime) will do that for process 1.

Answer 4

Each process has a address space of 4GB in a 32 bit system. Where is this real 4GB is managed by the OS. So in principle 2 different process can have same addresses that is local to the process.

Now when one process has to read the memory of another process it has to either communicate with the other process (memory mapped files etc.,) or use the Debug apis like OpenProcess/ReadProcessMemory.

What I am sure is one process cannot directly go and read the virtual memory of other process atleast in Win32 without the help of the OS.

Answer 5

Theoretically every process executed by user in any present popular OSes(Win,linux,unix,Sol etc) are initially allowed to use the address range of 4gig ( 0x00000000 t0 0xffffffff on 32 bit platform),whether its a simple hello world program or its complex web container hosting stackoverflow site.It means every process has its range starting from the same start address and ending with the same address space VIRTUALLY. So obviously every process has that same virtual addresses in their respective virtual address space range. So answer for your first question is YES.

Difference comes when OS execute any process, modern OSes are multitasking OS and they run more then on process at any point of time.So accommodating 4gig of every process in the main memory is not feasible at all. So OSes using paging system,in which they divide the virtual address range (0x00000000 to 0xffffffff) into a page of 4k size(not always). So before starting the process it actually load the required pages which needed at the initial time to the main memory and then load the another virtual page ranges as required. So loading of virtual memory to physical memory (main memory) is called memory mapping. In this process you map the page's virtual address range to physical address range( like ox00000000 to ox00001000 virtaul address range to 0x00300000 to 0x00301000 physical address range)based on the slot free in the main memory.So at any point of time only one virtual address range will be mapped to that particular physical address range,so answer for your second question is NO.

BUT

Shared Memory concept is an exception where all the process can share some of their virtual address range with each other,that will be mapped to a common physical address space.So in this case answer can be YES.

As an example on Linux every executable require libc.so library to execute the program executable.Every process load their required libraries and allocate them some virtual address page ranges in their address space. So now consider a scenario where you are executing 100's of process where each process require this library libc.so. So if OS allocate virtual address space in every process for this library libc.so,then you can imagine the level of duplication for library libc.so & its highly possible that at any point of time you will get multiple instance of libc.so address range pages in the main memory.So to make is redundant OS will load libc.so to specific virtual address space range of every process which is mapped to a fixed physical address range in main memory.So every process will refer to that fixed physical address range to execute any code in libc.so. So in this case every process share some physical address ranges as well.

But there is no chance of two process has same physical address at the same time in the user malloced virtual address range mapping.

Hope it helps.