How are global variables in shared libraries linked?

Question

Suppose I have shared library with this function where "i" is some global variable.

int foo() {
return i++;
}

When I call this function from multiple processes the value of "i" in each process is independent on the other processes.

This behavior is quite expected.

I was just wondering how is usually this behavior implemented by the linker? From my understanding the code is shared between processes, so the variable has to have the same virtual address in all address spaces of every program that uses this library. That condition seems quite difficult to accomplish to me so I guess I am missing something here and it is done differently.

Can I get some more detailed info on this subject?

Answer 1

The dynamic linking process at run time (much the same as the static linking process), allocates separate data (and bss) segments for each process, and maps those into the process address space. Only the text segments are shared between processes. This way, each process gets its own copy of static data.

Answer 2

the code is shared between processes, so the variable has to have the same virtual address in all address spaces of every program that uses this library

The code is not shared the way you think. Yes the dynamic shared object is loaded only once but the memory references or the stack or the heap that code in the so uses is not shared. Only the section that contains the code is shared.

Answer 3

Each process has it's own unique address space, so when a process access the variable it can have different values then the other process. If the process should share the same memory, they would have to specifically set this up. A shared library is not enough for that.