shared library address space

Question

While I was studying about shared library I read a statement

Although the code of a shared library is shared among multiple processes, its variables are not. Each process that uses the library has its own copies of the global and static variables that are defined within the library.

I just have few doubts.

1. Whether code part of each process are in separate address space?
2. Whether shared-library code part are in some some global(unique) address space.

I am just a starter so please help me understand.

Thanks!

Answer 1

Shared libraries are loaded into a process by memory-mapping the file into some portion of the process's address-space. When multiple processes load the same library, the OS simply lets them share the same physical RAM.

Portions of the library that can be modified, such as static globals, are generally loaded in copy-on-write mode, so that when a write is attempted, a page fault occurs, the kernel responds by copying the affected page to another physical page of RAM (for that process only), the mapping redirected to the new page, and then finally the write operation completes.

To answer your specific points:

1. All processes have their own address space. The sharing of physical memory between processes is invisible to each process (unless they do so deliberately via a shared memory API).
2. All data and code live in physical RAM, which is a kind of address-space. Most of the addresses you are likely see, however, are virtual memory addresses belonging to the address-space of one process or another, even if that "process" is the kernel.