What happens to static variables when libraries are statically linked

Question

Let's say I have library (A) implementing the singleton pattern (it has a static variable in its implementation).

(A) library is compiled as a static library.

Now, let's say I have in my probject:

• (B), another static library linking statically with (A).
• (C), another static library linking statically with (A).
• (D), a top level program linking with (B) and (C).

In the end, is my singleton really a singleton (and my variable really static)? Are (B) and (C) seing the same static variable from (A) (is it unic)? Or does the fact that (A) was statically linked twice embedded (A)'s code twice ending up with my static variable from (A) appearing twice in the final binary code? Then if (B) modifies the static variable value, (C) would not see the change?

Note: I experienced that when changing the libraries of project to be linked statically instead of dynamically. I'm just wondering if I did something wrong, or if that's a normal known behaviour.

Answer 1

static linked library, and static variables are not related.

A static variable, is not visible outside the current compilation scope (no symbol name is created in the .o file, so no other .o files can find that variable via the symbol name).

This means that the variable

static int foo; can exist in every compilation scope, and each one will be uniqe

Answer 2

First of all:

(B) and (C) do NOT link against (A). Static libs are compiled, not linked. When building (B) and (C) the compiler might need to see certain definitions from (A) but do not confuse this with linking. (A's) code is not copied into (B) or (C).

Secondly:

(D) will have to link against (A), (B) and (C). That means you get only one copy of (A's) code in (D).

Dynamic-link Library/Shared Object:

This of course would be different if (B) and (C) were dlls/sos instead. Dlls are linked and so if you build (B) and (C) as dlls and link them against (A) then you would have a separate copy of (A's) code in both (B) and (C).

Are (B) and (C) seing the same static variable from (A)

This depends on if your variable has external or internal linkage. The following header file contains a static int variable with interal linkage. This means that every translation unit that includes this file will get it's own copy of myVariable.

//MyHeader.h
#pragma once
static int myVariable = 0;