Static variable inside shared library

Question

My question is about static variable (static void*) created inside shared library (let's call this library 'S'), but it's an internal variable not shown outside, but every call of API depends on it. Now let's think about a case, when a program (let's call it main program) links to another two shared libraries and every one of them is linked with library S. Now what happens to this static variable for our main program? Does it have one instance? Two?

Answer 1

Suma's answer is correct. There will only be a single instance of the static variable. This is also why having static globals in shared libraries can be a huge problem. One real-world example where this can happen:

• Apache webserver which loads the following modules:
  • mod_php which is linked against
    • libxml2
  • mod_perl which loads
    • libxml2

Now if some PHP code modifies a global setting like a parser option in libxml2, the Perl code will also see these changes. This can lead to bugs that are extremely hard to diagnose. So you should avoid global state in your shared libraries at all cost.

(With libxml2 you can make most settings locally these days.)