Difference between 'global' and 'static global'

Question

A global variable's scope is in all the files, while a static global variable's scope is just the file where it is declared. Why so?

Where are global or static global variables stored in memory?

Answer 1

There is some confusion, since static in C can mean two different things. One is static storage duration, and the other is internal linkage. static used as a keyword on file-scope will give the function or object it is used with internal linkage.

Internal linkage for a function or object means that if you declare another function in another "file" (this is not really called "file", but rather translation unit - TU), then that declaration will refer to a different function: The name declared in that unit will "link" to a different entity than the name declared in that other translation unit, which was "internal" to that TU. The same applies to objects.

Whether or not a file-scope variable is declared with static, it will still have a static storage duration: That means it lives through the whole program, and dies when the program terminates. Another example of an object that has static storage duration is a string literal. Where objects that have static storage duration are stored isn't specified, but they are usually stored depending on whether they are initialized or not: Initialized file-scope variables are usually stored in a section called ".data", while non-initialized file-scope variables are usually stored in a section called ".bss". Remember that if the variable isn't initialized, it will be zero initialized at the start of the program: The ".bss" section is usually zero initialized by an implementation on program's startup.

I said "usually" everywhere, since where things are stored isn't specified. For example, some implementations could store string literals in a read-only section. And if you have a file-scope pointer and don't initialize it, the implementation initializes it to a null-pointer, which is not necessarily an object with all null bytes :)

Answer 2

They're both stored in the data segment; the difference is that the global has an externally-visible linker symbol, and the static global does not.