Variable Scope

Don’t confuse scope with lifetime. The scope of a declaration is a region of the program text; it is a compile-time property. The lifetime of a variable is the range of time during execution when the variable can be referred to by other parts of the program; it is a run-time property.

See also: Python Namespace, JavaScript Hoisting

Variables

Variables may be internal to a function, external but known only within a single source file, or visible to the entire program.

By default, external variables and functions have the property that all references to them by the same name, even from functions compiled separately, are references to the same thing. (The standard calls this property external linkage.)

Static

The static declaration, applied to an external variable or function, limits the scope of that object to the rest of the source file being compiled.

Lexical Scope

javascript - What is lexical scope? - Stack Overflow

• AKA static scope

void fun()
{
    int x = 5;
 
    void fun2()
    {
        printf("%d", x);
    }
}

• Contract: dynamic scope

void fun()
{
    printf("%d", x);
}
 
void dummy1()
{
    int x = 5;
    fun();
}
 
void dummy2()
{
    int x = 10;
    fun();
}

The first one is called static because it can be deduced at compile-time, and the second is called dynamic because the outer scope is dynamic and depends on the chain call of the functions.

Language Specific

C

Extern Keyword

How to correctly use the extern keyword in C - Stack Overflow

extern changes the linkage. With the keyword, the function / variable is assumed to be available somewhere else and the resolving is deferred to the linker.
There’s a difference between extern on functions and on variables.
For variables it doesn’t instantiate the variable itself, i.e. doesn’t allocate any memory. This needs to be done somewhere else. Thus it’s important if you want to import the variable from somewhere else.
For functions, this only tells the compiler that linkage is extern. As this is the default (you use the keyword static to indicate that a function is not bound using extern linkage) you don’t need to use it explicitly.

Point to the same variable, instead of a different copy.