C linkage

An identifier declared more than once can be made to refer to the same object or function by a process called linkage. ¹

There are three kinds of linkage:

• no linkage: each declaration denotes a unique entity
• external linkage: each declaration denotes the same object or function entity
• internal linkage: similar to external linkage but limited to a single translation unit

Linkage of Variables

A declaration’s linkage is determined by two factors: the presence of a storage-class specifier (static or extern), if any, and whether it is declared at block or file scope. By default, global variable and function declarations have external linkage, while local variable declarations have no linkage.

At file scope, the static specifier gives an identifier internal linkage. However, at local scope, it affects only the storage duration rather than the linkage. We can declare a static function at local scope because a function does not have storage duration.

If an identifier is declared with extern while a prior declaration with internal or external linkage is visible, it inherits the same linkage. If no prior declaration is visible or the prior declaration had no linkage, the extern declaration has external linkage. For example: ²

static int a; // internal linkage
extern int a; // inherited internal linkage
 
int main(void) {
  extern int b; // external linkage
  return b;
}
int b = 5; // external linkage

The extern keyword can be used to reintroduce a variable with external linkage into scope if it is shadowed by a local variable.

int a = 4;
 
int main(void) {
    int a = 3;
    {
        extern int a; // refer to the global variable a rather than the local variable
        return a;
    }
}

Reference

Footnotes

1. C23 Standard Draft 6.2.2 ↩

2. Writing a C Compiler: Build a Real Programming Language From Scratch Ch 10 ↩