C · 1 · Foundations4 / 35 · 11%

Storage Classes & Scope

auto, static, extern, register — who owns a variable, and for how long.

Examples: static, extern, const, scope rules

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Automatic Storage

Locals live on the stack and die when the block exits.

Example

example

void f(void) {
    int x = 1;      // automatic: created on entry
    {
        int y = 2;  // inner block scope
    }               // y destroyed here
}                   // x destroyed here

one frame per call

static — Lifetime vs Visibility

static means two different things depending on where you write it.

Example

example

static int counter = 0;   // file scope: private to this .c file

void tick(void) {
    static int calls = 0; // block scope: persists across calls
    calls++;
}

At file scope static = internal linkage. Inside a function static = permanent lifetime.

extern & Headers

extern declares a symbol defined in another translation unit.

Example

example

// config.h
extern int g_verbose;   // declaration only

// config.c
int g_verbose = 0;      // the single definition

WATCH OUT

Defining a global in a header (without extern) causes duplicate-symbol link errors.

const & Shadowing Pitfalls

Constants and the scoping traps that hide bugs.

Example

example

const double PI = 3.14159;   // read-only object
int x = 1;
void f(void) {
    int x = 2;   // shadows the global — the global is unreachable here
}

Common pitfalls

✕Shadowing a global with a local of the same name — compile with -Wshadow.
✕Assuming a static local resets between calls. It does not.
✕Using const on a pointer without deciding which side it protects.