C Academy · Lesson

static Variables

Persistent locals.

Lesson 2 of 413 steps

static Variables is a free C Academy lesson on CoddyKit. This is lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, and your progress syncs across the web and the CoddyKit app. The C Academy course includes 4 lessons in total.

The static Keyword

A static local variable keeps its value between calls to the function. Its lifetime is the whole program.

#include <stdio.h>
void f(void) {
    static int count = 0;
    count++;
    printf("count = %d\n", count);
}
int main(void) {
    f();
    f();
    f();
    return 0;
}

Initialized Once

A static variable is initialized only the first time the function runs. Later calls skip the initializer.

#include <stdio.h>
void f(void) {
    static int x = 100;
    printf("x = %d\n", x);
    x += 10;
}
int main(void) {
    f();
    f();
    return 0;
}

Default Zero

Unlike auto variables, a static variable without an initializer starts at 0.

#include <stdio.h>
void f(void) {
    static int n;
    printf("n = %d\n", n);
    n++;
}
int main(void) {
    f();
    f();
    return 0;
}

Counting Calls

A classic use is counting how many times a function has been invoked.

#include <stdio.h>
int nextId(void) {
    static int id = 0;
    return ++id;
}
int main(void) {
    printf("%d %d %d\n", nextId(), nextId(), nextId());
    return 0;
}

Static Lifetime, Local Scope

A static local still has local scope. Only the function can see it, but it persists across calls.

#include <stdio.h>
void tick(void) {
    static int t = 0;
    t++;
    printf("tick %d\n", t);
}
int main(void) {
    tick();
    tick();
    return 0;
}

Static vs auto

Compare: an auto counter resets every call, while a static one accumulates.

#include <stdio.h>
void autoCounter(void) {
    int c = 0;
    c++;
    printf("auto: %d\n", c);
}
int main(void) {
    autoCounter();
    autoCounter();
    return 0;
}

Static at File Scope

At file scope, static means internal linkage: the variable is private to its source file.

#include <stdio.h>
static int secret = 7;
int main(void) {
    printf("secret = %d\n", secret);
    return 0;
}

Caching With Static

Static variables can cache a computed value so it is calculated only once.

#include <stdio.h>
int getBase(void) {
    static int base = 0;
    if (base == 0) {
        base = 50;
        printf("computed once\n");
    }
    return base;
}
int main(void) {
    printf("%d\n", getBase());
    printf("%d\n", getBase());
    return 0;
}

Accumulating State

Static locals let a function remember a running total without a global.

#include <stdio.h>
int addToTotal(int v) {
    static int total = 0;
    total += v;
    return total;
}
int main(void) {
    printf("%d\n", addToTotal(10));
    printf("%d\n", addToTotal(5));
    return 0;
}

A Caution

Static state makes a function depend on history, which can be harder to test. Use it deliberately.

#include <stdio.h>
int stepNumber(void) {
    static int s = 0;
    return ++s;
}
int main(void) {
    printf("step %d\n", stepNumber());
    printf("step %d\n", stepNumber());
    return 0;
}

Combining With Loops

A static local inside a loop body keeps its value across both iterations and calls.

#include <stdio.h>
void run(void) {
    static int seen = 0;
    for (int i = 0; i < 2; i++)
        seen++;
    printf("seen = %d\n", seen);
}
int main(void) {
    run();
    run();
    return 0;
}

Quick Check

Test your understanding of static variables.

Answer it in the interactive lesson

Recap: static Variables

You learned about persistent locals.

static locals live for the whole program
They initialize once and default to 0
They keep local scope but persist values
At file scope static gives internal linkage
Great for counters and caches

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Frequently Asked Questions

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Persistent locals. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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