C Academy · Lesson

extern and Global

Sharing across files.

Lesson 3 of 413 steps

extern and Global is a free C Academy lesson on CoddyKit. This is lesson 3 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.

Global Variables

A variable declared outside any function has file scope and lives for the whole program. It is a global.

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

Globals Are Shared

Every function in the file can read and write a global. This makes them convenient but easy to misuse.

#include <stdio.h>
int score = 100;
void penalty(void) { score -= 25; }
void bonus(void) { score += 10; }
int main(void) {
    penalty();
    bonus();
    printf("score = %d\n", score);
    return 0;
}

Default Zero Initialization

Globals without an initializer start at 0 automatically.

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

The extern Keyword

extern declares a global that is defined elsewhere. It tells the compiler the variable exists without allocating it.

#include <stdio.h>
int shared = 5;
extern int shared;
int main(void) {
    printf("shared = %d\n", shared);
    return 0;
}

Declaration vs Definition

A definition allocates storage. A declaration just names something. extern declarations do not allocate.

#include <stdio.h>
int value = 42;
int main(void) {
    extern int value;
    printf("value = %d\n", value);
    return 0;
}

Sharing Across Files

In multi-file projects, one .c file defines the global and other files use extern in a header to see it.

#include <stdio.h>
extern int globalCount;
int globalCount = 3;
int main(void) {
    printf("globalCount = %d\n", globalCount);
    return 0;
}

Functions Can Be extern Too

Functions have external linkage by default, so a prototype is effectively an extern declaration of that function.

#include <stdio.h>
extern void hello(void);
void hello(void) {
    printf("Hello from extern\n");
}
int main(void) {
    hello();
    return 0;
}

Global Lifetime

Globals exist from program start to finish, so their values persist across all function calls.

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

Risks of Globals

Because any code can change a global, bugs become hard to trace. Prefer passing values as parameters when possible.

#include <stdio.h>
int config = 1;
int compute(int input) {
    return input * config;
}
int main(void) {
    printf("%d\n", compute(8));
    return 0;
}

Read-Only Globals

Combine const with a global to make a shared constant that no one can modify.

#include <stdio.h>
const int MAX_USERS = 50;
int main(void) {
    printf("MAX_USERS = %d\n", MAX_USERS);
    return 0;
}

Practical Pattern

A global config value read by several functions is a common, acceptable use.

#include <stdio.h>
int multiplier = 3;
int scale(int x) { return x * multiplier; }
int main(void) {
    printf("%d\n", scale(4));
    printf("%d\n", scale(7));
    return 0;
}

Quick Check

Test your understanding of extern and globals.

Answer it in the interactive lesson

Recap: extern and Global

You learned about globals and external linkage.

Globals are file-scope variables with program lifetime
They default to 0 and are shared by all functions
extern declares a global defined elsewhere
Definition allocates, declaration only names
Use globals sparingly to avoid hidden coupling

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Frequently Asked Questions

Is the “extern and Global” lesson free?

Yes — the full text of “extern and Global” is free to read here on the web. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO. The C Academy course includes 4 lessons in total.

What will I learn in “extern and Global”?

Sharing across files. 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.

Do I need any experience to start C Academy?

No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 3 of 4.

How long does the “extern and Global” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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