Scope

Scope determines where a variable can be accessed. Understanding scope helps you reason about variables inside and outside functions.

Local variables

Variables defined inside a function are local variables. They can only be accessed inside that function.

#include <stdio.h>

void swap(int a, int b) {
    int temporary = a;
    a = b;
    b = temporary;
    printf("inside swap(): %d %d\n", a, b);
}

int main(void) {
    int a = 1;
    int b = 2;
    printf("Before: %d %d\n", a, b);
    swap(a, b);
    printf("After: %d %d\n", a, b);
    return 0;
}

Output:

Before: 1 2
inside swap(): 2 1
After: 1 2

File-scope variables and static

A variable declared outside every function has file scope. Adding static keeps that name private to the current source file while allowing functions in that file to share its value.

A static variable has one value for the duration of the program. After one static function changes it, another static function reads the updated value.

#include <stdio.h>

static int score = 0;

static void add_score(int points) {
    score += points;
}

static int get_score(void) {
    return score;
}

int main(void) {
    add_score(10);
    add_score(5);

    printf("%d\n", get_score());

    return 0;
}

Output:

15

Use a file-scope static variable only for state that genuinely belongs to the module. Prefer focused functions for updates instead of assigning to it from arbitrary locations.