Lesson 14 · 14 of 20

Scope and static variables · C

Understand where each variable lives in C: globals, locals, block variables and static ones, which keep their value between calls to a function.

C · Scope and static variables in DMarlon Studio: the example's code and its output in the terminal

A variable's scope is the part of the program where it exists. total_visits sits outside every function: it is global, and every function after it can read and change it. Use globals sparingly, since anything can modify them.

Variables declared inside a function are local: they are born on each call and vanish when it ends. That is why local is always 1: each time it starts again at 0 and gets one added.

With `static`, a local variable is created only once and keeps its value between calls, although it can still only be seen inside its function. calls counts 1, 2, 3, and next_ticket hands out consecutive numbers from 101. ++number adds one and gives back the new value straight away.

Every { } block has its own scope. The lives inside the block is a different variable that hides the outer one while the block lasts; once outside, the outer one is visible again, still 3. In the same way, the i from the for no longer exists after the loop.

Code · main.c

#include <stdio.h>int total_visits = 0;  // global: every function below can use itvoid visit(const char page[]) {    static int calls = 0;  // static: keeps its value between calls    int local = 0;         // a normal local starts again on every call    calls++;    local++;    total_visits++;    printf("%-5s calls=%d local=%d\n", page, calls, local);}int next_ticket(void) {    static int number = 100;  // set to 100 only the first time    return ++number;          // add 1, then give the new value}int main(void) {    visit("home");    visit("shop");    visit("cart");    printf("Total visits: %d\n", total_visits);    for (int i = 0; i < 3; i++) printf("Ticket %d\n", next_ticket());    int lives = 3;    {   // a block has its own scope: this lives hides the outer one        int lives = 99;        printf("inside the block: lives = %d\n", lives);    }    printf("outside the block: lives = %d\n", lives);    return 0;}

Output · cc main.c -o main && ./main

home  calls=1 local=1
shop  calls=2 local=1
cart  calls=3 local=1
Total visits: 3
Ticket 101
Ticket 102
Ticket 103
inside the block: lives = 99
outside the block: lives = 3

Key points

A global variable is visible in the whole file.
A local variable is born and dies with each call.
static keeps the value between calls.
A { } block has its own scope.
An inner variable hides an outer one with the same name.

Exercise

Write a void log_message(const char text[]) function that numbers its messages with a static variable («1: start», «2: load»…) and call it four times.