Local Variables

Inside a function, you can create variables, just like in main. But such a variable exists only inside its function: it is created when the function runs, and it disappears when the function ends. Outside the function, the compiler doesn't know it:

#include <stdio.h>

void print_total(void) {
    int total = 5 + 3;
    printf("%d\n", total);
}

int main(void) {
    print_total();
    printf("%d\n", total);
    return 0;
}

The program doesn't compile:

main.c:10:20: error: use of undeclared identifier 'total'
   10 |     printf("%d\n", total);
      |                    ^~~~~
1 error generated.

On line 10, main tries to print the variable total, which exists only inside the function print_total. Use of undeclared identifier means that the program uses a name that the compiler doesn't know in this place. Parameters are the same: they exist only inside their function.

A variable created inside a function is local: it belongs only to that function. The part of a program where a variable exists is called its scope. You already know that a variable created in a block, such as the i of a for loop, exists only up to the } of that block. The body of a function is a block too. And it applies to main as well, because main is a function: other functions can't see its variables. When a function needs a value from main, pass it as an argument.

Why is it good that local variables stay inside their functions?

• Functions don't get in each other's way. Two functions can each have a variable named total, and those are two different variables: changing one doesn't change the other.
• To use a function, you only need to know what to give it (the arguments) and what it gives back (the return value). How it works inside is its own business.
• Every call starts afresh: the variables of the function are created again, with nothing left over from the previous call.

A parameter is a local variable too. When a function is called, its parameter gets a copy of the argument. When the function changes its parameter, it changes only the copy, and the variable in main stays the same:

#include <stdio.h>

void reset_score(int score) {
    score = 0;
}

int main(void) {
    int score = 10;
    reset_score(score);
    printf("%d\n", score);
    return 0;
}

This program prints:

10

The function set its parameter score to 0. But the variable score in main is a different variable that only has the same name, and it stayed 10.

So the rule is: values go into a function through its parameters and come out with return. When you want a function to give you a new value, return it, and store it where you call the function, for example score = add_bonus(score, 10);. In the next lesson, on pointers, you will learn another way: how a function can change a variable of the code that called it.