Function calls and the stack

When main calls add_tax, a new stack frame appears on top with the function's own variables. The argument price is a copy of cost: changing it inside the function doesn't touch cost in main. That's pass by value.

When the function returns, its frame disappears and the result lands in total.

#include <stdio.h>

double add_tax(double price) {
    double tax = price * 0.1;
    price = price + tax;
    return price;
}

int main(void) {
    double cost = 20.0;
    double total = add_tax(cost);
    printf("cost %.2f, total %.2f\n", cost, total);
    return 0;
}

Output:

cost 20.00, total 22.00

From the lesson: Functions