C Functions Explained Visually: Calls, Parameters and Returns

Start with working C code

The call add(3, 4) passes 3 and 4 by value; the function returns 7, which main stores in result.

#include <stdio.h>
int add(int a, int b) {
    return a + b;
}
int main(void) {
    int result = add(3, 4);
    printf("%d\n", result);
    return 0;
}

What happens during a function call?

When main calls add(3, 4), execution enters add. Its parameter variables a and b receive the values 3 and 4. The expression a + b evaluates to 7, then return sends the value to the caller.

The diagram is a conceptual call-stack trace. Exact compiler optimizations and physical stack layout may differ, but the sequence of evaluation and return value is the important concept.

Pass by value in C

Ordinary function parameters are initialized with copies of argument values. Changing a inside add does not change a separate variable in main. To modify an existing object owned by the caller, a function can receive a pointer to that object and dereference it safely.

The int before add declares its return type. The parameter types are written inside parentheses. A function declaration or definition must be visible before the function is called.

Exercises

1. Predict the output of printf("%d", add(5, 2));.
2. What values do a and b receive for add(9, 1)?
3. Explain what happens to the call's parameters after return.