Direct and indirect recursion

Direct recursion

A function calls itself directly in its own body.

int fact(int n) {
    if (n == 0) return 1;
    return n * fact(n - 1);     /* fact calls fact */
}

Call chain: fact → fact → fact ...

Indirect recursion

A function calls another function, which in turn calls the first one again, forming a cycle (A → B → A). The self-call happens through other functions.

void funB(int n);
void funA(int n) {
    if (n > 0) { printf("%d ", n); funB(n - 1); }
}
void funB(int n) {
    if (n > 1) { printf("%d ", n); funA(n / 2); }
}

Call chain: funA → funB → funA → ... For funA(20) the output is 20 19 9 8 4 3 1.