Write a program using a recursive call to get a natural number n and express it as a sum of natural numbers less than n

#include <stdio.h>

void f(int* a, int s, int n, int idx){
    int i;
    if(s==0){
        for(i=0; i<idx-1; i++)
            printf("%d+", a[i]);
        printf("%d\n", a[i]);
        return;
    }
    for(int i=1; i<n; i++)
        if(s-i>=0){
            a[idx]=i;
            f(a, s-i, n, idx+1);
        }
}

int main(){
    int n, *a;
    scanf("%d", &n);
    a[0]=0;
    f(a, n, n, 0);
}