Question on permutation using recursion

#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>


string subS(string fullS, int s,int len);
string permut(string s ,int n);
void swap(string s,int a,int b);

int main (int argc, string argv[])
{

    if (argc!=2)
    printf("usage: ./permutation 5");

    int n = strlen(argv[1]);
    permut(argv[1],n);

    // print permutations of string s , of length n


}
  string subS(string fullS, int s,int len)

   {
      string sub = fullS;

       for(int j=0;j<len;j++)
       {
           sub[j]=fullS[s];
           s=s+1;
       }
        sub[len]='\0';
       return sub;
   }


    string permut(string s ,int n)
    {
        if (s!=NULL)
        {


            if(n==1)
                return s;
            else
            {
                string dummy_s=s;// dummy variable so as not to pass the string
                

                string sub=subS(dummy_s,1,n-1);// get the substring of s starting at s[1] and of length n-1
                
                string smaller=permut(sub,n-1);// permute the substring
                
                string final = strncat(smaller,s,1); // append the first letter of s to the permuted string.will be using it to print the permutations of
                 
                

//generating the permutations by taking s[0] and appending it to "smaller".This is done using a string "final" and swapping s[0] with its left neigbour and printing the each of these strings 
                for (int i=n-1;i>0;i--)
                {
                    printf("%s\n",final);
                    swap(final,i,i-1);
                }
                
            }
        }
        return s;

    }

    void swap(string s,int a,int b)
    {
        if (a<=b)
            return;
        else
        {
            char c= s[a];
            s[a]=s[b];
            s[b]=c;

        }
    }