C++ programs to extract a string from given string without using inbuilt function.

/*-------------------------------------------------------------------------------------------------------------------------------
Author: Sujith
Program: Write C++ programs to extract a string from given string without using inbuilt function.
Prog Name: 1substr.cpp
Date: 21-04-2015
---------------------------------------------------------------------------------------------------------------------------------*/
#include<stdio.h>
#include<iostream>
#include<string.h>
using namespace std;
void stringsub(char s1[],int i,int j,char s2[]);
int main()
{
     char str1[20],str2[20];
      int i,j,len;
      cout<<"Enter the string";
      cin>>str1;
      len=strlen(str1);
      cout<<"Enter the position of the string";
      cin>>i;
        if(i>len)
        cout<<"Invalid poistion\n";
       else
      {
          cout<<"Enter the number of character";
          cin>>j;
          stringsub(str1,i,j,str2);
          cout<<str2;
      }
}
void stringsub(char s1[],int i,int j,char s2[])
{
   int k,l;
   l=i;
    for(k=0;k<j;k++)
    {
       s2[k]=s1[l];
       l++;
     }
    s2[k]='\0';
}
Output:
"1substr.cpp" [New] 37L, 740C written
[141740@localhost C++]$ g++ substr.cpp
[141740@localhost C++]$ ./a.out
Enter the string
commonsense
Enter the position of the string
6
Enter the number of character
5
sense
[141740@localhost C++]$