C++ program that returns the position for a substring

This is a C++ program that accepts string from user and search for a sub string (group of continuous characters). If the search is successful, the function should return the position of sub string and otherwise -1.
The program coding can be given as:

#include 
#include 
#include 
// Function to search a given string within a string and its position
int search(char str[20], char substr[10])
{
 int j = 0, i, flag = 0, pos;
 for(i=0; str[i]!='\0'; i++)
 {
  if (substr[j] =='\0')
   break;
  if (str[i] == substr[j] )
  {
   pos = i;
   flag = 1;
   if (substr[j] =='\0')
    break;
   j++;
  }
  else
   flag = 0;
 }
 for(j=0; substr[j]!='\0'; j++);
  if (flag == 1)
  {
   pos = (pos - j + 2);
    return(pos);
  }
  else
   return(-1);
}
void main()
{
 char nstr[20], nsub[10];
 int ctr = 0;
 cout << "Enter main string : ";
 gets(nstr);
 cout << "Enter search string : ";
 gets(nsub);
 ctr = search(nstr, nsub);
 if (ctr == -1)
  cout << "String does not found.";
 else
  cout << "String found at " << ctr << " position.";

}

Source:  www.LearnCandCpp.com