Function to find whether a given string is a substring of another string

Following function finds whether a given string is a substring of another string. If a given string is a substring of another string it returns 1 which means success, otherwise it will return 0.

int substr(char* str1, char* str2)
{
                  int m, n, i, j;
                  m=n=0;

                  //Find the lengths of strings
                  while(str1[m])
                             ++m;
                  while(str2[n])
                              ++n;

                   for(i=0; i<m-n+1;++i)
                   {
                              for(j=0; j<n;++j)
                                         if(str2[j] != str1[i+j])
                                                       break;
                                
                               if( j == n )
                                         return 1;        //Substring exists
                     }
          
                      return 0;         //Substring does not exists
}