C code for all substrings of a string

#include <stdio.h>
#include <string.h>
#include <malloc.h>
 
char* substring(char*, int, int);
 
main() 
{
   char string[100], *pointer;
   int position = 1, length = 1, temp, string_length;
 
   printf("Enter a string\n");
   gets(string);
 
   temp = string_length = strlen(string);
 
   printf("Substring of \"%s\" are\n", string);
 
   while (position <= string_length)
   {
      while (length <= temp)
      {
         pointer = substring(string, position, length);
         printf("%s\n", pointer);
         free(pointer);
         length++;
      }
      temp--;
      position++;
      length = 1;
   }
 
   return 0;
}
 
/* Use substring function given in above c program*/