Way #1

#include<stdio.h>

int string_length(char*);
void reverse(char*);

main()
{
   char string[100];

   printf("Enter a string\n");
   gets(string);

   reverse(string);

   printf("Reverse of entered string is \"%s\".\n", string);

   return 0;
}

void reverse(char *string)
{
   int length, c;
   char *begin, *end, temp;

   length = string_length(string);
   begin  = string;
   end    = string;

   for (c = 0; c < length - 1; c++)
      end++;

   for (c = 0; c < length/2; c++)
   {
      temp   = *end;
      *end   = *begin;
      *begin = temp;

      begin++;
      end--;
   }
}

int string_length(char *pointer)
{
   int c = 0;

   while( *(pointer + c) != '\0' )
      c++;

   return c;
}