#include <stdlib.h>
#include <string.h>

void reverse_string (char *instr)
{
   int len, i;
   char temp;

   if (!instr) return;

   len = strlen (instr) - 1;

   for (i = 0; i < len/2; i++)
      {
          temp = instr[i];
          instr[i] = instr[len - i];
          instr[len - i] = temp;
      }
}

void reverse_words (char *instr)
{
   int pos=0, start, end;
   char temp;

   if (!instr) return;

   while (instr[pos] != '\0')
      {  
         for (start = pos; instr[pos] != ' ' && instr[pos] != '\0'; pos++);
         
         end = pos;
   
         for (pos = start; pos < ((end-start)/2)+start; pos++)
            {
               temp = instr[pos];
               instr[pos] = instr[end-1-pos+start];
               instr[end-1-pos+start] = temp;
            }

         if (instr[end] == '\0')
            break;

         pos = end + 1;         
      }
}

int main()
{
   char mystring[100];

   strcpy (mystring, "Here is the string I would like to reverse.");
   printf ("%s\n", mystring);

   reverse_string (mystring);
   printf ("%s\n", mystring);

   reverse_string (mystring);
   printf ("%s\n", mystring);

   reverse_words (mystring);
   printf ("%s\n", mystring);

   reverse_words (mystring);
   printf ("%s\n", mystring);

   reverse_words (mystring);
   reverse_string (mystring);
   
   printf ("%s\n", mystring);

   return 0;
}

/* result:
Here is the string I would like to reverse.
.esrever ot ekil dluow I gnirts eht si ereH
Here is the string I would like to reverse.
ereH si eht gnirts I dluow ekil ot .esrever
Here is the string I would like to reverse.
reverse. to like would I string the is Here
*/