#include <stdio.h>
#include <ctype.h>
#include <string.h>
#define MAX_LEN 80
#define MAX_WORDS 10

int get_words (char *line, char tokens[MAX_WORDS][MAX_LEN+1]);
void replace_whitespace (char *str);

int main(void)
{ 
  int i;
  char *str, tokens[MAX_WORDS][MAX_LEN+1];

  printf("Enter a sentence: ");
  gets(str);

  get_words(str, tokens);

  printf("Reversal  of sentence: ");

  for (i = strlen(str); i > 0; i--)
    printf("%s ", str[i]);

return 0;
}

/* store the words from string line in the array 
         tokens, one word per entry. */

int get_words (char *line, char tokens[MAX_WORDS][MAX_LEN+1])
{
  int i, char_count = 0;
  char *p = line;
  int len;

  replace_whitespace(p);

  len = strlen(p);
  
  while (*p == '\0' && *p < len) 
  {
     p++;
     char_count++;
  }
  
  while (*p != '\0')
  {  
     strcpy(tokens[i], p);
     p += strlen(p);
     while(*p == '\0')
        p++;  
     i++;
  }


}

/* replaces every whitespace character in str with the null 
   character; uses the function isspace(char c) from ctype.h */

void replace_whitespace (char *str)  
{  
   int i;

   for (i = 0; i < strlen(str); i++)
     if (isspace(str[i]))
       str[i] = '\0';
        
}

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#define MAX_LEN 80
#define MAX_WORDS 10

int get_words (char *line, char tokens[MAX_WORDS][MAX_LEN+1]);
void replace_whitespace (char *str);

int main(void)
{ 
  int i;
  char *str, "str" doesn't point anywhere useful.
          tokens[MAX_WORDS][MAX_LEN+1];

  printf("Enter a sentence: ");
  gets(str);  gets() is dangerous, due to buffer overruns -- see the FAQ, use fgets() instead. See above, "str" does not point anywhere. 

  get_words(str, tokens);

  printf("Reversal  of sentence: ");

  for (i = strlen(str); i > 0; i--)
    printf("%s ", str[i]);

return 0;
}

/* store the words from string line in the array 
         tokens, one word per entry. */

int get_words (char *line, char tokens[MAX_WORDS][MAX_LEN+1])
{
  int i, char_count = 0;
  char *p = line;
  int len;

  replace_whitespace(p);  This is incorrect logic, how do you know where the last whitespace character was replaced?

  len = strlen(p);  This will give you the length of the first part (upto the first whitespace character). 
  
  while (*p == '\0' && *p < len) 
  {
     p++;
     char_count++;
  }
  
  while (*p != '\0')
  {  
     strcpy(tokens[i], p);
     p += strlen(p);
     while(*p == '\0')
        p++;  
     i++;
  }


}

/* replaces every whitespace character in str with the null 
   character; uses the function isspace(char c) from ctype.h */

void replace_whitespace (char *str)  
{  
   int i;

   By replacing whitespace with null terminators, you're changing the length of the string, thus this loop is useless. It will halt on the first whitespace character.
   for (i = 0; i < strlen(str); i++)
     if (isspace(str[i]))
       str[i] = '\0';
        
}

zac@neux:code (1) $ gcc rev.c -o rev
zac@neux:code (1) $ ./rev 
Token 1: This
Token 2: is
Token 3: an
Token 4: example
zac@neux:code (1) $ cat rev.c 
#include <stdio.h>

#define MAX_WORDS 4

int main(void)
{
   char str[] = "This is an example";
   char * tokens[MAX_WORDS] = {NULL};
   size_t i = 0;

   size_t numberOfWords = 4;        /* obviously you'd have to calculate this, and below at runtime */

   tokens[0] = str;           /* This */
   str[4] = '\0';

   tokens[1] = (str + 5);     /* is */
   str[7] = '\0';

   tokens[2] = (str + 8);     /* an */
   str[10] = '\0';

   tokens[3] = (str + 11);    /* example */

   for(i = 0; i < numberOfWords; ++i)
   {
      printf("Token %d: %s\n", i + 1, tokens[i]);
   }

   return 0;
}