A simple hash implementation.

hash.c

/*
 *
 * hash.c
 *
 * A simple hash implementation.
 *
 * It's not mine, the credit goes to The Book. 
 *
 */

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

/* table entry */
struct nlist {
  /* defined name */
  char *name;
  /* the whatever value */
  int value;
  /* next entry in the chain */
  struct nlist *next;
};

#define HASH_SIZE 101
/* pointer table */
static struct nlist *hashtab[HASH_SIZE];

/* hash: form hash value for string <s> */
unsigned hash(char *s)
{
  unsigned hashval;

  for (hashval = 0; *s != '\0'; s++)
    hashval = *s + 31 * hashval;

  return hashval % HASH_SIZE;
}

/* lookup: look for <s> in hashtab */
struct nlist *lookup(char *s)
{
  struct nlist *np;

  for (np = hashtab[hash(s)]; np != NULL; np = np->next)
    if (!strcmp(s, np->name))
      /* found */
      return np;

  /* not found */
  return NULL;
}

/* new: put (name, defn) in hashtab */
struct nlist *new(char *name, int value)
{
  struct nlist *np;
  unsigned hashval;

  if ((np = lookup(name)) == NULL){
    /* not found */
    np = malloc(sizeof(struct nlist));

    if (np == NULL || (np->name = strdup(name)) == NULL){
      return NULL;
    }

    hashval = hash(name);
    np->next = hashtab[hashval];
    hashtab[hashval] = np;
  }

  np->value = value;

  return np;
}

int main(void)
{
  /* discard the returned values */
  new("five", 5);
  new("three", 3);
  new("two", 2);

  printf("five->value:  %d\n", lookup("five")->value);
  printf("two->value:   %d\n", lookup("two")->value);
  printf("three->value: %d\n", lookup("three")->value);

  return 0;
}