uthash.h - A hash table for C structure

Overview

Any C structure can be stored in a hash table using uthash. Just add a UT_hash_handle to the structure and choose one or more fields in your structure to act as the key. Then use these macros to store, retrieve or delete items from the hash table.

  • If key is integer, using HASH_ADD_INT

  • If key is string， using HASH_ADD_STR

  • If key is pointer , using HASH_ADD_PTR

  • If key is another type, we can use HASH_ADD. (The macros above are all implemeted by HASH_ADD.)

Steps before using uthash

Step 1. Include header

#include "uthash.h"

Step 2. Define a structure with a pair of key-value

Note that the structure must have one UT_hash_handle member.

struct my_struct {
    int id;            /* we'll use this field as the key */
    char name[10];     /* value */
    UT_hash_handle hh; /* makes this structure hashable */
};

Step 3. Define a pointer to a hash table. And it points to the NULL.

struct myhash *users  = NULL;

Examples of using uthash

Here are the exmpales to show how to use uthash.

Example 1. Adding an item to a hash.

struct my_struct *users = NULL;

void add_user(struct my_struct *s) {
    HASH_ADD_INT( users, id, s );
}

Example 2. Looking up an item in a hash.

struct my_struct *find_user(int user_id) {
    struct my_struct *s;

    HASH_FIND_INT( users, &user_id, s );
    return s;
}

Example 3. Deleting an item from a hash.

void delete_user(struct my_struct *user) {
    HASH_DEL( users, user );
}

Example 4. Counting the items

int count_user(struct my_struct *user){
    HASH_COUNT( users );
}

Example 5. Sorting the hash table

int name_sort(struct my_struct *a, struct my_struct *b)
{
    return strcmp(a->name,b->name);
}

int id_sort(struct my_struct *a, struct my_struct *b)
{
    return (a->id - b->id);
}

void sort_by_name(struct my_struct *head)
{
    HASH_SORT(*head, name_sort);
}

void sort_by_id(struct my_struct *head)
{
    HASH_SORT(*head, id_sort);
}

Example 6. Traverse the hash table

void print_user_iterator(struct my_struct head)
{
    struct my_struct *s, *tmp;

    HASH_ITER(hh, head, s, tmp)
    {
        printf("user id %d: name %s\n", s->id, s->name);
        /* ... it is safe to delete and free s here */
    }
}