# uthash User Guide

Troy D. Hanson
`<thanson@users.sourceforge.net>`
version 1.2, November 2006

## A hash in C

This document is geared towards C programmers of UNIX systems. Since you're reading this, chances are that you know a hash is used for looking up items using a key. In scripting languages like Perl, hashes are used all the time. In C, hashes don't exist in the language itself. This software provides a hash for C structures.

### What can it do?

This software supports four basic operations on hashes.

1. add

2. find

3. delete

4. iterate/sort

### Is it fast?

Add, find and delete are normally constant-time operations. This is influenced by your key domain and the hash function.

This hash aims to be minimalistic and efficient. It's around 500 lines of C. It inlines automatically because it's implemented as macros. It's fast as long as the hash function is properly chosen to suit your keys. This is easy; see Appendix A: Choosing the hash function.

### Is it a library?

No, it's just a single header file: `uthash.h`. All you need to do is copy the header file into your project, and:

    #include "uthash.h"

You may then utilize the hash macros as explained below.

### Supported platforms

This software has been tested on Linux, Solaris, OpenBSD, Mac OSX and Cygwin. It probably runs on other Unix or Unix-like platforms too.

### BSD licensed

This software is made available under the BSD license. It is free and open source.

### Obtaining uthash

Please follow the link to download on the uthash website.

### Getting help

You can email the author at thanson@users.sourceforge.net if you have questions not answered by this document.

## Keys and values

A hash is comprised of structures. Each structure represents a key-value association. One or more fields act as the key; the structure itself is the value.

Example: Defining a structure that can be hashed

    #include "uthash.h"

    struct my_struct {
        int id;                    /* key */
        char name[10];
        UT_hash_handle hh;         /* makes this structure hashable */
    };

There are no restrictions on the data type or name of the key field(s).

It may be a surprise that this hash does not require your structure or your key field to have a particular datatype. How is it possible to implement a type-independent hash in a strongly-typed language? In a word, macros. All the hash code is in the form of macros, whose arguments are typeless.

Note | Just remember that, as with any hash, the keys have to be unique.

### The UT_hash_handle field

The `UT_hash_handle` field must be present in your structure in order to hash it. It is used for the internal bookkeeping that makes the hash work. It does not require initialization. It can be named anything, but you can simplify matters by naming it `hh`. This allows you to use the easier "convenience" macros to add, find and delete items.

## Adding, finding, deleting and iterating

This section introduces the `HASH_ADD`, `HASH_FIND`, `HASH_DELETE`, and `HASH_SORT` macros. This section demonstrates the macros by example. For a more succinct listing, see Appendix F: Macro Reference.

### Adding an item

Your hash must be declared as a `NULL`-initialized pointer to your structure. Then allocate and initialize your structure and call `HASH_ADD`. (Here we use the convenience macro `HASH_ADD_INT`, which is for keys of type `int`.)

Important | The hash must be initialized to `NULL`!

Example: Adding an item to a hash

    struct my_struct *users = NULL;    /* important! initialize to NULL */

    int add_user(int user_id, char *name) {
        struct my_struct *s;

        s = malloc(sizeof(struct my_struct));
        s->id = user_id;
        strcpy(s->name, name);
        HASH_ADD_INT( users, id, s );  /* id: name of key field */
    }

The second parameter to `HASH_ADD_INT` is the name of the key field. Here, this is `id`.

Does it look strange to you that `id`, which is the name of a field in the structure, can be passed as a standalone parameter to `HASH_ADD_INT`? Welcome to the world of macros. Rest assured that the C preprocessor expands this macro to valid C code.

Once an item has been added to the hash, do not change the value of its key. Instead, delete the item from the hash, change the key, and then re-add it.

Important | Key uniqueness Your application must enforce key uniqueness. In other words, do not add the same key to the hash more than once. If you do, hash behavior will be undefined.

You can test whether a key already exists in the hash using `HASH_FIND`.

### Finding an item

To look up an item in a hash, you must have its key. Then call `HASH_FIND`. (Here we use the convenience macro `HASH_FIND_INT` for keys of type `int`).

Example: Looking up an item in a hash by its key

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

        HASH_FIND_INT( users, &user_id, s );  /* s: output pointer */
        return s;
    }

Note | The middle argument is a pointer to the key. You can't pass a literal key value to `HASH_FIND`. Instead assign the literal value to a variable, and pass a pointer to the variable.

Here, `s` is the output variable of `HASH_FIND_INT`. It's a pointer to the sought item, or `NULL` if the key wasn't found in the hash.

### Deleting an item

To delete an item from a hash, you must have a pointer to the item.

Example: Deleting an item from a hash

    void delete_user(struct my_struct *user) {
        HASH_DEL( users, user);  /* user: pointer to deletee */
    }

Note | Deleting an item from a hash just removes it— it does not free it.

### Iterating and sorting

You can loop over the items in the hash by starting from the beginning and following the `hh.next` pointer.

Example: Iterating over all the items in a hash

    void print_users() {
        struct my_struct *s;

        for(s=users; s != NULL; s=s->hh.next) {
            printf("user id %d: name %s\n", s->id, s->name);
        }
    }

There is also an `hh.prev` pointer you could use to iterate backwards through the hash, starting from any known item.

Iterating backward and forward through the items in the hash is possible because of the `hh.prev` and `hh.next` fields. All the items in the hash can be reached by repeatedly following these pointers, thus the hash is also a doubly-linked list.

#### Sorted iteration

The items in the hash are, by default, traversed in the order they were added ("insertion order") when you follow the `hh.next` pointer. But you can sort the items into a new order using `HASH_SORT`. E.g.,

    HASH_SORT( users, name_sort );

The second argument is a pointer to a comparison function. It must accept two arguments which are pointers to two items to compare. Its return value should be less than zero, zero, or greater than zero, if the first item sorts before, equal to, or after the second item, respectively. (Just like `strcmp`).

Example: Sorting the items in the hash

    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() {
        HASH_SORT(users, name_sort);
    }

    void sort_by_id() {
        HASH_SORT(users, id_sort);
    }

When the items in the hash are sorted, the first item may also change position; so in the example above, `users` may point to a different item after calling `HASH_SORT`.

### Putting it all together: A complete Example

We'll repeat all the code and embellish it with a `main()` function to form a working example.

If this code was placed in a file called `example.c` in the same directory as `uthash.h`, it could be compiled and run like this:

    cc -o example example.c
    ./example

Follow the prompts to try the program, and type `Ctrl-C` when done.

Example: A complete program

    #include <stdio.h>   /* gets */
    #include <stdlib.h>  /* atoi, malloc */
    #include <string.h>  /* strcpy */
    #include "uthash.h"

    struct my_struct {
        int id;                    /* key */
        char name[10];
        UT_hash_handle hh;         /* makes this structure hashable */
    };

    struct my_struct *users = NULL;

    int add_user(int user_id, char *name) {
        struct my_struct *s;

        s = malloc(sizeof(struct my_struct));
        s->id = user_id;
        strcpy(s->name, name);
        HASH_ADD_INT( users, id, s );  /* id: name of key field */
    }

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

        HASH_FIND_INT( users, &user_id, s );  /* s: output pointer */
        return s;
    }

    void delete_user(struct my_struct *user) {
        HASH_DEL( users, user);  /* user: pointer to deletee */
    }

    void print_users() {
        struct my_struct *s;

        for(s=users; s != NULL; s=s->hh.next) {
            printf("user id %d: name %s\n", s->id, s->name);
        }
    }

    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() {
        HASH_SORT(users, name_sort);
    }

    void sort_by_id() {
        HASH_SORT(users, id_sort);
    }

    int main(int argc, char *argv[]) {
        char in[10];
        int id=1;
        struct my_struct *s;

        while (1) {
            printf("1. add user\n");
            printf("2. find user\n");
            printf("3. delete user\n");
            printf("4. sort items by name\n");
            printf("5. sort items by id\n");
            printf("6. print users\n");
            gets(in);
            switch(atoi(in)) {
                case 1:
                    printf("name?\n");
                    add_user(id++, gets(in));
                    break;
                case 2:
                    printf("id?\n");
                    s = find_user(atoi(gets(in)));
                    printf("user: %s\n", s ? s->name : "unknown");
                    break;
                case 3:
                    printf("id?\n");
                    s = find_user(atoi(gets(in)));
                    if (s) delete_user(s);
                    else printf("id unknown\n");
                    break;
                case 4:
                    sort_by_name();
                    break;
                case 5:
                    sort_by_id();
                    break;
                case 6:
                    print_users();
                    break;
            }
        }
    }

This program is included in the distribution in `tests/example.c`. You can run `make example` in that directory to compile it easily.

## Arbitrary keys

### An example with a string key

This is almost the same as using integer keys except the macros are named `HASH_ADD_STR` and `HASH_FIND_STR`.

Note | char[ ] vs. char* The string is within the structure in this example— `name` is a `char[10]` field. If instead our structure merely pointed to the key (i.e., `name` was declared `char *`), we'd use `HASH_ADD_KEYPTR`, described in Appendix F.

Example: A string-keyed hash

    #include <string.h>  /* strcpy */
    #include <stdlib.h>  /* malloc */
    #include <stdio.h>   /* printf */
    #include "uthash.h"

    struct my_struct {
        char name[10];             /* key */
        int id;
        UT_hash_handle hh;         /* makes this structure hashable */
    };

    int main(int argc, char *argv[]) {
        char **n, *names[] = { "joe", "bob", "betty", NULL };
        struct my_struct *s, *users = NULL;
        int i=0;

        for (n = names; *n != NULL; n++) {
            s = malloc(sizeof(struct my_struct));
            strcpy(s->name, *n);
            s->id = i++;
            HASH_ADD_STR( users, name, s );
        }

        HASH_FIND_STR( users, "betty", s);
        if (s) printf("betty's id is %d\n", s->id);
    }

This example is included in the distribution in `tests/test15.c`. It prints:

    betty's id is 2

Note | Remember, don't change an item's key after adding it to the hash. (Instead, delete the item from the hash, change the key and then re-add it to the hash).

### An example with a multi-field, binary key

Your key field can have any data type, and it can even be an aggregate of multiple contiguous fields.

The generalized version of the macros (e.g., `HASH_ADD`) must be used. In addition to the usual arguments, the generalized macros require the name of the `UT_hash_handle` field, and the key length.

To determine the key length when using a multi-field key, you must include any intervening structure padding the compiler adds for alignment purposes.

An easy way to calculate the key length is to use the `offsetof` macro from `<stddef.h>`. The formula is:

    key length =   offsetof(last_key_field)
                 + sizeof(last_key_field)
                 - offsetof(first_key_field)

In the example below, the `keylen` variable is set using this technique.

Example: A hash with a multi-field binary key

    #include <stdlib.h>    /* malloc       */
    #include <stddef.h>    /* offsetof     */
    #include <stdio.h>     /* printf       */
    #include <string.h>    /* memset       */
    #include "uthash.h"

    struct my_event {
        struct timeval tv;         /* key is aggregate of this field */
        char event_code;           /* and this field.                */
        int user_id;
        UT_hash_handle hh;         /* makes this structure hashable */
    };

    int main(int argc, char *argv[]) {
        struct my_event *e, ev, *events = NULL;
        int i, keylen;

        keylen =   offsetof(struct my_event, event_code) + sizeof(char)
                 - offsetof(struct my_event, tv);

        for(i = 0; i < 10; i++) {
            e = malloc(sizeof(struct my_event));
            memset(e,0,sizeof(struct my_event));
            e->tv.tv_sec = i * (60*60*24*365);          /* i years (sec)*/
            e->tv.tv_usec = 0;
            e->event_code = 'a'+(i%2);                   /* meaningless */
            e->user_id = i;

            HASH_ADD( hh, events, tv, keylen, e);
        }

        /* look for one specific event */
        memset(&ev,0,sizeof(struct my_event));
        ev.tv.tv_sec = 5 * (60*60*24*365);
        ev.tv.tv_usec = 0;
        ev.event_code = 'b';
        HASH_FIND( hh, events, &ev.tv, keylen, e );
        if (e) printf("found: user %d, unix time %ld\n", e->user_id, e->tv.tv_sec);
    }

This example is included in the distribution in `tests/test16.c`.

Notice the use of `memset` in the example above to initialize the structure by zero-filling it. This might seem unnecessary, since the next lines explicitly set each field. However, the `memset` is needed, because it zeroes out any padding that the compiler has inserted into the structure.

If we didn't zero-fill the structure, padding between the fields that comprise the key would be initialized with random values. This would make it impossible to lookup items by their key reliably. (The multi-field key comprises the fields and their intervening padding).

## Participating in multiple hashes

A structure can be in multiple hashes simultaneously. For example, you might want to hash a structure on both an ID field and on a unique username.

Example: A structure with two keys

    struct my_struct {
        int id;                    /* key 1 */
        char username[10];         /* key 2 */
        UT_hash_handle hh1,hh2;    /* makes this structure hashable */
    };

In the example above, the structure can now be added to two separate hashes; in one hash, `id` is its key, while in the other hash, `username` is the key. (There is no requirement that the two hashes have different key fields. They could both use the same key, such as `id`).

Note | You need to have a separate `UT_hash_handle` for each hash that your structure will participate in simultaneously. These are `hh1` and `hh2` in this example.

Example: A structure participating in two hashes

        struct my_struct *users_by_id = NULL, *users_by_name = NULL, *s;
        int i;
        char *name;

        s = malloc(sizeof(struct my_struct));
        s->id = 1;
        strcpy(s->username, "thanson");

        HASH_ADD(hh1, users_by_id, id, sizeof(int), s);
        HASH_ADD(hh2, users_by_name, username, strlen(s->username), s);

        /* lookup user by ID */
        i=1;
        HASH_FIND(hh1, users_by_id, &i, sizeof(int), s);
        if (s) printf("found id %d: %s\n", i, s->username);

        /* lookup user by username */
        name = "thanson";
        HASH_FIND(hh2, users_by_name, name, strlen(name), s);
        if (s) printf("found user %s: %d\n", name, s->id);

When participating in two or more hashes simultaneously, always use the same key field with the same hash handle. E.g., in this example, `hh1` is always used with the `id` key, and `hh2` is always used with the `username` key.

## Threaded programs

You can use this hash in a threaded program. But, the locking is up to you. You must protect the hash against concurrent modification. For every hash that you create in a threaded program, you should create a lock to protect it.

Concurrent hash lookups (`HASH_FIND`) are permitted, but modifications (`HASH_ADD`, `HASH_DEL`, and `HASH_SORT`) cannot be concurrent. Therefore, you can use a read/write lock which provides concurrent-read/exclusive-write semantics. Or you can use a mutex-style lock to prevent any concurrent access to the hash, but this is less efficient.

## Appendix A: Choosing the hash function

Internally this software uses a particular hash function, such as Bernstein's hash, to transform a key to a bucket number.

Example: Bernstein hash function

    #define HASH_BER(key,keylen,num_bkts,bkt,i,j,k)                        \
      bkt = 0;                                                             \
      while (keylen--)  bkt = (bkt * 33) + *key++;                         \
      bkt &= (num_bkts-1);

Several such hash functions are built-in. The default is Jenkin's hash, but your key domain may get better distribution with another function. You can use a specific hash function by compiling your program with `-DHASH_FUNCTION=HASH_xyz` where `xyz` is one of the symbolic names listed below. E.g.,

    cc -DHASH_FUNCTION=HASH_OAT -o program program.c

Table: Built-in hash functions
Symbol | Name
--- | ---
JEN | Jenkins (default)
BER | Bernstein
SAX | Shift-Add-Xor
OAT | One-at-a-time
FNV | Fowler/Noll/Vo
JSW | Julienne Walker

### Which hash function is best?

You can easily determine the best hash function for your key domain. To do so, you'll need to run your program once in a data-collection pass, and then run the collected data through an included analysis utility.

First you must build the analysis utility. From the top-level directory,

    cd tests/
    make

We'll use `test14.c` to demonstrate the data-collection and analysis steps (using `sh` syntax):

    cc -DHASH_EMIT_KEYS=3 -I../src -o test14 test14.c
    ./test14 3>test14.keys
    ./keystats test14.keys

The numeric value of `HASH_EMIT_KEYS` is a file descriptor. Any file descriptor that your program doesn't use for its own purposes can be used instead of 3.

Note | The data-collection mode enabled by `-DHASH_EMIT_KEYS=x` should not be used in production code.

The result of running the `keystats` command should look like this:

    fcn     hash_q     #items   #buckets      #dups      flags   add_usec  find_usec
    --- ---------- ---------- ---------- ---------- ---------- ---------- ----------
    BER   1.000000       1219        512          0         ok       1206        550
    FNV   1.000000       1219        512          0         ok       1734        660
    JEN   1.000000       1219        512          0         ok       2008        845
    OAT   0.998897       1219        512          0         ok       1941        833
    SAX   0.997514       1219       1024          0         ok       2217        674
    JSW   0.994652       1219        512          0         ok       1369        608

Usually, you should just pick the first hash function that is listed. Here, this is `BER`. This is the function that provides the most even distribution for your keys. If several have the same `hash_q`, then choose the fastest one according to the `find_usec` column.

Tip | Sometimes you might pick the fastest one even though its `hash_q` isn't the best. This is ok, particularly if its substantially faster (ideally hash functions have negligible performance overhead but in reality they can vary).

### keystats column reference

fcn

  symbolic name of hash function

hash_q

  a number between 0 and 1 that measures how evenly the items are distributed among the buckets (1 is best)

#items

  the number of keys that were read in from the emitted key file

#buckets

  the number of buckets in the hash after all the keys were added

#dups

  the number of duplicate keys encounted in the emitted key file. Duplicates keys are filtered out to maintain key uniqueness. (Duplicates are normal. For example, if the application adds an item to a hash, deletes it, then re-adds it, the key is written twice to the emitted file.)

flags

  this is either `ok`, or `noexpand` if the expansion inhibited flag is set, described in Appendix C. It is not recommended to use a hash function that has the `noexpand` flag set.

add_usec

  the clock time in microseconds required to add all the keys to a hash

find_usec

  the clock time in microseconds required to look up every key in the hash

## Appendix B: Statistic hash_q

The hash_q statistic measures how evenly the keys are distributed among the buckets. It is a value between 0 (worst) and 1 (best). You can access it in this way, if you're interested in observing how well your hash keys hold up to the ideal distribution.

Example: Observing the hash_q statistic

    /* using the example "users" hash from previous listings */

    void print_hashq() {
        if (users) printf("hash_q: %f\n", users->hh.tbl->hash_q);
    }

The hash_q statistic is updated whenever bucket expansion occurs (see Appendix C).

The n items in a hash are distributed into k buckets. Ideally each bucket would contain an equal share (n/k) of the items. (I.e., the maximum linear position of any item within a bucket would be n/k.) The hash_q statistic measures the fraction of items whose linear position is less than or equal to the maximum ideal position n/k.

Said another way, hash_q is the fraction of items which can be found using no more than the ideal number of steps, given the number of items and buckets.

A low value of `hash_q` may lead to reduced lookup (`HASH_FIND`) performance. You can change the hash function to one that distributes your keys more evenly; see Appendix A: Choosing the hash function.

## Appendix C: Bucket expansion

Internally this hash manages the number of buckets, with the goal of having enough buckets so that each one contains only small number of items.

When looking up an item by its key, this hash scans linearly through the items in the appropriate bucket. In order for the linear scan to run in constant time, the number of items in each bucket must be bounded. This is accomplished by increasing the number of buckets as needed.

This hash attempts to keep fewer than 10 items in each bucket. When an item is added that would cause a bucket to exceed this number, the number of buckets in the hash is doubled and the items are redistributed.

Note | Bucket expansion occurs automatically and invisibly as needed. There is no need for the application to know when it occurs.

### Bucket expansion hook

There is a hook for this event, primarily for uthash development.

Example: Bucket expansion hook

    #include "uthash.h"

    #undef uthash_expand_fyi
    #define uthash_expand_fyi(tbl) printf("expanded to %d buckets\n", tbl->num_buckets)

    ...

During bucket expansion, if the number of items in a bucket exceeds the ideal number, then the expansion threshold for that bucket will be increased (instead of being 10, it will be a multiple of 10). This prevents excessive bucket expansion for hash functions that tend to over-utilize a few buckets yet have good distribution overall.

### Bucket expansion inhibited hook

Some key domains may not yield a good bucket distribution using the default (or manually-specified) hash function.

If this occurs, statistic `hash_q` reflects the uneven distribution by tending closer to 0 than 1. This means that items are piling up in some buckets while other buckets are under-utilized. Bucket expansion then takes place as this hash tries to keep a bounded number of items in each bucket.

However, if items continue to be distributed unevenly after repeated bucket expansion, there is a point at which this hash in effect says, "This key domain is really incompatible with this hash function. I could spend all day expanding the buckets but it isn't helping to spread out the items in the hash, so I'm going to cut my losses and stop expanding the buckets."

#### Inhibition criteria

The decision to inhibit expansion is made by a heuristic rule: if two consecutive bucket expansions yield `hash_q` values below 0.5, then the bucket expansion inhibited flag is set for this hash.

Because this condition would cause `HASH_FIND` lookups to exhibit worse than constant-time performance, your application can choose to be made aware if it occurs. (Usually you don't need to worry about this, if you used the `keystats` utility during development to select a good hash for your keys).

Example: Bucket expansion inhibited hook

    #include "uthash.h"

    #undef uthash_noexpand_fyi
    #define uthash_noexpand_fyi printf("warning: bucket expansion inhibited\n");

    ...

Once set, the bucket expansion inhibited flag remains in effect as long as the hash has items in it.

## Appendix D: Hooks for memory allocation

By default this hash implementation uses `malloc` and `free` to manage memory. If your application uses its own custom allocator, this hash can use them too.

Example: Specifying alternate memory management functions

    #include "uthash.h"

    /* undefine the defaults */
    #undef uthash_bkt_malloc
    #undef uthash_bkt_free
    #undef uthash_tbl_malloc
    #undef uthash_tbl_free

    /* re-define, specifying alternate functions */
    #define uthash_bkt_malloc(sz) my_malloc(sz)  /* for UT_hash_bucket */
    #define uthash_bkt_free(ptr) my_free(ptr)
    #define uthash_tbl_malloc(sz) my_malloc(sz)  /* for UT_hash_table  */
    #define uthash_tbl_free(ptr) my_free(ptr)

    ...

The first pair allocates and frees `UT_hash_bucket` structures, while the second pair deals with `UT_hash_table` structures. While they don't need to have separate allocation/free functions (and indeed the default is just to use `malloc` and `free` anyway), having them separate permits easy integration with pool-type allocators. (In these allocators, there is a pool for each data structure.)

### Out of memory handling

If memory allocation fails (i.e., the malloc function returned `NULL`), the default behavior is to terminate the process by calling `exit(-1)`. This can be modified by re-defining the `uthash_fatal` macro.

    #undef uthash_fatal
    #define uthash_fatal(msg) my_fatal_function(msg);

The fatal function should terminate the process; uthash does not support "returning a failure" if memory cannot be allocated.

## Appendix E: Internal implementation notes

### Application and bucket order

The `UT_hash_handle` data structure includes `next`, `prev`, `hh_next` and `hh_prev` fields. The former two fields determine the "application" ordering (that is, iteration order that corresponds to the order the items were added). The latter two fields determine the "bucket" order. These link the `UT_hash_handles` together in a doubly-linked list that defines a bucket.

### Internal consistency check with -DHASH_DEBUG=1

If a program that uses this hash is compiled with `-DHASH_DEBUG=1`, a special internal consistency-checking mode is activated. In this mode, the integrity of the whole hash is checked following every add or delete operation.

Note | This is for debugging the uthash software only, not for use in production code.

In this mode, any internal errors in the hash data structure will cause a message to be printed to `stderr` and the program to exit.

Checks performed in `-DHASH_DEBUG=1` mode:

- the hash is walked in its entirety twice: once in bucket order and a second time in application order

- the total number of items encountered in both walks is checked against the stored number

- during the walk in bucket order, each item's `hh_prev` pointer is compared for equality with the last visited item

- during the walk in application order, each item's `prev` pointer is compared for equality with the last visited item

In the `tests/` directory, running `make debug` will run all the tests in this mode.

## Appendix F: Macro reference

### Generalized macros

These macros add, find, delete and sort the items in a hash.

Table: Generalized macros
macro | arguments
--- | ---
HASH_ADD | (hh_name, head, keyfield_name, key_len, item_ptr)
HASH_ADD_KEYPTR | (hh_name, head, key_ptr, key_len, item_ptr)
HASH_FIND | (hh_name, head, key_ptr, key_len, item_ptr)
HASH_DELETE | (hh_name, head, item_ptr)
HASH_SORT | (head, cmp)

Note | `HASH_ADD_KEYPTR` is used when the structure contains a pointer to the key, rather than the key itself.

### Convenience macros

The convenience macros do the same thing as the generalized macros, but require fewer arguments. They have key and naming restrictions (see below).

Table: Convenience macros
macro | arguments
--- | ---
HASH_ADD_INT | (head, keyfield_name, item_ptr)
HASH_FIND_INT | (head, key_ptr, item_ptr)
HASH_ADD_STR | (head, keyfield_name, item_ptr)
HASH_FIND_STR | (head, key_ptr, item_ptr)
HASH_DEL | (head, item_ptr)

#### Key and naming restrictions for convenience macros

In order to use the convenience macros,

1. the structure's `UT_hash_handle` field must be named `hh`, and

2. the key field must be of type `int` or `char[]`

`HASH_DEL` does not require the second condition.

### Argument descriptions

hh_name

  name of the `UT_hash_handle` field in the structure. Conventionally called `hh`.

head

  the structure pointer variable which acts as the "head" of the hash. So named because it points to the first item which is added to the hash.

keyfield_name

  the name of the key field in the structure. (In the case of a multi-field key, this is the first field of the key). If you're new to macros, it might seem strange to pass the name of a field as a parameter. See note.

key_len

  the length of the key field in bytes. E.g. for an integer key, this is `sizeof(int)`, while for a string key it's `strlen(key)`. (For a multi-field key, see the notes in this guide on calculating key length).

key_ptr

  for `HASH_FIND`, this is a pointer to the key to look up in the hash (since it's a pointer, you can't directly pass a literal value here). For `HASH_ADD_KEYPTR`, this is the address of the key of the item being added.

item_ptr

  pointer to the structure being added, deleted or looked up. This is an input parameter for HASH_ADD and HASH_DELETE macros, and an output parameter for HASH_FIND.

cmp

  pointer to comparison function which accepts two arguments (pointers to items to compare) and returns an int specifying whether the first item should sort before, equal to, or after the second item (like `strcmp`).