Hash Tables

Overview

// typedefs

typedef struct apr_hash_t apr_hash_t;
typedef struct apr_hash_index_t apr_hash_index_t;

typedef unsigned int (*apr_hashfunc_t)(
    const char *key,
    apr_ssize_t *klen
    );

typedef int() apr_hash_do_callback_fn_t(
    void *rec,
    const void *key,
    apr_ssize_t klen,
    const void *value
    );

// global functions

unsigned int
apr_hashfunc_default(
    const char* key,
    apr_ssize_t* klen
);

apr_hash_t*
apr_hash_make(apr_pool_t* pool);

apr_hash_t*
apr_hash_make_custom(
    apr_pool_t* pool,
    apr_hashfunc_t hash_func
);

apr_hash_t*
apr_hash_copy(
    apr_pool_t* pool,
    const apr_hash_t* h
);

void
apr_hash_set(
    apr_hash_t* ht,
    const void* key,
    apr_ssize_t klen,
    const void* val
);

void*
apr_hash_get(
    apr_hash_t* ht,
    const void* key,
    apr_ssize_t klen
);

apr_hash_index_t*
apr_hash_first(
    apr_pool_t* p,
    apr_hash_t* ht
);

apr_hash_index_t*
apr_hash_next(apr_hash_index_t* hi);

void
apr_hash_this(
    apr_hash_index_t* hi,
    const void** key,
    apr_ssize_t* klen,
    void** val
);

const void*
apr_hash_this_key(apr_hash_index_t* hi);

apr_ssize_t
apr_hash_this_key_len(apr_hash_index_t* hi);

void*
apr_hash_this_val(apr_hash_index_t* hi);

unsigned int
apr_hash_count(apr_hash_t* ht);

void
apr_hash_clear(apr_hash_t* ht);

apr_hash_t*
apr_hash_overlay(
    apr_pool_t* p,
    const apr_hash_t* overlay,
    const apr_hash_t* base
);

apr_hash_t*
apr_hash_merge(
    apr_pool_t* p,
    const apr_hash_t* h1,
    const apr_hash_t* h2,
    void*(*)(apr_pool_t*p, const void*key, apr_ssize_t klen, const void*h1_val, const void*h2_val, const void*data) merger,
    const void* data
);

int
apr_hash_do(
    apr_hash_do_callback_fn_t* comp,
    void* rec,
    const apr_hash_t* ht
);

apr_pool_t*
apr_hash_pool_get(const apr_hash_t* thehash);

// macros

#define APR_HASH_KEY_STRING

Detailed Documentation

Typedefs

typedef struct apr_hash_t apr_hash_t

Abstract type for hash tables.

typedef struct apr_hash_index_t apr_hash_index_t

Abstract type for scanning hash tables.

typedef unsigned int (*apr_hashfunc_t)(
    const char *key,
    apr_ssize_t *klen
    )

Callback functions for calculating hash values.

Parameters:

key	The key.
klen	The length of the key, or APR_HASH_KEY_STRING to use the string length. If APR_HASH_KEY_STRING then returns the actual key length.

typedef int() apr_hash_do_callback_fn_t(
    void *rec,
    const void *key,
    apr_ssize_t klen,
    const void *value
    )

Declaration prototype for the iterator callback function of apr_hash_do().

Iteration continues while this callback function returns non-zero. To export the callback function for apr_hash_do() it must be declared in the _NONSTD convention.

Parameters:

rec	The data passed as the first argument to apr_hash_[v]do()
key	The key from this iteration of the hash table
klen	The key length from this iteration of the hash table
value	The value from this iteration of the hash table

Global Functions

unsigned int
apr_hashfunc_default(
    const char* key,
    apr_ssize_t* klen
)

The default hash function.

apr_hash_t*
apr_hash_make(apr_pool_t* pool)

Create a hash table.

Parameters:

pool	The pool to allocate the hash table out of

Returns:

The hash table just created

apr_hash_t*
apr_hash_make_custom(
    apr_pool_t* pool,
    apr_hashfunc_t hash_func
)

Create a hash table with a custom hash function

Parameters:

pool	The pool to allocate the hash table out of
hash_func	A custom hash function.

Returns:

The hash table just created

apr_hash_t*
apr_hash_copy(
    apr_pool_t* pool,
    const apr_hash_t* h
)

Make a copy of a hash table Makes a shallow copy

Parameters:

pool	The pool from which to allocate the new hash table
h	The hash table to clone

Returns:

The hash table just created

void
apr_hash_set(
    apr_hash_t* ht,
    const void* key,
    apr_ssize_t klen,
    const void* val
)

Associate a value with a key in a hash table. If the value is NULL the hash entry is deleted.

Parameters:

ht	The hash table
key	Pointer to the key
klen	Length of the key. Can be APR_HASH_KEY_STRING to use the string length.
val	Value to associate with the key

void*
apr_hash_get(
    apr_hash_t* ht,
    const void* key,
    apr_ssize_t klen
)

Look up the value associated with a key in a hash table.

Parameters:

ht	The hash table
key	Pointer to the key
klen	Length of the key. Can be APR_HASH_KEY_STRING to use the string length.

Returns:

Returns NULL if the key is not present.

apr_hash_index_t*
apr_hash_first(
    apr_pool_t* p,
    apr_hash_t* ht
)

Start iterating over the entries in a hash table. There is no restriction on adding or deleting hash entries during an iteration (although the results may be unpredictable unless all you do is delete the current entry) and multiple iterations can be in progress at the same time.

int sum_values(apr_pool_t *p, apr_hash_t *ht)
{
    apr_hash_index_t *hi;
    void *val;
    int sum = 0;
    for (hi = apr_hash_first(p, ht); hi; hi = apr_hash_next(hi)) {
        apr_hash_this(hi, NULL, NULL, &val);
        sum += *(int *)val;
    }
    return sum;
}

Parameters:

p	The pool to allocate the apr_hash_index_t iterator. If this pool is NULL, then an internal, non-thread-safe iterator is used.
ht	The hash table

Returns:

The iteration state

apr_hash_index_t*
apr_hash_next(apr_hash_index_t* hi)

Continue iterating over the entries in a hash table.

Parameters:

hi	The iteration state

Returns:

a pointer to the updated iteration state. NULL if there are no more entries.

void
apr_hash_this(
    apr_hash_index_t* hi,
    const void** key,
    apr_ssize_t* klen,
    void** val
)

Get the current entry’s details from the iteration state. The return pointers should point to a variable that will be set to the corresponding data, or they may be NULL if the data isn’t interesting.

Parameters:

hi	The iteration state
key	Return pointer for the pointer to the key.
klen	Return pointer for the key length.
val	Return pointer for the associated value.

const void*
apr_hash_this_key(apr_hash_index_t* hi)

Get the current entry’s key from the iteration state.

Parameters:

hi	The iteration state

Returns:

The pointer to the key

apr_ssize_t
apr_hash_this_key_len(apr_hash_index_t* hi)

Get the current entry’s key length from the iteration state.

Parameters:

hi	The iteration state

Returns:

The key length

void*
apr_hash_this_val(apr_hash_index_t* hi)

Get the current entry’s value from the iteration state.

Parameters:

hi	The iteration state

Returns:

The pointer to the value

unsigned int
apr_hash_count(apr_hash_t* ht)

Get the number of key/value pairs in the hash table.

Parameters:

ht	The hash table

Returns:

The number of key/value pairs in the hash table.

void
apr_hash_clear(apr_hash_t* ht)

Clear any key/value pairs in the hash table.

Parameters:

ht	The hash table

apr_hash_t*
apr_hash_overlay(
    apr_pool_t* p,
    const apr_hash_t* overlay,
    const apr_hash_t* base
)

Merge two hash tables into one new hash table. The values of the overlay hash override the values of the base if both have the same key. Both hash tables must use the same hash function.

Parameters:

p	The pool to use for the new hash table
overlay	The table to add to the initial table
base	The table that represents the initial values of the new table

Returns:

A new hash table containing all of the data from the two passed in

apr_hash_t*
apr_hash_merge(
    apr_pool_t* p,
    const apr_hash_t* h1,
    const apr_hash_t* h2,
    void*(*)(apr_pool_t*p, const void*key, apr_ssize_t klen, const void*h1_val, const void*h2_val, const void*data) merger,
    const void* data
)

Merge two hash tables into one new hash table. If the same key is present in both tables, call the supplied merge function to produce a merged value for the key in the new table. Both hash tables must use the same hash function.

Parameters:

p	The pool to use for the new hash table
h1	The first of the tables to merge
h2	The second of the tables to merge
merger	A callback function to merge values, or NULL to make values from h1 override values from h2 (same semantics as apr_hash_overlay())
data	Client data to pass to the merger function

Returns:

A new hash table containing all of the data from the two passed in

int
apr_hash_do(
    apr_hash_do_callback_fn_t* comp,
    void* rec,
    const apr_hash_t* ht
)

Iterate over a hash table running the provided function once for every element in the hash table. The

Parameters:

comp	function will be invoked for every element in the hash table.
comp	The function to run
rec	The data to pass as the first argument to the function
ht	The hash table to iterate over

Returns:

FALSE if one of the comp() iterations returned zero; TRUE if all iterations returned non-zero

See also:

apr_hash_do_callback_fn_t

apr_pool_t*
apr_hash_pool_get(const apr_hash_t* thehash)

Get a pointer to the pool which the hash table was created in

Macros

#define APR_HASH_KEY_STRING

When passing a key to apr_hash_set or apr_hash_get, this value can be passed to indicate a string-valued key, and have apr_hash compute the length automatically.

apr_hash will use strlen(key) for the length. The NUL terminator is not included in the hash value (why throw a constant in?). Since the hash table merely references the provided key (rather than copying it), apr_hash_this() will return the NUL-term’d key.