tcl_hash(3)

NAME

Tcl_InitHashTable, Tcl_InitCustomHashTable, Tcl_InitOb
jHashTable, Tcl_DeleteHashTable, Tcl_CreateHashEntry,
Tcl_DeleteHashEntry, Tcl_FindHashEntry, Tcl_GetHashValue,
Tcl_SetHashValue, Tcl_GetHashKey, Tcl_FirstHashEntry,
Tcl_NextHashEntry, Tcl_HashStats - procedures to manage
hash tables

SYNOPSIS

#include <tcl.h>
Tcl_InitHashTable(tablePtr, keyType)
Tcl_InitCustomHashTable(tablePtr, keyType, typePtr)
Tcl_InitObjHashTable(tablePtr)
Tcl_DeleteHashTable(tablePtr)
Tcl_HashEntry *
Tcl_CreateHashEntry(tablePtr, key, newPtr)
Tcl_DeleteHashEntry(entryPtr)
Tcl_HashEntry *
Tcl_FindHashEntry(tablePtr, key)
ClientData
Tcl_GetHashValue(entryPtr)
Tcl_SetHashValue(entryPtr, value)
char *
Tcl_GetHashKey(tablePtr, entryPtr)
Tcl_HashEntry *
Tcl_FirstHashEntry(tablePtr, searchPtr)
Tcl_HashEntry *
Tcl_NextHashEntry(searchPtr)
CONST char *
Tcl_HashStats(tablePtr)

ARGUMENTS

Tcl_HashTable *tablePtr (in) Address of hash

table structure
(for all proce
dures but
Tcl_InitHashTable, this must have
been initialized
by previous call
to
Tcl_InitHashTable).

int keyType (in) Kind of keys to

use for new hash
table. Must be
either
TCL_STRING_KEYS,
TCL_ONE_WORD_KEYS,
TCL_CUS
TOM_TYPE_KEYS,
TCL_CUS
TOM_PTR_KEYS, or
an integer value
greater than 1.

Tcl_HashKeyType *typePtr (in) Address of struc

ture which defines
the behaviour of
the hash table.

CONST char *key (in) Key to use for

probe into table.
Exact form depends
on keyType used to create table.

int *newPtr (out) The word at *new_

Ptr is set to 1 if
a new entry was
created and 0 if
there was already
an entry for key.

Tcl_HashEntry *entryPtr (in) Pointer to hash

table entry.

ClientData value (in) New value to

assign to hash
table entry. Need
not have type
ClientData, but
must fit in same
space as Client
Data.

Tcl_HashSearch *searchPtr (in) Pointer to record

to use to keep
track of progress
in enumerating all
the entries in a
hash table.

DESCRIPTION

A hash table consists of zero or more entries, each con
sisting of a key and a value. Given the key for an entry,
the hashing routines can very quickly locate the entry,
and hence its value. There may be at most one entry in a
hash table with a particular key, but many entries may
have the same value. Keys can take one of four forms:
strings, one-word values, integer arrays, or custom keys
defined by a Tcl_HashKeyType structure (See section THE
TCL_HASHKEYTYPE STRUCTURE below). All of the keys in a given table have the same form, which is specified when
the table is initialized.

The value of a hash table entry can be anything that fits
in the same space as a ``char *'' pointer. Values for
hash table entries are managed entirely by clients, not by
the hash module itself. Typically each entry's value is a
pointer to a data structure managed by client code.

Hash tables grow gracefully as the number of entries
increases, so that there are always less than three
entries per hash bucket, on average. This allows for fast
lookups regardless of the number of entries in a table.

The core provides three functions for the initialization
of hash tables, Tcl_InitHashTable, Tcl_InitObjHashTable
and Tcl_InitCustomHashTable.

Tcl_InitHashTable initializes a structure that describes a new hash table. The space for the structure is provided
by the caller, not by the hash module. The value of key_
Type indicates what kinds of keys will be used for all
entries in the table. All of the key types described later
are allowed, with the exception of TCL_CUSTOM_TYPE_KEYS and TCL_CUSTOM_PTR_KEYS.

Tcl_InitObjHashTable is a wrapper around Tcl_InitCus tomHashTable and initializes a hash table whose keys are Tcl_Obj *.

Tcl_InitCustomHashTable initializes a structure that describes a new hash table. The space for the structure is
provided by the caller, not by the hash module. The value
of keyType indicates what kinds of keys will be used for all entries in the table. KeyType must have one of the following values:

TCL_STRING_KEYS Keys are null-terminated ASCII

strings. They are passed to
hashing routines using the
address of the first character of
the string.

TCL_ONE_WORD_KEYS Keys are single-word values;

they are passed to hashing rou
tines and stored in hash table
entries as ``char *'' values.
The pointer value is the key; it
need not (and usually doesn't)
actually point to a string.

TCL_CUSTOM_TYPE_KEYS Keys are of arbitrary type, and

are stored in the entry. Hashing
and comparison is determined by
typePtr. The Tcl_HashKeyType structure is described in the
section THE TCL_HASHKEYTYPE STRUCTURE below.

TCL_CUSTOM_PTR_KEYS Keys are pointers to an arbitrary

type, and are stored in the
entry. Hashing and comparison is
determined by typePtr. The Tcl_HashKeyType structure is
described in the section THE
TCL_HASHKEYTYPE STRUCTURE below.

other If keyType is not one of the

above, then it must be an integer
value greater than 1. In this
case the keys will be arrays of
``int'' values, where keyType gives the number of ints in each
key. This allows structures to
be used as keys. All keys must
have the same size. Array keys
are passed into hashing functions
using the address of the first
int in the array.

Tcl_DeleteHashTable deletes all of the entries in a hash table and frees up the memory associated with the table's
bucket array and entries. It does not free the actual
table structure (pointed to by tablePtr), since that mem ory is assumed to be managed by the client. Tcl_Delete HashTable also does not free or otherwise manipulate the values of the hash table entries. If the entry values
point to dynamically-allocated memory, then it is the
client's responsibility to free these structures before
deleting the table.

Tcl_CreateHashEntry locates the entry corresponding to a particular key, creating a new entry in the table if there
wasn't already one with the given key. If an entry
already existed with the given key then *newPtr is set to zero. If a new entry was created, then *newPtr is set to a non-zero value and the value of the new entry will be
set to zero. The return value from Tcl_CreateHashEntry is a pointer to the entry, which may be used to retrieve and
modify the entry's value or to delete the entry from the
table.

Tcl_DeleteHashEntry will remove an existing entry from a table. The memory associated with the entry itself will
be freed, but the client is responsible for any cleanup
associated with the entry's value, such as freeing a
structure that it points to.

Tcl_FindHashEntry is similar to Tcl_CreateHashEntry except that it doesn't create a new entry if the key doesn't
exist; instead, it returns NULL as result.

Tcl_GetHashValue and Tcl_SetHashValue are used to read and write an entry's value, respectively. Values are stored
and retrieved as type ``ClientData'', which is large
enough to hold a pointer value. On almost all machines
this is large enough to hold an integer value too.

Tcl_GetHashKey returns the key for a given hash table entry, either as a pointer to a string, a one-word (``char
*'') key, or as a pointer to the first word of an array of
integers, depending on the keyType used to create a hash table. In all cases Tcl_GetHashKey returns a result with type ``char *''. When the key is a string or array, the
result of Tcl_GetHashKey points to information in the table entry; this information will remain valid until the
entry is deleted or its table is deleted.

Tcl_FirstHashEntry and Tcl_NextHashEntry may be used to scan all of the entries in a hash table. A structure of
type ``Tcl_HashSearch'', provided by the client, is used
to keep track of progress through the table.
Tcl_FirstHashEntry initializes the search record and returns the first entry in the table (or NULL if the table
is empty). Each subsequent call to Tcl_NextHashEntry returns the next entry in the table or NULL if the end of
the table has been reached. A call to Tcl_FirstHashEntry followed by calls to Tcl_NextHashEntry will return each of the entries in the table exactly once, in an arbitrary
order. It is unadvisable to modify the structure of the
table, e.g. by creating or deleting entries, while the
search is in progress.

Tcl_HashStats returns a dynamically-allocated string with overall information about a hash table, such as the number
of entries it contains, the number of buckets in its hash
array, and the utilization of the buckets. It is the
caller's responsibility to free the result string by pass
ing it to ckfree.

The header file tcl.h defines the actual data structures
used to implement hash tables. This is necessary so that
clients can allocate Tcl_HashTable structures and so that
macros can be used to read and write the values of
entries. However, users of the hashing routines should
never refer directly to any of the fields of any of the
hash-related data structures; use the procedures and
macros defined here.

THE TCL_HASHKEYTYPE STRUCTURE

Extension writers can define new hash key types by defin
ing four procedures, initializing a Tcl_HashKeyType struc
ture to describe the type, and calling Tcl_InitCus tomHashTable. The Tcl_HashKeyType structure is defined as follows:

typedef struct Tcl_HashKeyType {

int version;
int flags;
Tcl_HashKeyProc *hashKeyProc;
Tcl_CompareHashKeysProc *compareKeysProc;
Tcl_AllocHashEntryProc *allocEntryProc;
Tcl_FreeHashEntryProc *freeEntryProc;

} Tcl_HashKeyType;

The version member is the version of the table. If this structure is extended in future then the version can be
used to distinguish between different structures. It
should be set to TCL_HASH_KEY_TYPE_VERSION.

The flags member is one or more of the following values
OR'ed together:

TCL_HASH_KEY_RANDOMIZE_HASH

There are some things, pointers
for example which don't hash well
because they do not use the lower
bits. If this flag is set then
the hash table will attempt to
rectify this by randomising the
bits and then using the upper N
bits as the index into the table.

The hashKeyProc member contains the address of a function called to calculate a hash value for the key.

typedef unsigned int (Tcl_HashKeyProc) (

Tcl_HashTable *tablePtr,
VOID *keyPtr);

If this is NULL then keyPtr is used and TCL_HASH_KEY_RAN DOMIZE_HASH is assumed.

The compareKeysProc member contains the address of a func tion called to compare two keys.

typedef int (Tcl_CompareHashKeysProc) (VOID

*keyPtr,

Tcl_HashEntry *hPtr);

If this is NULL then the keyPtr pointers are compared. If the keys don't match then the function returns 0, other
wise it returns 1.

The allocEntryProc member contains the address of a func tion called to allocate space for an entry and initialise
the key.

typedef Tcl_HashEntry *(Tcl_AllocHashEntryProc) (

Tcl_HashTable *tablePtr, VOID *keyPtr);

If this is NULL then Tcl_Alloc is used to allocate enough
space for a Tcl_HashEntry and the key pointer is assigned
to key.oneWordValue. String keys and array keys use this
function to allocate enough space for the entry and the
key in one block, rather than doing it in two blocks. This
saves space for a pointer to the key from the entry and
another memory allocation. Tcl_Obj * keys use this func
tion to allocate enough space for an entry and increment
the reference count on the object. If

The freeEntryProc member contains the address of a func tion called to free space for an entry.

typedef void (Tcl_FreeHashEntryProc) (Tcl_HashEntry

*hPtr);

If this is NULL then Tcl_Free is used to free the space
for the entry. Tcl_Obj * keys use this function to decre
ment the reference count on the object.

KEYWORDS

hash table, key, lookup, search, value

Tcl Tcl_Hash(3)