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Tcl Library Procedures Tcl_Hash(3TCL)

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NAME

Tcl_InitHashTable, Tcl_InitCustomHashTable,

Tcl_InitObjHashTable, Tcl_DeleteHashTable,

Tcl_CreateHashEntry, Tcl_DeleteHashEntry, Tcl_FindHashEntry,

Tcl_GetHashValue, Tcl_SetHashValue, Tcl_GetHashKey,

Tcl_FirstHashEntry, Tcl_NextHashEntry, Tcl_HashStats - pro-

cedures to manage hash tables

SYNOPSIS

#include

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 procedures but

Tcl_InitHashTable,

this must have been Tcl Last change: 1

Tcl Library Procedures Tcl_Hash(3TCL)

initialized by pre-

vious 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_CUSTOM_TYPE_KEYS,

TCL_CUSTOM_PTR_KEYS,

or an integer value greater than 1.

Tcl_HashKeyType *typePtr (in) Address of structure

which defines the behaviour of the hash table. CONST char *key (in) Key to use for probe into table. Exact

form depends on key-

Type used to create table. int *newPtr (out) The word at *newPtr 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 ClientData.

Tcl_HashSearch *searchPtr (in) Pointer to record to

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

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DESCRIPTION

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Tcl Library Procedures Tcl_Hash(3TCL)

A hash table consists of zero or more entries, each consist-

ing 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 keyType 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_InitCustomHashTable 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 follow-

ing values:

TCL_STRING_KEYS Keys are null-terminated 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 routines and Tcl Last change: 3

Tcl Library Procedures Tcl_Hash(3TCL)

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 struc-

ture 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 deter-

mined 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 memory is

assumed to be managed by the client. Tcl_DeleteHashTable

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

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Tcl Library Procedures Tcl_Hash(3TCL)

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 associ-

ated 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 unadvis-

able 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 passing it to ckfree. The header file tcl.h defines the actual data structures used to implement hash tables. This is necessary so that Tcl Last change: 5

Tcl Library Procedures Tcl_Hash(3TCL)

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 defining

four procedures, initializing a Tcl_HashKeyType structure to

describe the type, and calling Tcl_InitCustomHashTable. 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_RANDOMIZE_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, otherwise Tcl Last change: 6

Tcl Library Procedures Tcl_Hash(3TCL)

it returns 1. The allocEntryProc member contains the address of a function 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 func-

tion 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 function to allo-

cate enough space for an entry and increment the reference count on the object. If The freeEntryProc member contains the address of a function 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 decrement the

reference count on the object. KEYWORDS hash table, key, lookup, search, value

ATTRIBUTES

See attributes(5) for descriptions of the following attri-

butes:

_______________________________________

| ATTRIBUTE TYPE | ATTRIBUTE VALUE|

|____________________|__________________|_

| Availability | runtime/tcl-8 |

|____________________|__________________|_

| Interface Stability| Uncommitted |

|____________________|_________________|

NOTES Source for Tcl is available on http://opensolaris.org. Tcl Last change: 7