Provided by: tcl8.4-doc_8.4.20-8_all 
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 -
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
procedures 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_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 keyType 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.
_________________________________________________________________
DESCRIPTION
A hash table consists of zero or more entries, each consisting 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 following 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 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 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 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 passing 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 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 function 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 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
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 function to allocate 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