Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)______________________________________________________________________________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)
char *
Tcl_HashStats(tablePtr)ARGUMENTS
Tcl_HashTable *tablePtr (in) Address of hash table struc‐
ture (for all procedures but
Tcl_InitHashTable, this must
have been 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_CUS‐
TOM_TYPE_KEYS, TCL_CUS‐
TOM_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 ta‐
ble. 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 enu‐
merating 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_InitCus‐
tomHashTable.
Tcl_InitHashTable initializes a structure that describes a new hash ta‐
ble. 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_CUS‐
TOM_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 does
not) actually point to a string.
TCL_CUSTOM_TYPE_KEYS Keys are of arbitrary type, and are stored in
the entry. Hashing and comparison is deter‐
mined 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 compari‐
son is determined by typePtr. The Tcl_HashKey‐
Type 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 val‐
ues of the hash table entries. If the entry values point to dynami‐
cally-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 was not 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 does
not 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 informa‐
tion 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”, pro‐
vided by the client, is used to keep track of progress through the ta‐
ble. Tcl_FirstHashEntry initializes the search record and returns the
first entry in the table (or NULL if the table is empty). Each subse‐
quent 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 inadvisable to modify the structure of the table, e.g. by creat‐
ing or deleting entries, while the search is in progress, with the
exception of deleting the entry returned by Tcl_FirstHashEntry or
Tcl_NextHashEntry.
Tcl_HashStats returns a dynamically-allocated string with overall
information about a hash table, such as the number of entries it con‐
tains, 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 imple‐
ment 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 proce‐
dures, 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 0 or one or more of the following values OR'ed
together:
TCL_HASH_KEY_RANDOMIZE_HASH
There are some things, pointers for example
which do not 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
randomizing the bits and then using the upper
N bits as the index into the table.
TCL_HASH_KEY_SYSTEM_HASH This flag forces Tcl to use the memory alloca‐ │
tion procedures provided by the operating sys‐ │
tem when allocating and freeing memory used to │
store the hash table data structures, and not │
any of Tcl's own customized memory allocation │
routines. This is important if the hash table │
is to be used in the implementation of a cus‐ │
tom set of allocation routines, or something │
that a custom set of allocation routines might │
depend on, in order to avoid any circular │
dependency.
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 do
not 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 initialize the key and clientData.
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, the key pointer is assigned to key.oneWordValue and the
clientData is set to NULL. 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 refer‐
ence count on the object.
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
TclTcl_Hash(3)
