Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)
____________________________________________________________________________________________________________
NAME
Tcl_InitHashTable, Tcl_InitCustomHashTable, Tcl_InitObjHashTable, Tcl_DeleteHashTable, Tcl_Create-HashEntry, Tcl_CreateHashEntry,
HashEntry, Tcl_DeleteHashEntry, Tcl_FindHashEntry, Tcl_GetHashValue, Tcl_SetHashValue, Tcl_GetH-ashKey, Tcl_GetHashKey,
ashKey, 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 previ-ous previous
ous 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 keyType
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 enumer-ating enumerating
ating 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. Typi-cally Typically
cally 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_Ini-tObjHashTable Tcl_InitObjHashTable
tObjHashTable and Tcl_InitCustomHashTable.
Tcl_InitHashTable initializes a structure that describes a new hash table. The space for the struc-ture structure
ture 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 val-ues: values:
ues:
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 compar-ison comparison
ison 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 key-Type keyType
Type 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 struc-tures structures
tures 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 oneword
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_FirstHashEn-try Tcl_FirstHashEntry
try 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 utiliza-tion utilization
tion 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 nec-essary necessary
essary 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 ver-sion version
sion 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 func-tion function
tion to decrement the reference count on the object.
KEYWORDS
hash table, key, lookup, search, value
Tcl Tcl_Hash(3)
|