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)

void *
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
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
behavior of the hash
table.

const void *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 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
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 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 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 inadvisable to modify the structure of the table, e.g. by
creating 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
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 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
allocation procedures provided by the
operating system 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 custom 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 reference count on the value.

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 value.

KEYWORDS

hash table, key, lookup, search, value

Tcl Tcl_Hash(3)