Tcl_FindHashEntry(3tcl)
Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)
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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.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 structure
(for all procedures
but
Tcl_InitHashTable,
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Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)
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_CUSTOM_TYPE_KEYS,
TCL_CUSTOM_PTR_KEYS,
or an integer value
greater than 1.
Tcl_HashKeyType *typePtr (in) Address of struc-
ture 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 Client-
Data, 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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Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)
DESCRIPTION
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
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Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)
are passed to hashing routines and
stored in hash table entries as
"char *" values. The pointer value
is the key; it need not (and usu-
ally 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 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
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.
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Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)
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 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 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 inadvis-
able 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.
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Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)
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 pro- |
vided by the operating system when |
allocating and freeing memory used |
to store the hash table data struc- |
tures, and not any of Tcl's own |
customized memory allocation rou- |
tines. 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 |
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Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)
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 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 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 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
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