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 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 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
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