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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  pro-
						      cedures		   but
						      Tcl_InitHashTable,  this
						      must  have been initial-
						      ized 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	behav-
						      ior 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 en-
						      try 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  Client-
						      Data.

       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 en-
       try'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 ta-
       bles,   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  en-
       tries.	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 en-
       try'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  ex-
       ception	of  deleting  the  entry  returned  by	Tcl_FirstHashEntry  or
       Tcl_NextHashEntry.

       Tcl_HashStats returns a dynamically-allocated string with  overall  in-
       formation  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-re-
       lated 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  to-
       gether:

       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 de-
				pendency.

       The hashKeyProc member contains the address of  a  function  called  to
       calculate a hash	value for the key.

	      typedef TCL_HASH_TYPE 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 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)

NAME | SYNOPSIS | ARGUMENTS | DESCRIPTION | THE TCL_HASHKEYTYPE STRUCTURE | KEYWORDS

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