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Tcl_Hash(3)		    Tcl Library Procedures		   Tcl_Hash(3)


       Tcl_InitHashTable,	Tcl_DeleteHashTable,	  Tcl_CreateHashEntry,
       Tcl_DeleteHashEntry, Tcl_FindHashEntry, Tcl_GetHashValue,  Tcl_SetHash‐
       Value, Tcl_GetHashKey, Tcl_FirstHashEntry, Tcl_NextHashEntry, Tcl_Hash‐
       Stats - procedures to manage hash tables

       #include <tcl.h>

       Tcl_InitHashTable(tablePtr, keyType)


       Tcl_HashEntry *
       Tcl_CreateHashEntry(tablePtr, key, newPtr)


       Tcl_HashEntry *
       Tcl_FindHashEntry(tablePtr, key)


       Tcl_SetHashValue(entryPtr, value)

       char *
       Tcl_GetHashKey(tablePtr, entryPtr)

       Tcl_HashEntry *
       Tcl_FirstHashEntry(tablePtr, searchPtr)

       Tcl_HashEntry *

       char *

       Tcl_HashTable	*tablePtr    (in)      Address of hash table structure
					       (for    all    procedures   but
					       Tcl_InitHashTable,  this	  must
					       have been initialized by previ‐
					       ous call to Tcl_InitHashTable).

       int		keyType	     (in)      Kind of keys  to	 use  for  new
					       hash  table.   Must  be	either
					       TCL_ONE_WORD_KEYS,  or an inte‐
					       ger value greater than 1.

       char		*key	     (in)      Key to use for probe  into  ta‐
					       ble.   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 ta‐
					       ble 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 enu‐
					       merating all the entries	 in  a
					       hash table.

       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 three forms:	 strings, one-
       word values, or integer arrays.	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  man‐
       aged  entirely  by  clients,  not by the hash module itself.  Typically
       each entry's value is a pointer to a data structure managed  by	client

       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

       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.  KeyType must have one of the fol‐
       lowing values:

       TCL_STRING_KEYS		Keys are null-terminated ASCII 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.

       other			If   keyType   is   not	  TCL_STRING_KEYS   or
				TCL_ONE_WORD_KEYS,  then it must be an integer
				value greater than 1.  In this case  the  keys
				will  be  arrays of ``int'' values, where key‐
				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 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 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 (``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

       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 unadvisable to modify the structure of the table, e.g.  by creat‐
       ing 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  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 free.

       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-
       related data structures; use the procedures and macros defined here.

       hash table, key, lookup, search, value

Tcl								   Tcl_Hash(3)

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