Provided by: tcl8.4-doc_8.4.19-4ubuntu3_all bug

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)

       CONST 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  doesn't)
                                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 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  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 unadvisable to  modify  the  structure  of  the  table,  e.g.   by
       creating 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 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 one or more of the following values OR'ed together:

       TCL_HASH_KEY_RANDOMIZE_HASH
                                There are some things, pointers for example which don't 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  randomising  the
                                bits and then using the upper N bits as the index into the table.

       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 don't 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 initialise the key.
              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  and
       the  key  pointer  is  assigned  to key.oneWordValue.  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.  If

       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