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Tcl_FindHashEntry(3tcl)




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 -  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.

Tcl                       Last change:                          5

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

Tcl                       Last change:                          7


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