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Tcl_Hash(3)                                Tcl Library Procedures                                Tcl_Hash(3)



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NAME
       Tcl_InitHashTable,  Tcl_InitCustomHashTable,  Tcl_InitObjHashTable,  Tcl_DeleteHashTable, Tcl_Create-HashEntry, Tcl_CreateHashEntry,
       HashEntry,  Tcl_DeleteHashEntry,  Tcl_FindHashEntry,  Tcl_GetHashValue,  Tcl_SetHashValue,  Tcl_GetH-ashKey, Tcl_GetHashKey,
       ashKey, 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 previ-ous previous
                                                ous 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 key-Type keyType
                                                Type 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 enu-merating enumerating
                                                merating 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.   Typi-cally Typically
       cally 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_Ini-tObjHashTable Tcl_InitObjHashTable
       tObjHashTable and Tcl_InitCustomHashTable.

       Tcl_InitHashTable initializes a structure that describes a new hash table.  The space for the  struc-ture structure
       ture 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 val-ues: values:
       ues:

       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 compar-ison comparison
                                ison 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  struc-tures structures
       tures 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 oneword
       word ("char *") key, or as a pointer to the first word of an array of integers, depending on the key-Type keyType
       Type  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  utiliza-tion utilization
       tion  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 nec-essary necessary
       essary  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_HashKey-Type Tcl_HashKeyType
       Type 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 ver-sion version
       sion   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 implemen- |
                                tation  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 alloca-tion. allocation.
       tion. Tcl_Obj* keys use this function to allocate enough space for an entry and increment the  refer-ence reference
       ence 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 func-tion function
       tion to decrement the reference count on the object.

KEYWORDS
       hash table, key, lookup, search, value



Tcl                                                                                              Tcl_Hash(3)