Tcl_Hash(3) Tcl Library Procedures Tcl_Hash(3)______________________________________________________________________________NAME
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
Tcl_CreateHashEntry(tablePtr, key, newPtr)
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
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
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‐
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