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NAME
Tcl_InitHashTable, 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_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 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,
or an integer value
greater than 1.
char *key (in) Key to use for probe
into table. Exact
form depends on key-
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
enumerating all the
entries in a hash
table.
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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 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 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.
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. KeyType must have one of the following values:
TCL_STRING_KEYS Keys are null-terminated ASCII
strings. They are passed to hash-
ing 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
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 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 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 unadvis-
able 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.
KEYWORDS
hash table, key, lookup, search, value
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