Tcl_Hash(3Tcl) Tcl_Hash(3Tcl)
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
#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)
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, or an integer
value greater than 1.
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Tcl_Hash(3Tcl) Tcl_Hash(3Tcl)
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.
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 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 hashing routines using the address
of the first character of the string.
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Tcl_Hash(3Tcl) Tcl_Hash(3Tcl)
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 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
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Tcl_Hash(3Tcl) Tcl_Hash(3Tcl)
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 free.
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.
hash table, key, lookup, search, value
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HASH(3) UNIX System V (August 18, 1994) HASH(3)
NAME [Toc] [Back]
hash - hash database access method
SYNOPSIS [Toc] [Back]
#include <sys/types.h>
#include <db.h>
DESCRIPTION [Toc] [Back]
The routine dbopen is the library interface to database
files. One of the supported file formats is hash files.
The general description of the database access methods is in
dbopen(3), this manual page describes only the hash specific
information.
The hash data structure is an extensible, dynamic hashing
scheme.
The access method specific data structure provided to dbopen
is defined in the <db.h> include file as follows:
typedef struct {
u_int bsize;
u_int ffactor;
u_int nelem;
u_int cachesize;
u_int32_t (*hash)(const void *, size_t);
int lorder;
} HASHINFO;
The elements of this structure are as follows:
bsize
Bsize defines the hash table bucket size, and is, by
default, 256 bytes. It may be preferable to increase
the page size for disk-resident tables and tables with
large data items.
ffactor
Ffactor indicates a desired density within the hash
table. It is an approximation of the number of keys
allowed to accumulate in any one bucket, determining
when the hash table grows or shrinks. The default
value is 8.
nelem
Nelem is an estimate of the final size of the hash
table. If not set or set too low, hash tables will
expand gracefully as keys are entered, although a
slight performance degradation may be noticed. The
default value is 1.
cachesize
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HASH(3) UNIX System V (August 18, 1994) HASH(3)
A suggested maximum size, in bytes, of the memory
cache. This value is only advisory, and the access
method will allocate more memory rather than fail.
hash Hash is a user defined hash function. Since no hash
function performs equally well on all possible data,
the user may find that the built-in hash function does
poorly on a particular data set. User specified hash
functions must take two arguments (a pointer to a byte
string and a length) and return a 32-bit quantity to be
used as the hash value.
lorder
The byte order for integers in the stored database
metadata. The number should represent the order as an
integer; for example, big endian order would be the
number 4,321. If lorder is 0 (no order is specified)
the current host order is used. If the file already
exists, the specified value is ignored and the value
specified when the tree was created is used.
If the file already exists (and the O_TRUNC flag is not
specified), the values specified for the parameters bsize,
ffactor, lorder and nelem are ignored and the values
specified when the tree was created are used.
If a hash function is specified, hash_open will attempt to
determine if the hash function specified is the same as the
one with which the database was created, and will fail if it
is not.
Backward compatible interfaces to the routines described in
dbm(3), and ndbm(3) are provided, however these interfaces
are not compatible with previous file formats.
ERRORS [Toc] [Back]
The hash access method routines may fail and set errno for
any of the errors specified for the library routine
dbopen(3).
SEE ALSO [Toc] [Back]
btree(3), dbopen(3), mpool(3), recno(3)
Dynamic Hash Tables, Per-Ake Larson, Communications of the
ACM, April 1988.
A New Hash Package for UNIX, Margo Seltzer, USENIX
Proceedings, Winter 1991.
BUGS [Toc] [Back]
Only big and little endian byte order is supported.
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HASH(3) UNIX System V (August 18, 1994) HASH(3)
This version of berkeley db (1.85) is free software which is
not developed nor maintained by SGI. It is known to have
some bugs that are unlikely to get fixed (See NOTES below)
in particular, the following hash operations are known to
have problems, up to corrupting databases, and should be
avoided according to http://www.sleepycat.com/db.185.html:
o Overwriting or deleting overflow hash key/data pairs
(pairs with items larger than the page size).
o Intermixing hash cursor operations with deletes.
NOTES [Toc] [Back]
The default hash function in this version of db is the
Fowler/Vo/Noll hash which gives better distributions (less
collisions) on average than the publicly released version.
This version of berkeley db is 1.85. A newer enhanced
version db-2.x requires licensing. The db 2.x version is
more stable, faster, and supports concurrent accesses plus
many other new features. Check out
http://www.sleepycat.com/ for details.
Page 3 (printed 4/30/98)
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