HASHINIT(9) | Kernel Developer's Manual | HASHINIT(9) |
hashinit
, hashdone
—
#include <sys/systm.h>
void *
hashinit
(u_int chains,
enum hashtype htype, bool
waitok, u_long *hashmask);
void
hashdone
(void
*hashtbl, enum hashtype
htype, u_long
hashmask);
hashinit
() function allocates and initializes space
for a simple chaining hash table. The number of slots will be the least power
of two not smaller than chains. The customary choice for
chains is the maximum number of elements you intend to
store divided by your intended load factor. The
LIST...
or TAILQ...
macros of
queue(3) can be used to
manipulate the chains; pass HASH_LIST
or
HASH_TAILQ
as htype to indicate
which. Each slot will be initialized as the head of an empty chain of the
proper type. Because different data structures from
queue(3) can define head
structures of different sizes, the total size of the allocated table can vary
with the choice of htype.
If waitok is true, hashinit can wait until enough memory is available. Otherwise, it immediately fails if there is not enough memory is available.
A value will be stored into *hashmask suitable for masking any computed hash, to obtain the index of a chain head in the allocated table.
The hashdone
() function deallocates the
storage allocated by hashinit
() and pointed to by
hashtbl, given the same htype
and hashmask that were passed to and returned from
hashinit
(). If the table contains any nonempty chain
when hashdone
() is called, the result is
undefined.
hashinit
() should be cast as
pointer to an array of LIST_HEAD
or
TAILQ_HEAD
as appropriate.
hashinit
() returns NULL
on
failure.
hashinit
() function was present, without the
htype or mflags arguments, in
4.4BSD-Alpha. It was independent of
queue(3) and simply allocated and
nulled a table of pointer-sized slots. It sized the table to the
largest power of two not greater than
chains; that is, it built in a load factor between 1 and
2.
NetBSD 1.0 was the first
NetBSD release to have a
hashinit
() function. It resembled that from
4.4BSD but made each slot a
LIST_HEAD
from
queue(3). For
NetBSD 1.3.3 it had been changed to size the table
to the least power of two not less than or equal to
chains. By NetBSD 1.4 it had
the mflags argument and the current sizing rule.
NetBSD 1.5 had the
hashdone
() function. By NetBSD
1.6 hashinit
() supported
LIST
or TAILQ
chains
selected with htype.
FreeBSD has a
hashinit
() with behavior equivalent (as of
FreeBSD 6.1) to that in NetBSD
1.0, and a hashdestroy
() that behaves as
hashdone
() but checks that all chains are empty
first. OpenBSD has a
hashinit
() comparable (as of
OpenBSD 3.9) to that of NetBSD
1.4. This manual page was added for NetBSD
4.0.
July 1, 2008 | NetBSD 9.0 |