HASHINIT(9) BSD Kernel Developer's Manual HASHINIT(9)NAME
hashinit, hashdone — kernel hash table construction and destruction
SYNOPSIS
#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);
DESCRIPTION
The 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 fac‐
tor. The LIST... or TAILQ... macros of queue(3) can be used to manipu‐
late 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 avail‐
able.
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.
RETURN VALUES
The value returned by hashinit() should be cast as pointer to an array of
LIST_HEAD or TAILQ_HEAD as appropriate. hashinit() returns NULL on fail‐
ure.
SEE ALSOqueue(3), hash(9), malloc(9)CODE REFERENCES
These functions are implemented in sys/kern/subr_hash.c.
HISTORY
A 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() sup‐
ported 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() compa‐
rable (as of OpenBSD 3.9) to that of NetBSD 1.4. This manual page was
added for NetBSD 4.0.
BUGS
The only part of the work of implementing a hash table that these func‐
tions relieve is the part that isn't much work.
BSD July 1, 2008 BSD
