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HASHINIT(9)                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
     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.

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
     failure.

CODE REFERENCES
     These functions are implemented in sys/kern/subr_hash.c.

SEE ALSO
     queue(3), hash(9), malloc(9)

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()
     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.

BUGS
     The only part of the work of implementing a hash table that these
     functions relieve is the part that isn't much work.

NetBSD 7.1.2                     July 1, 2008                     NetBSD 7.1.2