hashinit(9) - NetBSD Manual Pages

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HASHINIT(9)            NetBSD 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 ALSO

     queue(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.

NetBSD 6.0.6                     July 1, 2008                     NetBSD 6.0.6