freebsd-skq/share/man/man9/hashinit.9

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.Dd October 10, 2004
.Dt HASHINIT 9
.Os
.Sh NAME
.Nm hashinit , hashinit_flags , hashdestroy , phashinit
.Nd manage kernel hash tables
.Sh SYNOPSIS
.In sys/malloc.h
.In sys/systm.h
.In sys/queue.h
.Ft "void *"
.Fn hashinit "int nelements" "struct malloc_type *type" "u_long *hashmask"
.Ft void
.Fo hashinit_flags
.Fa "int nelements" "struct malloc_type *type" "u_long *hashmask" "int flags"
.Fc
.Ft void
.Fn hashdestroy "void *hashtbl" "struct malloc_type *type" "u_long hashmask"
.Ft "void *"
.Fn phashinit "int nelements" "struct malloc_type *type" "u_long *nentries"
.Sh DESCRIPTION
The
.Fn hashinit ,
.Fn hashinit_flags
and
.Fn phashinit
functions allocate space for hash tables of size given by the argument
.Fa nelements .
.Pp
The
.Fn hashinit
function allocates hash tables that are sized to largest power of two
less than or equal to argument
.Fa nelements .
The
.Fn phashinit
function allocates hash tables that are sized to the largest prime
number less than or equal to argument
.Fa nelements .
The
.Fn hashinit_flags
function operates like
.Fn hashinit
but also accepts an additional argument
.Fa flags
which control various options during allocation.
Allocated hash tables are contiguous arrays of
.Xr LIST_HEAD 3
entries, allocated using
.Xr malloc 9 ,
and initialized using
.Xr LIST_INIT 3 .
The malloc arena to be used for allocation is pointed to by argument
.Fa type .
.Pp
The
.Fn hashdestroy
function frees the space occupied by the hash table pointed to by argument
.Fa hashtbl .
Argument
.Fa type
determines the malloc arena to use when freeing space.
The argument
.Fa hashmask
should be the bit mask returned by the call to
.Fn hashinit
that allocated the hash table.
The argument
.Fa flags
must be used with one of the following values.
.Pp
.Bl -tag -width ".Dv HASH_NOWAIT" -offset indent -compact
.It Dv HASH_NOWAIT
Any malloc performed by the
.Fn hashinit_flags
function will not be allowed to wait, and therefore may fail.
.It Dv HASH_WAITOK
Any malloc performed by the
.Fn hashinit_flags
function is allowed to wait for memory.
.El
.Sh IMPLEMENTATION NOTES
The largest prime hash value chosen by
.Fn phashinit
is 32749.
.Sh RETURN VALUES
The
.Fn hashinit
function returns a pointer to an allocated hash table and sets the
location pointed to by
.Fa hashmask
to the bit mask to be used for computing the correct slot in the
hash table.
.Pp
The
.Fn phashinit
function returns a pointer to an allocated hash table and sets the
location pointed to by
.Fa nentries
to the number of rows in the hash table.
.Sh EXAMPLES
A typical example is shown below:
.Bd -literal -offset indent
\&...
static LIST_HEAD(foo, foo) *footable;
static u_long foomask;
\&...
footable = hashinit(32, M_FOO, &foomask);
.Ed
.Pp
Here we allocate a hash table with 32 entries from the malloc arena
pointed to by
.Dv M_FOO .
The mask for the allocated hash table is returned in
.Va foomask .
A subsequent call to
.Fn hashdestroy
uses the value in
.Va foomask :
.Bd -literal -offset indent
\&...
hashdestroy(footable, M_FOO, foomask);
.Ed
.Sh DIAGNOSTICS
The
.Fn hashinit
and
.Fn phashinit
functions will panic if argument
.Fa nelements
is less than or equal to zero.
.Pp
The
.Fn hashdestroy
function will panic if the hash table
pointed to by
.Fa hashtbl
is not empty.
.Sh SEE ALSO
.Xr LIST_HEAD 3 ,
.Xr malloc 9
.Sh BUGS
There is no
.Fn phashdestroy
function, and using
.Fn hashdestroy
to free a hash table allocated by
.Fn phashinit
usually has grave consequences.