/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.