HASHINIT(9)            FreeBSD Kernel Developer's Manual           HASHINIT(9)

NAME
     hashinit, hashinit_flags, hashdestroy, phashinit, phashinit_flags -
     manage kernel hash tables

SYNOPSIS
     #include <sys/malloc.h>
     #include <sys/systm.h>
     #include <sys/queue.h>

     void *
     hashinit(int nelements, struct malloc_type *type, u_long *hashmask);

     void
     hashinit_flags(int nelements, struct malloc_type *type, u_long *hashmask,
         int flags);

     void
     hashdestroy(void *hashtbl, struct malloc_type *type, u_long hashmask);

     void *
     phashinit(int nelements, struct malloc_type *type, u_long *nentries);

     phashinit_flags(int nelements, struct malloc_type *type,
         u_long *nentries, int flags);

DESCRIPTION
     The hashinit(), hashinit_flags(), phashinit() and phashinit_flags()
     functions allocate space for hash tables of size given by the argument
     nelements.

     The hashinit() function allocates hash tables that are sized to largest
     power of two less than or equal to argument nelements.  The phashinit()
     function allocates hash tables that are sized to the largest prime number
     less than or equal to argument nelements.  The hashinit_flags() function
     operates like hashinit() but also accepts an additional argument flags
     which control various options during allocation.  phashinit_flags()
     function operates like phashinit() but also accepts an additional
     argument flags which control various options during allocation.
     Allocated hash tables are contiguous arrays of LIST_HEAD(3) entries,
     allocated using malloc(9), and initialized using LIST_INIT(3).  The
     malloc arena to be used for allocation is pointed to by argument type.

     The hashdestroy() function frees the space occupied by the hash table
     pointed to by argument hashtbl.  Argument type determines the malloc
     arena to use when freeing space.  The argument hashmask should be the bit
     mask returned by the call to hashinit() that allocated the hash table.
     The argument flags must be used with one of the following values.

           HASH_NOWAIT      Any malloc performed by the hashinit_flags() and
                            phashinit_flags() function will not be allowed to
                            wait, and therefore may fail.
           HASH_WAITOK      Any malloc performed by hashinit_flags() and
                            phashinit_flags() function is allowed to wait for
                            memory.  This is also the behavior of hashinit()
                            and phashinit().

IMPLEMENTATION NOTES
     The largest prime hash value chosen by phashinit() is 32749.

RETURN VALUES
     The hashinit() function returns a pointer to an allocated hash table and
     sets the location pointed to by hashmask to the bit mask to be used for
     computing the correct slot in the hash table.

     The phashinit() function returns a pointer to an allocated hash table and
     sets the location pointed to by nentries to the number of rows in the
     hash table.

EXAMPLES
     A typical example is shown below:

           ...
           static LIST_HEAD(foo, foo) *footable;
           static u_long foomask;
           ...
           footable = hashinit(32, M_FOO, &foomask);

     Here we allocate a hash table with 32 entries from the malloc arena
     pointed to by M_FOO.  The mask for the allocated hash table is returned
     in foomask.  A subsequent call to hashdestroy() uses the value in
     foomask:

           ...
           hashdestroy(footable, M_FOO, foomask);

DIAGNOSTICS
     The hashinit() and phashinit() functions will panic if argument nelements
     is less than or equal to zero.

     The hashdestroy() function will panic if the hash table pointed to by
     hashtbl is not empty.

SEE ALSO
     LIST_HEAD(3), malloc(9)

BUGS
     There is no phashdestroy() function, and using hashdestroy() to free a
     hash table allocated by phashinit() usually has grave consequences.

FreeBSD 13.1-RELEASE-p6         April 29, 2016         FreeBSD 13.1-RELEASE-p6