)
is the library interface to database files.
One of the supported file formats is hash files.
The general description of the database access methods is in
dbopen(3),
this manual page describes only the hash specific information.
The hash data structure is an extensible, dynamic hashing scheme.
The access method specific data structure provided to
dbopen()
is defined in the
<
db.h
>
include file as follows:
typedef struct {
u_int bsize;
u_int ffactor;
u_int nelem;
u_int cachesize;
uint32_t (*hash)(const void *, size_t);
int lorder;
} HASHINFO;
The elements of this structure are as follows:
bsize
bsize
defines the hash table bucket size, and defaults to 4096 for in-memory tables.
If
bsize
is 0 (no bucket size is specified) a bucket size is chosen based on the
underlying file system I/O block size.
It may be preferable to increase the page size for disk-resident
tables and tables with large data items.
ffactor
ffactor
indicates a desired density within the hash table.
It is an approximation of the number of keys allowed to accumulate in
any one bucket, determining when the hash table grows or shrinks.
The default value is 8.
nelem
nelem
is an estimate of the final size of the hash table.
If not set or set too low, hash tables will expand gracefully as keys
are entered, although a slight performance degradation may be
noticed.
The default value is 1.
cachesize
hash
hash
is a user defined hash function.
Since no hash function performs equally well on all possible data, the
user may find that the built-in hash function does poorly on a
particular data set.
User specified hash functions must take two arguments (a pointer to a
byte string and a length) and return a 32-bit quantity to be used as
the hash value.
lorder
lorder
is 0 (no order is specified) the current host order is used.
If the file already exists, the specified value is ignored and the
value specified when the tree was created is used.
If the file already exists (and the
O_TRUNC
flag is not specified), the values specified for the parameters
bsize
,
ffactor
,
lorder
,
and
nelem
are ignored and the values specified when the tree was created are
used.
If a hash function is specified,
hash_open()
will attempt to determine if the hash function specified is the same
as the one with which the database was created, and will fail if it is
not.