NetBSD/lib/libc/db/db.3

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.\"	@(#)db.3	5.16 (Berkeley) 4/2/91
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.TH DB 3  "April 2, 1991"
.UC 7
.SH NAME
btree_open, hash_open, recno_open \- database access methods
.SH SYNOPSIS
.nf
.ft B
#include <sys/types.h>
#include <db.h>

DB *
btree_open(const char *file, int flags, int mode,
.ti +5
const BTREEINFO * openinfo);

DB *
hash_open(const char *file, int flags, int mode,
.ti +5
const HASHINFO * openinfo);

DB *
recno_open(const char *file, int flags, int mode,
.ti +5
const RECNOINFO * openinfo);
.ft R
.fi
.SH DESCRIPTION
.IR Btree_open ,
.IR hash_open ,
and
.I recno_open
are access method interfaces to database files in btree, hashed, and
flat-file formats, respectively.
The btree format is a representation of a sorted, balanced tree structure.
The hashed format is an extensible, dynamic hashing scheme.
The flat-file format is a UNIX file with fixed or variable length
lines.
These formats are described in more detail below.
.PP
Access to all file types is based on key/data pairs.
.PP
Each routine opens
.I file
for reading and/or writing.
Databases never intended to be preserved on disk may be created by setting
the file parameter to NULL.
The
.I flags
and
.I mode arguments
are as specified to the
.IR open (2)
routine, however, only the O_CREAT, O_EXCL, O_RDONLY, O_RDWR, O_TRUNC
and O_WRONLY flags are meaningful.
The argument
.I openinfo
is a pointer to an access method specific structure described below.
.PP
The open routines return a pointer to a DB structure on success and NULL
on error.
The DB structure contains at least the following fields:
.sp
.nf
typedef struct {
.RS
int (*close)(const DB *db);
int (*sync)(const DB *db);
int (*del)(const DB *db, const DBT *key, u_int flags);
int (*get)(const DB *db, DBT *key, DBT *data, u_int flags);
int (*put)(const DB *db, const DBT *key, const DBT *data,
.ti +5
u_int flags);
int (*seq)(const DB *db, DBT *key, DBT *data, u_int flags);
int type;
void *openinfo;
.RE
} DB;
.fi
.PP
The elements of this structure consist of a pointer to an access method
specific structure and a set of routines which perform various functions.
All of these routines take a pointer to a structure as returned by
one of the open routines, one or more pointers to key/data structures,
and, optionally, a flag value.
.TP
openinfo
A pointer to an internal structure specific to the access method.
.TP
type
The type of the underlying access method; either DB_BTREE, DB_HASH
or DB_RECNO.
.TP
close
A pointer to a routine to flush any cached information to disk, free any
allocated resources, and close the database file.
Since key/data pairs may be cached in memory, failing to close the
file with a
.I close
routine may result in inconsistent or lost information.
.I Close
routines return -1 on error (setting
.IR errno )
and 0 on success.
.TP
del
A pointer to a routine to remove key/data pairs from the database.
.I Delete
routines return -1 on error (setting
.IR errno ),
0 on success, and 1 if the specified
.I key
was not in the file.
.TP
get
A pointer to a routine which is the interface for keyed retrieval from
the database.
The address and length of the data associated with the specified
.I key
are returned in the structure referenced by
.IR data .
.I Get
routines return -1 on error (setting
.IR errno ),
0 on success, and 1 if the
.I key
was not in the file.
.TP
put
A pointer to a routine to store key/data pairs in the database.
.IP
The parameter
.I flag
must be set to one of the following values:
.RS
.TP
R_IAFTER
Append the data immediately after the data referenced by
.IR key ,
creating a new key/data pair.
(This implies that the access method is able to create new keys,
i.e. the keys are ordered and independent, for example, record numbers.
Applicable only to the
.B RECNO
access method.)
.TP
R_IBEFORE
Insert the data immediately before the data referenced by
.IR key ,
creating a new key/data pair.
(This implies that the access method is able to create new keys,
i.e. the keys are ordered and independent, for example, record numbers.
Applicable only to the
.B RECNO
access method.)
.TP
R_NOOVERWRITE
Enter the new key/data pair only if the key does not previously exist.
.TP
R_PUT
Enter the new key/data pair and replace any previously existing key.
.RE
.IP
.I Put
routines return -1 on error (setting
.IR errno ),
0 on success, and 1 if the R_NOOVERWRITE
.I flag
was set and the key already exists in the file.
.TP
seq
A pointer to a routine which is the interface for sequential
retrieval from the database.
The address and length of the key are returned in the structure
referenced by
.IR key ,
and the address and length of the data are returned in the
structure referenced
by
.IR data .
.IP
Sequential key/data pair retrieval may begin at any time, and the
position of the ``cursor'' is not affected by calls to the
.IR del ,
.IR get ,
.IR put ,
or
.I sync
routines.
Modifications to the database during a sequential scan will be reflected
in the scan, i.e. records inserted behind the cursor will not be returned
while records inserted in front of the cursor will be returned.
.IP
The flag value must be set to one of the following values:
.RS
.TP
R_CURSOR
The data associated with the specified key is returned.
This differs from the
.I get
routines in that it sets the ``cursor'' to the location of the
key as well.
(This implies that the access method has a implicit order which does
not change.
Applicable only to the
.B BTREE
and
.B RECNO
access methods.)
.TP
R_FIRST
The first key/data pair of the database is returned.
.TP
R_LAST
The last key/data pair of the database is returned.
(This implies that the access method has a implicit order which does
not change.
Applicable only to the
.B BTREE
and
.B RECNO
access methods.)
.TP
R_NEXT
Retrieve the key/data pair immediately after the key/data pair most recently
retrieved using the
.I seq
routine.
The cursor is moved to the returned key/data pair.
If
.I flag
is set to R_NEXT the first time the
.I seq
routine is called, the first key/data pair of the database is returned.
.TP
R_PREV
Retrieve the key/data pair immediately before the key/data pair most recently
retrieved using the
.I seq
routine.
The cursor is moved to the returned key/data pair.
If
.I flag
is set to R_PREV the first time the
.I seq
routine is called, the last key/data pair of the database is returned.
(This implies that the access method has a implicit order which does
not change.
Applicable only to the
.B BTREE
and
.B RECNO
access methods.)
.RE
.IP
.I Seq
routines return -1 on error (setting
.IR errno ),
0 on success, 1 if there are no more key/data pairs available.
If the
.B RECNO
access method is being used, and if the database file is a character special
file and no complete key/data pairs are currently available, the
.I seq
routines return 2.
.TP
sync
A pointer to a routine to flush any cached information to disk.
If the database is in memory only, the
.I sync
routine has no effect and will always succeed.
.I Sync
routines return -1 on error (setting
.IR errno )
and 0 on success.
.SH "KEY/DATA PAIRS"
Access to all file types is based on key/data pairs.
Both keys and data are represented by the following data structure:
.PP
typedef struct {
.RS
void *data;
.br
size_t size;
.RE
} DBT;
.PP
The elements of the DBT structure are defined as follows:
.TP
data
A pointer to a byte string.
.TP
size
The length of the byte string.
.PP
Key/data strings must fit into available memory.
.SH BTREE
One of the access methods is a btree: a sorted, balanced tree structure
with associated key/data pairs.
.PP
The access method specific data structure provided to
.I btree_open
is as follows:
.PP
typedef struct {
.RS
u_long flags;
.br
u_int psize;
.br
u_int cachesize;
.br
int (*compare)(const void *, const void *);
.br
int lorder;
.RE
} BTREEINFO;
.PP
The elements of this structure are defined as follows:
.TP
flags
The flag value is specified by
.IR or 'ing
any of the following values:
.RS
.TP
R_DUP
On insertion,
if the key to be inserted already exists,
permit insertion anyway.
This flag permits duplicate keys in the tree.
By default,
duplicates are not permitted,
and attempts to insert them will fail.
Note, the order of retrieval of key/data pairs with duplicate keys is
undefined.
.RE
.TP
cachesize
A suggested maximum size, in bytes, of the memory cache.
Setting this value to zero specifies that an appropriate amount of memory
should be used.
Since every search examines the root page of the tree, caching the most
recently used pages substantially improves access time.
In addition, physical writes are delayed as long as possible, so a moderate
cache can reduce the number of I/O operations significantly.
Obviously, using a cache increases the likelihood of corruption or lost data
if the system crashes while a tree is being modified.
However, caching 10
pages decreases the creation time of a large tree by between two and three
orders of magnitude.
.TP
compare
Compare is a user defined comparison function.
It must return an integer less than, equal to, or greater than zero if the
first argument is considered to be respectively less than, equal to, or
greater than the second.
The same comparison function must be used on a given tree every time it
is opened.
If no comparison function is specified,
.IR strcmp (3)
is used.
.TP
lorder
The byte order for 4-byte integers in the stored database metadata.
The number should represent the order as an integer; for example, 
big endian order would be the number 4,321.
If
.I 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.
(Obviously, portability of the data forming the key/data pairs is the
concern of the application program.)
.TP
psize
Page size is the size in bytes of the pages used for nodes in the tree.
If the  file already exists, the specified value is ignored and the
value specified when the tree was created is used.
If
.I psize
is zero, an appropriate page size is chosen (based on the system memory
and/or file system constraints), but will never be less than 512 bytes.
.PP
If the pointer to the
.I openinfo
data structure is NULL, the
.I btree_open
routine will use appropriate values.
.PP
If the database file already exists, and the O_TRUNC flag is not specified
to
.IR btree_open ,
the parameter
.I psize
ignored.
.PP
Key structures may reference byte strings of slightly less than one-half the
tree's page size only (see
.IR psize ).
Data structures may reference byte strings of essentially unlimited length.
.PP
Searches, insertions, and deletions in a btree will all complete in
O lg N.
.PP
Forward sequential scans of a tree are from the least key to the greatest.
.PP
Space freed up by deleting key/data pairs from a btree is never reclaimed,
although it is normally made available for reuse.
The exception to this is that space occupied by large data items (those
greater than one quarter the size of a page) is neither reclaimed nor reused.
This means that the btree storage structure is grow-only.
The only solutions are to avoid excessive deletions, or to create a fresh
tree periodically from a scan of an existing one.
.SH HASH
One of the access methods is hashed access and storage.
The access method specific data structure provided to
.I hash_open
is as follows:
.sp
typedef struct {
.RS
u_long (*hash)(const void *, const size_t);
.br
u_int cachesize;
.br
int bsize;
.br
int ffactor;
.br
int lorder;
.br
int nelem;
.RE
} HASHINFO;
.PP
The elements of this structure are defined as follows:
.TP
bsize
.I Bsize
defines the hash table bucket size, and is, by default, 256 bytes.
It may be preferable to increase the page size for disk-resident tables and
tables with large data items.
.TP
cachesize
A suggested maximum size, in bytes, of the memory cache.
Setting this value to zero specifies that an appropriate amount of memory
should be used.
.TP
ffactor
.I 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.
.TP
hash
.I 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 an u_long to be used as the hash value.
.TP
lorder
The byte order for 4-byte integers in the stored database metadata.
The number should represent the order as an integer; for example, 
big endian order would be the number 4,321.
If
.I 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.
(Obviously, portability of the data forming the key/data pairs is the
concern of the application program.)
.TP
nelem
.I Nelem
is an estimate of the final size of the hash table.
If not set, the default value is 1.
If not set or set too low, hash tables will expand gracefully as keys
are entered, although a slight performance degradation may be noticed.
.PP
If the pointer to the
.I openinfo
data structure is NULL, the
.I hash_open
routine will use appropriate values.
.PP
If the hash table already exists, and the O_TRUNC flag is not
specified to
.IR hash_open ,
the parameters
.IR bsize ,
.IR ffactor ,
and
.I nelem
are ignored.
.PP
If a hash function is specified,
.I 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.
.PP
Both key and data structures may reference byte strings of essentially
unlimited length.
.PP
Backward compatible interfaces to the routines described in
.IR dbm (3),
.IR hsearch (3),
and
.IR ndbm (3)
are provided, however, these interfaces are not compatible with
previous file formats.
.SH RECNO
One of the access methods is either variable or fixed-length records,
the former delimited by a specific byte value.
The access method specific data structure provided to
.I recno_open
is as follows:
.sp
typedef struct {
.RS
u_long flags;
.br
u_int cachesize;
.br
size_t reclen;
.br
u_char bval;
.RE
} RECNOINFO;
.PP
The elements of this structure are defined as follows:
.TP
flags
The flag value is specified by
.IR or 'ing
any of the following values:
.RS
.TP
R_FIXEDLEN
The records are fixed-length, not byte delimited.
The structure element
.I reclen
specifies the length of the record, and the structure element
.I bval
is used as the pad character.
.TP
R_SNAPSHOT
This flag requires that a snapshot of the file be taken when
.I recno_open
is called, instead of permitting any unmodified records to be
read from the original file.
.RE
.TP
cachesize
A suggested maximum size, in bytes, of the memory cache.
Setting this value to zero specifies that an appropriate amount of memory
should be used.
.TP
reclen
The length of a fixed-length record.
.TP
bval
The delimiting byte to be used to mark the end of a record for
variable-length records, and the pad character for fixed-length
records.
.PP
Variable-length and fixed-length data files require
.I key
structures to reference the following structure:
.sp
typedef struct {
.RS
u_long length;
.br
u_long number;
.br
u_long offset;
.br
u_char valid;
.RE
} RECNOKEY;
.PP
The elements of this structure are defined as follows:
.TP
length
The length of the record.
.TP
number
The record number.
.TP
offset
The offset in the file at which the record is located.
.TP
valid
A flag value which indicates the validity of the other fields in the
structure.
The flag value is specified by
.IR or 'ing
one or more of the following values:
.RS
.TP
R_LENGTH
The record length is valid.
.TP
R_NUMBER
The record number is valid.
.TP
R_OFFSET
The byte offset is valid.
.RE
.PP
If the record retrieval is successful, the record number, byte offset and
record length are set in the RECNOKEY structure referenced by the caller's
.I key
structure.
.PP
Data structures may reference byte strings of essentially unlimited length.
.SH ERRORS
The
.I open
routines may fail and set
.I errno
for any of the errors specified for the library routines
.IR open (2)
and
.IR malloc (3)
or the following:
.TP
[EFTYPE]
A file used by one of the
.I open
routines is incorrectly formatted.
.TP
[EINVAL]
A parameter has been specified (hash function, pad byte etc.) that is
incompatible with the current file specification or there is a mismatch
between the version number of file and the software.
.PP
The
.I close
routines may fail and set
.I errno
for any of the errors specified for the library routines
.IR close (2),
.IR read (2),
.IR write (2),
.IR free (3),
or
.IR fsync (2).
.PP
The
.IR del ,
.IR get ,
.I put
and
.I seq
routines may fail and set
.I errno
for any of the errors specified for the library routines
.IR read (2),
.IR write (2),
.IR free (3)
or
.IR malloc (3).
.PP
The
.I sync
routines may fail and set
.I errno
for any of the errors specified for the library routine
.IR fsync (2).
.SH "SEE ALSO"
.IR "Dynamic Hash Tables" ,
Per-Ake Larson, Communications of the ACM, April 1988.
.br
.IR "A New Hash Package for UNIX" ,
Margo Seltzer, USENIX Proceedings, Winter 1991.
.SH BUGS
The typedef DBT is a mnemonic for ``data base thang'', and was used
because noone could think of a reasonable name that wasn't already used.
.PP
None of the access methods provide any form of concurrent access,
locking, or transactions.
.PP
Only big and little endian byte order is supported.