freebsd-dev/lib/libc/db/man/dbopen.3

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.\"	@(#)dbopen.3	8.5 (Berkeley) 1/2/94
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.Dd September 10, 2010
.Dt DBOPEN 3
.Os
.Sh NAME
.Nm dbopen
.Nd "database access methods"
.Sh SYNOPSIS
.In sys/types.h
.In db.h
.In fcntl.h
.In limits.h
.Ft DB *
.Fn dbopen "const char *file" "int flags" "int mode" "DBTYPE type" "const void *openinfo"
.Sh DESCRIPTION
The
.Fn dbopen
function
is the library interface to database files.
The supported file formats are btree, hashed and UNIX file oriented.
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 byte stream file with fixed or variable length
records.
The formats and file format specific information are described in detail
in their respective manual pages
.Xr btree 3 ,
.Xr hash 3
and
.Xr recno 3 .
.Pp
The
.Fn dbopen
function
opens
.Fa file
for reading and/or writing.
Files never intended to be preserved on disk may be created by setting
the
.Fa file
argument to
.Dv NULL .
.Pp
The
.Fa flags
and
.Fa mode
arguments
are as specified to the
.Xr open 2
routine, however, only the
.Dv O_CREAT , O_EXCL , O_EXLOCK , O_NOFOLLOW , O_NONBLOCK ,
.Dv O_RDONLY , O_RDWR , O_SHLOCK , O_SYNC
and
.Dv O_TRUNC
flags are meaningful.
(Note, opening a database file
.Dv O_WRONLY
is not possible.)
.\"Three additional options may be specified by
.\".Em or Ns 'ing
.\"them into the
.\".Fa flags
.\"argument.
.\".Bl -tag -width indent
.\".It Dv DB_LOCK
.\"Do the necessary locking in the database to support concurrent access.
.\"If concurrent access is not needed or the database is read-only this
.\"flag should not be set, as it tends to have an associated performance
.\"penalty.
.\".It Dv DB_SHMEM
.\"Place the underlying memory pool used by the database in shared
.\"memory.
.\"Necessary for concurrent access.
.\".It Dv DB_TXN
.\"Support transactions in the database.
.\"The
.\".Dv DB_LOCK
.\"and
.\".Dv DB_SHMEM
.\"flags must be set as well.
.\".El
.Pp
The
.Fa type
argument is of type
.Ft DBTYPE
(as defined in the
.In db.h
include file) and
may be set to
.Dv DB_BTREE , DB_HASH
or
.Dv DB_RECNO .
.Pp
The
.Fa openinfo
argument is a pointer to an access method specific structure described
in the access method's manual page.
If
.Fa openinfo
is
.Dv NULL ,
each access method will use defaults appropriate for the system
and the access method.
.Pp
The
.Fn dbopen
function
returns a pointer to a
.Ft DB
structure on success and
.Dv NULL
on error.
The
.Ft DB
structure is defined in the
.In db.h
include file, and contains at
least the following fields:
.Bd -literal
typedef struct {
	DBTYPE type;
	int (*close)(DB *db);
	int (*del)(const DB *db, const DBT *key, u_int flags);
	int (*fd)(const DB *db);
	int (*get)(const DB *db, const DBT *key, DBT *data, u_int flags);
	int (*put)(const DB *db, DBT *key, const DBT *data,
	     u_int flags);
	int (*sync)(const DB *db, u_int flags);
	int (*seq)(const DB *db, DBT *key, DBT *data, u_int flags);
} DB;
.Ed
.Pp
These elements describe a database type and a set of functions performing
various actions.
These functions take a pointer to a structure as returned by
.Fn dbopen ,
and sometimes one or more pointers to key/data structures and a flag value.
.Bl -tag -width indent
.It Va type
The type of the underlying access method (and file format).
.It Va close
A pointer to a routine to flush any cached information to disk, free any
allocated resources, and close the underlying file(s).
Since key/data pairs may be cached in memory, failing to sync the file
with a
.Va close
or
.Va sync
function may result in inconsistent or lost information.
.Va close
routines return -1 on error (setting
.Va errno )
and 0 on success.
.It Va del
A pointer to a routine to remove key/data pairs from the database.
.Pp
The
.Fa flags
argument
may be set to the following value:
.Bl -tag -width indent
.It Dv R_CURSOR
Delete the record referenced by the cursor.
The cursor must have previously been initialized.
.El
.Pp
.Va delete
routines return -1 on error (setting
.Va errno ) ,
0 on success, and 1 if the specified
.Fa key
was not in the file.
.It Va fd
A pointer to a routine which returns a file descriptor representative
of the underlying database.
A file descriptor referencing the same file will be returned to all
processes which call
.Fn dbopen
with the same
.Fa file
name.
This file descriptor may be safely used as an argument to the
.Xr fcntl 2
and
.Xr flock 2
locking functions.
The file descriptor is not necessarily associated with any of the
underlying files used by the access method.
No file descriptor is available for in memory databases.
.Va \&Fd
routines return -1 on error (setting
.Va errno ) ,
and the file descriptor on success.
.It Va 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
.Fa key
are returned in the structure referenced by
.Fa data .
.Va get
routines return -1 on error (setting
.Va errno ) ,
0 on success, and 1 if the
.Fa key
was not in the file.
.It Va put
A pointer to a routine to store key/data pairs in the database.
.Pp
The
.Fa flags
argument
may be set to one of the following values:
.Bl -tag -width indent
.It Dv R_CURSOR
Replace the key/data pair referenced by the cursor.
The cursor must have previously been initialized.
.It Dv R_IAFTER
Append the data immediately after the data referenced by
.Fa key ,
creating a new key/data pair.
The record number of the appended key/data pair is returned in the
.Fa key
structure.
(Applicable only to the
.Dv DB_RECNO
access method.)
.It Dv R_IBEFORE
Insert the data immediately before the data referenced by
.Fa key ,
creating a new key/data pair.
The record number of the inserted key/data pair is returned in the
.Fa key
structure.
(Applicable only to the
.Dv DB_RECNO
access method.)
.It Dv R_NOOVERWRITE
Enter the new key/data pair only if the key does not previously exist.
.It Dv R_SETCURSOR
Store the key/data pair, setting or initializing the position of the
cursor to reference it.
(Applicable only to the
.Dv DB_BTREE
and
.Dv DB_RECNO
access methods.)
.El
.Pp
.Dv R_SETCURSOR
is available only for the
.Dv DB_BTREE
and
.Dv DB_RECNO
access
methods because it implies that the keys have an inherent order
which does not change.
.Pp
.Dv R_IAFTER
and
.Dv R_IBEFORE
are available only for the
.Dv DB_RECNO
access method because they each imply that the access method is able to
create new keys.
This is only true if the keys are ordered and independent, record numbers
for example.
.Pp
The default behavior of the
.Va put
routines is to enter the new key/data pair, replacing any previously
existing key.
.Pp
.Va put
routines return -1 on error (setting
.Va errno ) ,
0 on success, and 1 if the
.Dv R_NOOVERWRITE
flag
was set and the key already exists in the file.
.It Va 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
.Fa key ,
and the address and length of the data are returned in the
structure referenced
by
.Fa data .
.Pp
Sequential key/data pair retrieval may begin at any time, and the
position of the
.Dq cursor
is not affected by calls to the
.Va del ,
.Va get ,
.Va put ,
or
.Va 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.
.Pp
The
.Fa flags
argument
.Em must
be set to one of the following values:
.Bl -tag -width indent
.It Dv R_CURSOR
The data associated with the specified key is returned.
This differs from the
.Va get
routines in that it sets or initializes the cursor to the location of
the key as well.
(Note, for the
.Dv DB_BTREE
access method, the returned key is not necessarily an
exact match for the specified key.
The returned key is the smallest key greater than or equal to the specified
key, permitting partial key matches and range searches.)
.It Dv R_FIRST
The first key/data pair of the database is returned, and the cursor
is set or initialized to reference it.
.It Dv R_LAST
The last key/data pair of the database is returned, and the cursor
is set or initialized to reference it.
(Applicable only to the
.Dv DB_BTREE
and
.Dv DB_RECNO
access methods.)
.It Dv R_NEXT
Retrieve the key/data pair immediately after the cursor.
If the cursor is not yet set, this is the same as the
.Dv R_FIRST
flag.
.It Dv R_PREV
Retrieve the key/data pair immediately before the cursor.
If the cursor is not yet set, this is the same as the
.Dv R_LAST
flag.
(Applicable only to the
.Dv DB_BTREE
and
.Dv DB_RECNO
access methods.)
.El
.Pp
.Dv R_LAST
and
.Dv R_PREV
are available only for the
.Dv DB_BTREE
and
.Dv DB_RECNO
access methods because they each imply that the keys have an inherent
order which does not change.
.Pp
.Va seq
routines return -1 on error (setting
.Va errno ) ,
0 on success and 1 if there are no key/data pairs less than or greater
than the specified or current key.
If the
.Dv DB_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
.Va seq
routines return 2.
.It Va sync
A pointer to a routine to flush any cached information to disk.
If the database is in memory only, the
.Va sync
routine has no effect and will always succeed.
.Pp
The
.Fa flags
argument may be set to the following value:
.Bl -tag -width indent
.It Dv R_RECNOSYNC
If the
.Dv DB_RECNO
access method is being used, this flag causes
the
.Va sync
routine to apply to the btree file which underlies the
recno file, not the recno file itself.
(See the
.Va bfname
field of the
.Xr recno 3
manual page for more information.)
.El
.Pp
.Va sync
routines return -1 on error (setting
.Va errno )
and 0 on success.
.El
.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:
.Bd -literal
typedef struct {
	void *data;
	size_t size;
} DBT;
.Ed
.Pp
The elements of the
.Ft DBT
structure are defined as follows:
.Bl -tag -width "data"
.It Va data
A pointer to a byte string.
.It Va size
The length of the byte string.
.El
.Pp
Key and data byte strings may reference strings of essentially unlimited
length although any two of them must fit into available memory at the same
time.
It should be noted that the access methods provide no guarantees about
byte string alignment.
.Sh ERRORS
The
.Fn dbopen
routine may fail and set
.Va errno
for any of the errors specified for the library routines
.Xr open 2
and
.Xr malloc 3
or the following:
.Bl -tag -width Er
.It Bq Er EFTYPE
A file is incorrectly formatted.
.It Bq Er EINVAL
An argument has been specified (hash function, pad byte etc.) that is
incompatible with the current file specification or which is not
meaningful for the function (for example, use of the cursor without
prior initialization) or there is a mismatch between the version
number of file and the software.
.El
.Pp
The
.Va close
routines may fail and set
.Va errno
for any of the errors specified for the library routines
.Xr close 2 ,
.Xr read 2 ,
.Xr write 2 ,
.Xr free 3 ,
or
.Xr fsync 2 .
.Pp
The
.Va del ,
.Va get ,
.Va put
and
.Va seq
routines may fail and set
.Va errno
for any of the errors specified for the library routines
.Xr read 2 ,
.Xr write 2 ,
.Xr free 3
or
.Xr malloc 3 .
.Pp
The
.Va fd
routines will fail and set
.Va errno
to
.Er ENOENT
for in memory databases.
.Pp
The
.Va sync
routines may fail and set
.Va errno
for any of the errors specified for the library routine
.Xr fsync 2 .
.Sh SEE ALSO
.Xr btree 3 ,
.Xr hash 3 ,
.Xr mpool 3 ,
.Xr recno 3
.Rs
.%T "LIBTP: Portable, Modular Transactions for UNIX"
.%A Margo Seltzer
.%A Michael Olson
.%R "USENIX proceedings"
.%D Winter 1992
.Re
.Sh BUGS
The typedef
.Ft DBT
is a mnemonic for
.Dq "data base thang" ,
and was used
because noone could think of a reasonable name that was not already used.
.Pp
The file descriptor interface is a kluge and will be deleted in a
future version of the interface.
.Pp
None of the access methods provide any form of concurrent access,
locking, or transactions.