gecko-dev/db/man/man.roff/db_cursor.3

.ds TYPE C
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.\"	Sleepycat Software.  All rights reserved.
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.\"	@(#)db_cursor.so	10.23 (Sleepycat) 4/10/98
.\"
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.\" See the file LICENSE for redistribution information.
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.\" The general information text macro.
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.ie '\*[TYPE]'C'\{The DB library is a family of groups of functions that provides a modular
programming interface to transactions and record-oriented file access.
The library includes support for transactions, locking, logging and file
page caching, as well as various indexed access methods.
Many of the functional groups (e.g., the file page caching functions)
are useful independent of the other DB functions,
although some functional groups are explicitly based on other functional
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.el\{The DB library is a family of classes that provides a modular
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are useful independent of the other DB classes,
although some classes are explicitly based on other classes
(e.g., transactions and logging).
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For a general description of the DB package, see
.IR db_intro (3).
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.\" The library error macro, the local error macro.
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.de Ea
[EAGAIN]
A lock was unavailable.
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[EBUSY]
The shared memory region was in use and the force flag was not set.
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[EAGAIN]
The shared memory region was locked and (repeatedly) unavailable.
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[EINVAL]
An invalid flag value or parameter was specified.
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[EACCES]
An attempt was made to modify a read-only database.
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The DB_THREAD flag was specified and spinlocks are not implemented for
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[EPERM]
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.IR db_archive (1),
.IR db_checkpoint (1),
.IR db_deadlock (1),
.IR db_dump (1),
.IR db_load (1),
.IR db_recover (1),
.IR db_stat (1),
.IR db_intro (3),
.ie '\*[TYPE]'C'\{\
.IR db_appinit (3),
.IR db_cursor (3),
.IR db_dbm (3),
.IR db_internal (3),
.IR db_lock (3),
.IR db_log (3),
.IR db_mpool (3),
.IR db_open (3),
.IR db_thread (3),
.IR db_txn (3)
\}
.el\{\
.IR db_internal (3),
.IR db_thread (3),
.IR Db (3),
.IR Dbc (3),
.IR DbEnv (3),
.IR DbException (3),
.IR DbInfo (3),
.IR DbLock (3),
.IR DbLockTab (3),
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.IR DbLsn (3),
.IR DbMpool (3),
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.IR DbMpoolFile (3),
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.IR Dbt (3),
.IR DbTxn (3),
.IR DbTxnMgr (3)
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Create any underlying files, as necessary.
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.\" The return values of the functions macros.
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.\" Rt is the standard two-value return, returning errno, 0, or < 0.
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.\" The TXN id macro.
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.IP
If the file is being accessed under transaction protection,
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.IR txn_begin ,
otherwise, NULL.
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.\" This macro takes two arguments:
.\"	+ the interface, e.g., "transaction region"
.\"	+ the prefix (for C++, this is the class name)
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The
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See the source code for the
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..
.\" Underlying function paragraph, for C++.
.\" This macro takes three arguments:
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The
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..
.TH DB_CURSOR 3 "April 10, 1998"
.UC 7
.SH NAME
db_cursor \- database sequential access functions
.SH SYNOPSIS
.nf
.ft B
#include <db.h>

int
DBcursor->c_close(DBC *cursor);

int
DBcursor->c_del(DBC *cursor, u_int32_t flags);

int
DBcursor->c_get(DBC *cursor, DBT *key, DBT *data, u_int32_t flags);

int
DBcursor->c_put(DBC *, DBT *key, DBT *data, u_int32_t flags);
.ft R
.fi
.SH DESCRIPTION
.Gn
.PP
This manual page describes the specific details of the cursor support
for the DB access methods.
.PP
The
.I db_cursor
functions are the library interface supporting sequential access to the
records stored by the access methods of the DB library.
Cursors are created by calling the
.I cursor
function of a DB structure returned by
.IR db_open (3),
which returns a pointer to a DBC structure.
.PP
Each cursor maintains positioning information within a set of key/data pairs.
In the presence of transactions, cursors are only valid within the
context of a single transaction, the one specified during the
.I cursor
call described in
.IR db_open (3).
All cursor operations will be executed in the context of that transaction.
Before aborting or committing a transaction, all cursors used within that
transaction must be closed.
In the presence of transactions, the application must call
.I txn_abort
if any of the cursor operations returns that a deadlock (EAGAIN) or
system failure occurred.
.PP
When locking is enabled,
page locks are retained between consecutive cursor calls.
For this reason, in the presence of locking,
applications should discard cursors as soon as they are done with them.
Calling the db
.I close
function (see
.IR db_open (3))
discards any cursors opened in the context of a particular DB structure
returned by the
.I db_open
call.
.PP
The cursor has an associated set of functions to access elements in the
database, accessed through the DBC structure.
The elements of the DBC structure are defined as follows:
.TP 5
int (*c_close)(DBC *cursor);
A pointer to a function that discards the cursor.
No further references to the cursor should be made.
.IP
.Rt c_close
.TP 5
int (*c_del)(DBC *cursor, u_int32_t flags);
A pointer to a function that deletes the key/data pair currently
referenced by the cursor.
.IP
.Fl
.IP
The cursor position is unchanged after a delete and subsequent calls
to cursor functions expecting the cursor to reference an existing
key will fail.
.IP
.Rc c_del
and DB_KEYEMPTY if the element has already been deleted.
.TP 5
int (*c_get)(DBC *cursor, DBT *key, DBT *data, u_int32_t flags);
.br
A pointer to a function that retrieves key/data pairs from the database.
The address and length of the key are returned in the structure referenced
by
.I key
(except for the case of the DB_SET flag where the
.I key
structure is unchanged),
and the address and length of the data are returned in the structure
referenced by
.IR data .
.sp
Modifications to the database during a sequential scan will be reflected
in the scan,
i.e. records inserted behind a cursor will not be returned while records
inserted in front of a cursor will be returned.
.sp
In recno databases, missing entries
(i.e., entries that were never explicitly created or that were created
and then deleted),
will be skipped during a sequential scan.
.sp
If multiple threads or processes insert items into the same database file
without using locking, the results are undefined.
For more detail, see the section below on cursor stability.
.IP
The parameter
.I flags
must be set to exactly one of the following values:
.RS
.TP 5
DB_FIRST
The cursor is set to reference the first key/data pair of the database,
and that pair is returned.
In the presence of duplicate key values,
the first data item in the set of duplicates is returned.
.IP
If the database is empty,
the
.I c_get
function will return DB_NOTFOUND.
.TP 5
DB_LAST
The cursor is set to reference the last key/data pair of the database,
and that pair is returned.
In the presence of duplicate key values,
the last data item in the set of duplicates is returned.
.IP
If the database is empty,
the
.I c_get
function will return DB_NOTFOUND.
.TP 5
DB_NEXT
If the cursor is not yet initialized, DB_NEXT is identical to DB_FIRST.
.IP
Otherwise,
move the cursor to the next key/data pair of the database,
and that pair is returned.
In the presence of duplicate key values,
the value of the key may not change.
.IP
If the cursor is already on the last record in the database,
the
.I c_get
function will return DB_NOTFOUND.
.TP 5
DB_PREV
If the cursor is not yet initialized, DB_PREV is identical to DB_LAST.
.IP
Otherwise,
move the cursor to the previous key/data pair of the database,
and that pair is returned.
In the presence of duplicate key values,
the value of the key may not change.
.IP
If the cursor is already on the first record in the database,
the
.I c_get
function will return DB_NOTFOUND.
.TP 5
DB_CURRENT
Return the key/data pair currently referenced by the cursor.
.IP
If the cursor key/data pair has been deleted,
the
.I c_get
function will return DB_KEYEMPTY.
.IP
If the cursor is not yet initialized,
the
.I c_get
function will return EINVAL.
.TP 5
DB_SET
Move the cursor to the specified key/data pair of the database,
and return the datum associated with the given key.
.IP
In the presence of duplicate key values,
.I c_get
will return the first data item for the given key.
.IP
If the database is a recno database and the requested key exists,
but was never explicitly created by the application or was later
deleted, the
.I c_get
function returns DB_KEYEMPTY.
.IP
If no matching keys are found,
the
.I c_get
function will return DB_NOTFOUND.
.TP 5
DB_SET_RANGE
The DB_SET_RANGE flag is identical to the DB_SET flag,
except that the key is returned as well as the data item,
and, in the case of the btree access method,
the returned key/data pair is the smallest key greater than or
equal to the specified key (as determined by the comparison function),
permitting partial key matches and range searches.
.TP 5
DB_SET_RECNO
Move the cursor to the specific numbered record of the database,
and return the associated key/data pair.
The
.I data
field of the specified
.I key
must be a pointer to a memory location from which a
.I db_recno_t
may be read, as described in
.IR db_dbt (3).
This memory location will be read to determine the record to be retrieved.
.sp
For DB_SET_RECNO to be specified, the underlying database must be of type
btree and it must have been created with the DB_RECNUM flag (see
.IR db_open (3)).
.TP 5
DB_GET_RECNO
Return the record number associated with the cursor.
The record number
will be returned in the data DBT as described in
.IR db_dbt (3).
The
.I key
parameter is ignored.
.sp
For DB_GET_RECNO to be specified, the underlying database must be of type
btree and it must have been created with the DB_RECNUM flag (see
.IR db_open (3)).
.RE
.IP
.Ro c_get
.IP
If
.I c_get
fails for any reason, the state of the cursor will be unchanged.
.TP 5
int (*c_put)(DBC *, DBT *key, DBT *data, u_int32_t flags);
.br
A pointer to a function that stores key/data pairs into the database.
.IP
The
.I flags
parameter must be set to exactly one of the following values:
.RS
.TP 5
DB_AFTER
In the case of the btree and hash access methods,
insert the data element as a duplicate element of the key referenced
by the cursor.
The new element appears immediately after the current cursor position.
It is an error to specify DB_AFTER if the underlying btree or hash database
was not created with the DB_DUP flag.
The
.I key
parameter is ignored.
.IP
In the case of the recno access method,
it is an error to specify DB_AFTER if the underlying recno database was
not created with the DB_RENUMBER flag.
If the DB_RENUMBER flag was specified, a new key is created,
all records after the inserted item are automatically renumbered,
and the key of the new record is returned in the structure referenced
by the parameter
.IR key .
The initial value of the
.I key
parameter is ignored.
See
.IR db_open (3)
for more information.
.IP
If the cursor is not yet initialized,
the
.I c_put
function will return EINVAL.
.TP 5
DB_BEFORE
In the case of the btree and hash access methods,
insert the data element as a duplicate element of the key referenced
by the cursor.
The new element appears immediately before the current cursor position.
It is an error to specify DB_BEFORE if the underlying btree or hash database
was not created with the DB_DUP flag.
The
.I key
parameter is ignored.
.IP
In the case of the recno access method,
it is an error to specify DB_BEFORE if the underlying recno database was
not created with the DB_RENUMBER flag.
If the DB_RENUMBER flag was specified, a new key is created,
the current record and all records after it are automatically renumbered,
and the key of the new record is returned in the structure referenced by
the parameter
.IR key .
The initial value of the
.I key
parameter is ignored.
See
.IR db_open (3)
for more information.
.IP
If the cursor is not yet initialized,
the
.I c_put
function will return EINVAL.
.TP 5
DB_CURRENT
Overwrite the data of the key/data pair referenced by the cursor with the
specified data item.
.IP
The
.I key
parameter is ignored.
.IP
If the cursor is not yet initialized,
the
.I c_put
function will return EINVAL.
.TP 5
DB_KEYFIRST
In the case of the btree and hash access methods,
insert the specified key/data pair into the database.
If the key already exists in the database,
the inserted data item is added as the first of the data items for that key.
.IP
The DB_KEYFIRST flag may not be specified to the recno access method.
.TP 5
DB_KEYLAST
Insert the specified key/data pair into the database.
If the key already exists in the database,
the inserted data item is added as the last of the data items for that key.
.IP
The DB_KEYLAST flag may not be specified to the recno access method.
.RE
.IP
If the cursor record has been deleted,
the
.I c_put
function will return DB_KEYEMPTY.
.IP
.Ro c_put
.IP
If
.I c_put
fails for any reason, the state of the cursor will be unchanged.
If
.I c_put
succeeds and an item is inserted into the database,
the cursor is always positioned to reference the newly inserted item.
.SH "CURSOR STABILITY"
.PP
In the absence of locking, no guarantees are made about the stability
of cursors in different processes or threads.
However,
the btree and recno access methods guarantee that cursor operations,
interspersed with other cursor or non-cursor operations in the same
thread of control (i.e., thread or single-threaded process),
will always return keys in order and will return each non-deleted
key/data pair exactly once.
Because the hash access method uses a dynamic hashing algorithm,
it cannot guarantee any form of stability in the presence of inserts and
deletes unless locking is performed.
.PP
If locking was specified when the DB file was opened,
but transactions are not in effect,
the access methods provide repeatable reads with respect to the cursor.
That is, a DB_CURRENT call on the cursor is guaranteed to return the same
record as was returned on the last call to the cursor.
.PP
In the presence of transactions, the access method calls between
.I txn_begin
and
.I txn_abort
or
.I txn_commit
provide degree 3 consistency.
For all access methods,
a cursor scan of the database performed within the context of a transaction
is guaranteed to return each key/data pair once and only once,
except in the following case.
If, while performing a cursor scan using the hash access method,
the transaction performing the scan inserts a new pair into the database,
it is possible that duplicate key/data pairs will be returned.
.SH ERRORS
.Ee DBcursor->c_close
.na
.Nh
calloc(3), 
fcntl(2), 
fflush(3), 
lock_get(3), 
lock_id(3), 
lock_put(3), 
lock_vec(3), 
log_put(3), 
malloc(3), 
memcpy(3), 
memmove(3), 
memp_fget(3), 
memp_fput(3), 
memp_fset(3), 
memset(3), 
realloc(3), 
and
strerror(3). 
.Hy
.ad
.PP
.Ec DBcursor->c_close
.TP 5
.Ea
.TP 5
.Ep
.PP
.Ee DBcursor->c_del
.na
.Nh
DB->del(3), 
calloc(3), 
fcntl(2), 
fflush(3), 
lock_get(3), 
lock_id(3), 
lock_put(3), 
lock_vec(3), 
log_put(3), 
malloc(3), 
memcpy(3), 
memmove(3), 
memp_fget(3), 
memp_fput(3), 
memp_fset(3), 
memset(3), 
realloc(3), 
and
strerror(3). 
.Hy
.ad
.PP
.Ec DBcursor->c_del
.TP 5
.Ea
.TP 5
.Ei
.TP 5
.Ep
.PP
.Ee DBcursor->c_get
.na
.Nh
DB->get(3), 
calloc(3), 
fcntl(2), 
fflush(3), 
lock_get(3), 
lock_id(3), 
lock_put(3), 
lock_vec(3), 
log_put(3), 
malloc(3), 
memcmp(3), 
memcpy(3), 
memmove(3), 
memp_fget(3), 
memp_fput(3), 
memp_fset(3), 
memset(3), 
realloc(3), 
and
strerror(3). 
.Hy
.ad
.PP
.Ec DBcursor->c_get
.TP 5
.Ea
.TP 5
.Ei
.sp
The DB_THREAD flag was specified to the
.IR db_open (3)
function and neither the DB_DBT_MALLOC or DB_DBT_USERMEM flags were set
in the DBT.
.TP 5
.Ep
.PP
.Ee DBcursor->c_put
.na
.Nh
calloc(3), 
fcntl(2), 
fflush(3), 
lock_get(3), 
lock_id(3), 
lock_put(3), 
lock_vec(3), 
log_put(3), 
malloc(3), 
memcmp(3), 
memcpy(3), 
memmove(3), 
memp_fget(3), 
memp_fput(3), 
memp_fset(3), 
memset(3), 
realloc(3), 
and
strerror(3). 
.Hy
.ad
.PP
.Ec DBcursor->c_put
.TP 5
.Es
.TP 5
.Ea
.TP 5
.Ei
.TP 5
.Ep
.SH "SEE ALSO"
.Sa