#include <db_cxx.h>
int
DbMpoolFile::get(db_pgno_t *pgnoaddr,
DbTxn *txnid, u_int32_t flags, void **pagep);
The DbMpoolFile::get() method returns pages from the cache.
All pages returned by DbMpoolFile::get() will be retained (that is,
latched) in the cache until a subsequent call to
DbMpoolFile::put()
.
There is no deadlock detection among latches so care must be taken
in the application if the DB_MPOOL_DIRTY or DB_MPOOL_EDIT flags are
used as these get exclusive latches on the pages.
The returned page is size_t type aligned.
Fully or partially created pages have all their bytes set to a nul byte, unless the DbMpoolFile::set_clear_len() method was called to specify other behavior before the file was opened.
The DbMpoolFile::get() method will return
DB_PAGE_NOTFOUND if the requested page does not exist and DB_MPOOL_CREATE
was not set. Unless otherwise specified, the DbMpoolFile::get()
method either returns a non-zero error value or throws an
exception that encapsulates a non-zero error value on
failure, and returns 0 on success.
The flags parameter must be set to 0 or by bitwise inclusively OR'ing together one or more of the following values:
If the specified page does not exist, create it. In this case, the pgin method, if specified, is called.
The page will be modified and must be written to the source file
before being evicted from the cache. For files open with the
DB_MULTIVERSION
flag set, a new copy of the page will be made if this is the first
time the specified transaction is modifying it.
A page fetched with the DB_MPOOL_DIRTY
flag will be
exclusively latched
until a subsequent call to
DbMpoolFile::put()
.
The page will be modified and must be written to the source file
before being evicted from the cache. No copy of the page will be made,
regardless of the
DB_MULTIVERSION
setting. This flag is only intended for use in situations where a
transaction handle is not available, such as during aborts or
recovery.
A page fetched with the DB_MPOOL_EDIT flag will be
exclusively latched until
a subsequent call to
DbMpoolFile::put()
.
Return the last page of the source file, and copy its page number into the memory location to which pgnoaddr refers.
Create a new page in the file, and copy its page number into the
memory location to which pgnoaddr
refers. In this case, the pgin_fcn callback, if specified on
DbEnv::memp_register()
, is
not called.
The DB_MPOOL_CREATE, DB_MPOOL_LAST, and
DB_MPOOL_NEW flags are mutually exclusive.
If the flags parameter is set to
DB_MPOOL_LAST or DB_MPOOL_NEW, the
page number of the created page is copied into the memory location to
which the pgnoaddr parameter refers.
Otherwise, the pgnoaddr parameter is the
page to create or retrieve.
Page numbers begin at 0; that is, the first page in the file is page number 0, not page number 1.
If the operation is part of an application-specified transaction, the txnid parameter is a transaction handle returned from DbEnv::txn_begin() ; otherwise NULL. A transaction is required if the file is open for multiversion concurrency control by passing DB_MULTIVERSION to DbMpoolFile::open() and the DB_MPOOL_DIRTY, DB_MPOOL_CREATE or DB_MPOOL_NEW flags were specified. Otherwise it is ignored.
The DbMpoolFile::get()
method may fail and throw a DbException
exception, encapsulating one of the following non-zero errors, or return one
of the following non-zero errors:
The DB_MPOOL_DIRTY or
DB_MPOOL_EDIT flag was set and the source file was
not opened for writing.
The page reference count has overflowed. (This should never happen unless there is a bug in the application.)
If the DB_MPOOL_NEW flag was set, and the source
file was not opened for writing; more than one of
DB_MPOOL_CREATE, DB_MPOOL_LAST,
and DB_MPOOL_NEW was set; or if an invalid flag
value or parameter was specified.
For transactions configured with DB_TXN_SNAPSHOT, the page has been modified since the transaction began.