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< th colspan = "3" align = "center" > DbEnv::memp_register()< / th >
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The DbMpoolFile Handle
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< h2 class = "title" style = "clear: both" > < a id = "mempregister" > < / a > DbEnv::memp_register()< / h2 >
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< pre class = "programlisting" > #include < db_cxx.h>
extern "C" {
typedef int (*pgin_fcn_type)(DB_ENV *dbenv,
db_pgno_t pgno, void *pgaddr, DBT *pgcookie);
typedef int (*pgout_fcn_type)(DB_ENV *dbenv,
db_pgno_t pgno, void *pgaddr, DBT *pgcookie);
};
int
DbEnv::memp_register(int ftype,
pgin_fcn_type pgin_fcn, pgout_fcn_type pgout_fcn);< / pre >
< p >
The < code class = "methodname" > DbEnv::memp_register()< / code > method registers page-in and page-out
functions for files of type < span class = "bold" > < strong > ftype< / strong > < / span > in the cache.
< / p >
< p >
If the < span class = "bold" > < strong > pgin_fcn< / strong > < / span > function is non-NULL,
it is called each time a page is read into the cache from a file
of type < span class = "bold" > < strong > ftype< / strong > < / span > , or a page is created
for a file of type < span class = "bold" > < strong > ftype< / strong > < / span > (see the
DB_MPOOL_CREATE flag for the
< a class = "xref" href = "mempfget.html" title = "DbMpoolFile::get()" > DbMpoolFile::get()< / a > method).
< / p >
< p >
If the < span class = "bold" > < strong > pgout_fcn< / strong > < / span > function is
non-NULL, it is called each time a page is written to a file of type
< span class = "bold" > < strong > ftype< / strong > < / span > .
< / p >
< p >
The purpose of the < code class = "methodname" > DbEnv::memp_register()< / code > function is to support
processing when pages are entered into, or flushed from, the cache. For example, this
functionality might be used to do byte-endian conversion as pages are read from, or written
to, the underlying file.
< / p >
< p >
A file type must be specified to make it possible for unrelated
threads or processes that are sharing a cache, to evict each other's
pages from the cache. During initialization, applications should call
< code class = "methodname" > DbEnv::memp_register()< / code > for each type of file requiring input or
output processing that will be sharing the underlying cache. (No
registry is necessary for the standard Berkeley DB access method types
because < a class = "xref" href = "dbopen.html" title = "Db::open()" > Db::open()< / a >
registers them separately.)
< / p >
< p >
If a thread or process does not call < code class = "methodname" > DbEnv::memp_register()< / code > for a
file type, it is impossible for it to evict pages for any file
requiring input or output processing from the cache. For this reason,
< code class = "methodname" > DbEnv::memp_register()< / code > should always be called by each application
sharing a cache for each type of file included in the cache, regardless
of whether or not the application itself uses files of that type.
< / p >
< p >
The < code class = "methodname" > DbEnv::memp_register()< / code > < span >
< span >
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.
< / span >
< / span >
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< h3 class = "title" > < a id = "id3624798" > < / a > Parameters< / h3 >
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< h4 class = "title" > < a id = "id3624801" > < / a > ftype< / h4 >
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The < span class = "bold" > < strong > ftype< / strong > < / span > parameter specifies the
type of file for which the page-in and page-out functions will be
called.
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< p >
The < span class = "bold" > < strong > ftype< / strong > < / span > value for a file must be a
non-zero positive number less than 128 (0 and negative numbers are
reserved for internal use by the Berkeley DB library).
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< h4 class = "title" > < a id = "id3624709" > < / a > pgin_fcn, pgout_fcn< / h4 >
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< / div >
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< p >
The page-in and page-out functions.
< / p >
< p >
The < span class = "bold" > < strong > pgin_fcn< / strong > < / span > and < span class = "bold" > < strong > pgout_fcn< / strong > < / span > functions are called with a reference
to the current database environment, the page number being read or
written, a pointer to the page being read or written, and any
parameter < span class = "bold" > < strong > pgcookie< / strong > < / span > that was specified
to the < a class = "xref" href = "mempset_pgcookie.html" title = "DbMpoolFile::set_pgcookie()" > DbMpoolFile::set_pgcookie()< / a >
method.
< / p >
< p >
The < span class = "bold" > < strong > pgin_fcn< / strong > < / span > and < span class = "bold" > < strong > pgout_fcn< / strong > < / span > functions should return 0 on success,
and a non-zero value on failure, in which case the shared Berkeley DB
library function calling it will also fail, returning that non-zero
value. The non-zero value should be selected from values outside of
the Berkeley DB library namespace.
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< a class = "link" href = "env.html" title = "Chapter > DbEnv< / a > , < a class = "link" href = "memp.html" title = "Chapter > DbMpoolFile< / a >
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< h3 class = "title" > < a id = "id3624930" > < / a > See Also< / h3 >
2011-09-13 17:44:24 +00:00
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< a class = "xref" href = "memp.html#memplist" title = "Memory Pools and Related Methods" > Memory Pools and Related Methods< / a >
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