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<h1>DbMpoolFile::open</h1>
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<h3><pre>
#include &lt;db_cxx.h&gt;
<p>
static int
DbMpoolFile::open(DbEnv *env, const char *file, u_int32_t flags, int mode,
size_t pagesize, DB_MPOOL_FINFO *finfop, DbMpoolFile **mpf);
</pre></h3>
<h1>Description</h1>
<p>The DbMpoolFile::open method opens a file in the pool specified by the
<a href="../api_cxx/dbenv_class.html">DbEnv</a> <b>env</b>, copying the <a href="../api_cxx/mempfile_class.html">DbMpoolFile</a> pointer
representing it into the memory location referenced by <b>mpf</b>.
<p>The <b>file</b> argument is the name of the file to be opened.
If <b>file</b> is NULL, a private file is created that cannot be
shared with any other process (although it may be shared with
other threads).
<p>The <b>flags</b> and <b>mode</b> arguments specify how files will be opened
and/or created if they do not already exist.
<p>The <b>flags</b> value must be set to 0 or by bitwise inclusively <b>OR</b>'ing together one or more
of the following values.
<p><dl compact>
<p><dt><a name="DB_CREATE">DB_CREATE</a><dd>Create any underlying files, as necessary. If the files do not already
exist and the DB_CREATE flag is not specified, the call will fail.
<p><dt><a name="DB_NOMMAP">DB_NOMMAP</a><dd>Always copy this file into the local cache instead of potentially mapping
it into process memory (see the description of the
<a href="../api_cxx/env_set_mp_mmapsize.html">DbEnv::set_mp_mmapsize</a> method for further information).
<p><dt><a name="DB_RDONLY">DB_RDONLY</a><dd>Open any underlying files for reading only. Any attempt to write the file
using the pool functions will fail, regardless of the actual permissions
of the file.
</dl>
<p>On UNIX systems, or in IEEE/ANSI Std 1003.1 (POSIX) environments, all files created by function DbMpoolFile::open
are created with mode <b>mode</b> (as described in <b>chmod</b>(2)) and
modified by the process' umask value at the time of creation (see
<b>umask</b>(2)). The group ownership of created files is based on
the system and directory defaults, and is not further specified by Berkeley DB.
If <b>mode</b> is 0, files are created readable and writeable by both
owner and group. On Windows systems, the mode argument is ignored.
<p>The <b>pagesize</b> argument is the size, in bytes, of the unit of transfer
between the application and the pool, although it is not necessarily the
unit of transfer between the pool and the source file.
<p>Files opened in the pool may be further configured based on the
<b>finfop</b> argument to DbMpoolFile::open (which is a pointer to a
structure of type DB_MPOOL_FINFO). No references to the <b>finfop</b>
structure are maintained by Berkeley DB, so it may be discarded when the
DbMpoolFile::open function returns. In order to ensure compatibility
with future releases of Berkeley DB, all fields of the DB_MPOOL_FINFO structure
that are not explicitly set should be initialized to 0 before the first
time the structure is used. Do this by declaring the structure external
or static, or by calling the C library routine <b>bzero</b>(3) or
<b>memset</b>(3).
<p>The fields of the DB_MPOOL_FINFO structure used by DbMpoolFile::open are
described below. If <b>finfop</b> is NULL or any of its fields are
set to their default value, defaults appropriate for the system are used.
<p><dl compact>
<p><dt>int <a name="ftype">ftype</a>;<dd>The <b>ftype</b> field should be the same as a <b>ftype</b> argument
previously specified to the <a href="../api_cxx/memp_register.html">DbEnv::memp_register</a> function, unless no
input or output processing of the file's pages are necessary, in which
case it should be 0. (See the description of the <a href="../api_cxx/memp_register.html">DbEnv::memp_register</a>
function for more information.)
<p><dt>DBT *<a name="pgcookie">pgcookie</a>;<dd>The <b>pgcookie</b> field contains the byte string that is passed to the
<b>pgin</b> and <b>pgout</b> functions for this file, if any. If no
<b>pgin</b> or <b>pgout</b> functions are specified, the
<b>pgcookie</b> field should be NULL. (See the description of the
<a href="../api_cxx/memp_register.html">DbEnv::memp_register</a> function for more information.)
<p><dt>u_int8_t *<a name="fileid">fileid</a>;<dd>The <b>fileid</b> field is a unique identifier for the file. If the
<b>fileid</b> field is non-NULL, it must reference a DB_FILE_ID_LEN
length array of bytes that will be used to uniquely identify the file.
<p>The mpool functions must be able to uniquely identify files in order that
multiple processes wanting to share a file will correctly identify it in
the pool.
<p>On most UNIX/POSIX systems, the <b>fileid</b> field will not need to be
set and the mpool functions will simply use the file's device and inode
numbers for this purpose. On Windows systems, the mpool functions use
the values returned by GetFileInformationByHandle() by default -- these
values are known to be constant between processes and over reboot in the
case of NTFS (where they are the NTFS MFT indexes).
<p>On other filesystems, (e.g., FAT or NFS) these default values are not
necessarily unique between processes or across system reboots.
<b>Applications wanting to maintain a shared memory buffer pool
between processes or across system reboots, where the pool contains pages
from files stored on such filesystems, must specify a unique file
identifier to the DbMpoolFile::open call and each process opening or
registering the file must provide the same unique identifier.</b>
<p>This should not be necessary for most applications. Specifically, it is
not necessary if the memory pool is not shared between processes and is
re-instantiated after each system reboot, or the application is using the
Berkeley DB access methods instead of calling the pool functions explicitly, or
the files in the memory pool are stored on filesystems where the default
values as described above are invariant between process and across system
reboots.
<p><dt>int32_t <a name="lsn_offset">lsn_offset</a>;<dd>The <b>lsn_offset</b> field is the zero-based byte offset in the page of
the page's log sequence number (LSN), or -1 if no LSN offset is specified.
(See the description of the <a href="../api_cxx/memp_sync.html">DbEnv::memp_sync</a> function for more
information.)
<p><dt>u_int32_t <a name="clear_len">clear_len</a>;<dd>The <b>clear_len</b> field is the number of initial bytes in a page
that should be set to zero when the page is created as a result of the
DB_MPOOL_CREATE or DB_MPOOL_NEW flags being specified to <a href="../api_cxx/memp_fget.html">DbMpoolFile::get</a>.
If <b>finfop</b> is NULL or <b>clear_len</b> is 0, the entire page is
cleared.
</dl>
<p>The DbMpoolFile::open 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.
<h1>Errors</h1>
<p>The DbMpoolFile::open method may fail and throw an exception or return a non-zero error for the following conditions:
<p><dl compact>
<p><dt>EINVAL<dd>An invalid flag value or parameter was specified.
<p>The file has already been entered into the pool, and the <b>pagesize</b>
value is not the same as when the file was entered into the pool, or the
length of the file is not zero or a multiple of the <b>pagesize</b>.
<p>The DB_RDONLY flag was specified for an in-memory pool.
</dl>
<p><dl compact>
<p><dt>ENOMEM<dd>The maximum number of open files has been reached.
</dl>
<p>The DbMpoolFile::open method may fail and throw an exception or return a non-zero error for errors specified for other Berkeley DB and C library or system methods.
If a catastrophic error has occurred, the DbMpoolFile::open method may fail and either
return <a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a> or throw an exception encapsulating
<a href="../ref/program/errorret.html#DB_RUNRECOVERY">DB_RUNRECOVERY</a>, in which case all subsequent Berkeley DB calls will fail
in the same way.
<h3>Classes</h3>
<a href="../api_cxx/dbenv_class.html">DbEnv</a>, <a href="../api_cxx/mempfile_class.html">DbMpoolFile</a>
<h1>See Also</h1>
<a href="../api_cxx/env_set_mp_mmapsize.html">DbEnv::set_mp_mmapsize</a>,
<a href="../api_cxx/memp_fclose.html">DbMpoolFile::close</a>,
<a href="../api_cxx/memp_fget.html">DbMpoolFile::get</a>,
<a href="../api_cxx/memp_fopen.html">DbMpoolFile::open</a>,
<a href="../api_cxx/memp_fput.html">DbMpoolFile::put</a>,
<a href="../api_cxx/memp_fset.html">DbMpoolFile::set</a>,
<a href="../api_cxx/memp_fsync.html">DbMpoolFile::sync</a>,
<a href="../api_cxx/memp_register.html">DbEnv::memp_register</a>,
<a href="../api_cxx/memp_stat.html">DbEnv::memp_stat</a>,
<a href="../api_cxx/memp_sync.html">DbEnv::memp_sync</a>
and
<a href="../api_cxx/memp_trickle.html">DbEnv::memp_trickle</a>.
</tt>
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