1<!--$Id: memp_set_fileid.so,v 10.14 2005/04/14 13:08:06 sue Exp $--> 2<!--Copyright (c) 1997,2008 Oracle. All rights reserved.--> 3<!--See the file LICENSE for redistribution information.--> 4<html> 5<head> 6<title>Berkeley DB: DbMpoolFile::set_fileid</title> 7<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit."> 8<meta name="keywords" content="embedded,database,programmatic,toolkit,btree,hash,hashing,transaction,transactions,locking,logging,access method,access methods,Java,C,C++"> 9</head> 10<body bgcolor=white> 11<table width="100%"><tr valign=top> 12<td> 13<b>DbMpoolFile::set_fileid</b> 14</td> 15<td align=right> 16<a href="/api_cxx/api_core.html"><img src="/images/api.gif" alt="API"></a> 17<a href="/ref/toc.html"><img src="/images/ref.gif" alt="Ref"></a></td> 18</tr></table> 19<hr size=1 noshade> 20<tt> 21<b><pre> 22#include <db_cxx.h> 23<p> 24int 25DbMpoolFile::set(u_int8_t *fileid); 26<p> 27int DbMpoolFile::get_fileid(u_int8_t *fileid); 28</pre></b> 29<hr size=1 noshade> 30<b>Description: DbMpoolFile::set_fileid</b> 31<p>The DbMpoolFile::set_fileid method specifies a unique identifier for the file. 32(The shared memory buffer pool functions must be able to uniquely 33identify files in order that multiple processes wanting to share a file 34will correctly identify it in the pool.)</p> 35<p>On most UNIX/POSIX systems, the <b>fileid</b> field will not need to 36be set, and the memory pool functions will use the file's device and 37inode numbers for this purpose. On Windows systems, the memory pool 38functions use the values returned by GetFileInformationByHandle() by 39default -- these values are known to be constant between processes and 40over reboot in the case of NTFS (in which they are the NTFS MFT 41indices).</p> 42<p>On other filesystems (for example, FAT or NFS), these default values 43are not necessarily unique between processes or across system reboots. 44<b>Applications wanting to maintain a shared memory buffer pool 45between processes or across system reboots, in which the pool contains 46pages from files stored on such filesystems, must specify a unique file 47identifier using the DbMpoolFile::set_fileid method, and each process opening 48the file must provide the same unique identifier.</b></p> 49<p>This call should not be necessary for most applications. Specifically, 50it is not necessary if the memory pool is not shared between processes 51and is reinstantiated after each system reboot, if the application is 52using the Berkeley DB access methods instead of calling the pool functions 53explicitly, or if the files in the memory pool are stored on filesystems 54in which the default values as described previously are invariant 55between process and across system reboots.</p> 56<p>The DbMpoolFile::set_fileid method configures a file in the memory pool, not only 57operations performed using the specified <a href="/api_cxx/mempfile_class.html">DbMpoolFile</a> handle.</p> 58<p>The DbMpoolFile::set_fileid method may not be called after the <a href="/api_cxx/memp_fopen.html">DbMpoolFile::open</a> method is 59called. 60</p> 61<p>The DbMpoolFile::set_fileid method 62either returns a non-zero error value 63or throws an exception that encapsulates a non-zero error value on 64failure, and returns 0 on success. 65</p> 66<b>Parameters</b> <br> 67 <b>fileid</b><ul compact><li>The <b>fileid</b> parameter is the unique identifier for the file. 68Unique file identifiers must be a DB_FILE_ID_LEN length array of bytes.</ul> 69<br> 70<hr size=1 noshade> 71<b>Description: DbMpoolFile::get_fileid</b> 72<p>The DbMpoolFile::get_fileid method copies the file's identifier into the 73memory location referenced by <b>fileid</b>.</p> 74<p>The DbMpoolFile::get_fileid method 75either returns a non-zero error value 76or throws an exception that encapsulates a non-zero error value on 77failure, and returns 0 on success. 78</p> 79<p>The DbMpoolFile::get_fileid method may be called at any time during the life of the 80application.</p> 81<hr size=1 noshade> 82<br><b>Class</b> 83<a href="/api_cxx/env_class.html">DbEnv</a>, <a href="/api_cxx/mempfile_class.html">DbMpoolFile</a> 84<br><b>See Also</b> 85<a href="/api_cxx/memp_list.html">Memory Pools and Related Methods</a> 86</tt> 87<table width="100%"><tr><td><br></td><td align=right> 88<a href="/api_cxx/api_core.html"><img src="/images/api.gif" alt="API"></a><a href="/ref/toc.html"><img src="/images/ref.gif" alt="Ref"></a> 89</td></tr></table> 90<p><font size=1>Copyright (c) 1996,2008 Oracle. All rights reserved.</font> 91</body> 92</html> 93
