381 lines
10 KiB
HTML
381 lines
10 KiB
HTML
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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="Copyright" content="Copyright (c) 2006 by IBM Corporation">
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<title>msync()--Synchronize Modified Data with Mapped File</title>
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<!-- Begin Header Records -->
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<!-- All rights reserved. Licensed Materials Property of IBM -->
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<!-- US Government Users Restricted Rights -->
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<!-- Use, duplication or disclosure restricted by -->
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<!-- Change History: -->
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<!-- YYMMDD USERID Change description -->
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<!-- MSYNC SCRIPT A converted by B2H R4.1 (346) (CMS) by V2CDIJAB at -->
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<!-- RCHVMW2 on 19 Sep 2000 at 14:21:52 -->
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<!-- 011108 JTROUS Changes from CL Review 1, V5R2 -->
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<!-- 0205?? JET This file has undergone html cleanup -->
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<!-- 020918 Rosckes: Add ENOTAVAIL errno description, V5R3 -->
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<!-- 050404 JTROUS: fix enums, no change flag, V5R4 -->
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<!-- 050404 JTROUS Add note on tagged pointers, V5R4 -->
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<!-- End Header Records -->
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<link rel="stylesheet" type="text/css" href="../rzahg/ic.css">
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</head>
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<body>
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<!-- Java sync-link -->
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<script language="Javascript" src="../rzahg/synch.js" type="text/javascript">
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</script>
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<a name="Top_Of_Page"></a>
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<h2>msync()--Synchronize Modified Data with Mapped File</h2>
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<div class="box" style="width: 60%;">
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<br>
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Syntax<br>
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<pre>
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#include <sys/types.h>
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#include <sys/mman.h>
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int msync( void <em>*addr</em>,
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size_t <em>len</em>,
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int <em>flags</em> );
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</pre>
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Service Program Name: QP0LLIB1<br>
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<!-- iddvc RMBR -->
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<br>
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Default Public Authority: *USE<br>
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<!-- iddvc RMBR -->
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<br>
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Threadsafe: Yes<br>
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<!-- iddvc RMBR -->
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<br>
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</div>
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<p>The <strong>msync()</strong> function can be used to write modified data
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from a shared mapping (created using the <strong>mmap()</strong> function) to
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non-volatile storage or invalidate privately mapped pages. The data located
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through mapping address <em>addr</em> for a length of <em>len</em> are either
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written to disk, or invalidated, depending on the value of <em>flags</em> and
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the private or shared nature of the mapping.</p>
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<br>
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<h3>Parameters</h3>
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<dl>
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<dt><strong><em>addr</em></strong></dt>
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<dd>The starting address of the memory region to be synchronized to permanent
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storage. The specified address must be a multiple of the page size.<br>
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<br>
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</dd>
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<dt><strong><em>len</em></strong></dt>
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<dd>The number of bytes affected. The length must not be zero. If the length is
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not a multiple of the page size the system will round this value to the next
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page boundary.<br>
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<br>
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</dd>
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<dt><strong><em>flags</em></strong></dt>
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<dd>The desired synchronization.
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<p>The following table shows the symbolic constants allowed for the flags
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parameter.</p>
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<table border cellpadding="5" width="80%">
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<tr>
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<th align="left" valign="top">Symbolic Constant</th>
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<th align="left" valign="top">Decimal<br>
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Value</th>
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<th align="left" valign="bottom">Description</th>
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</tr>
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<tr>
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<td align="left" valign="top" width="20%">MS_ASYNC</td>
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<td align="center" valign="top" width="15%">1</td>
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<td align="left" valign="top" width="65%">Perform asynchronous writes.</td>
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</tr>
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<tr>
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<td align="left" valign="top">MS_SYNC</td>
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<td align="center" valign="top">2</td>
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<td align="left" valign="top">Perform synchronous writes.</td>
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</tr>
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<tr>
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<td align="left" valign="top">MS_INVALIDATE</td>
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<td align="center" valign="top">4</td>
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<td align="left" valign="top">Invalidate privately cached data</td>
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</tr>
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</table>
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<p>The MS_SYNC and MS_ASYNC options are mutually exclusive. The MS_SYNC and
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MS_ASYNC options are ignored if the memory map was created with the MAP_PRIVATE
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option.</p>
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<p>The MS_INVALIDATE option is used to discard changes made to a memory map
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created with the MAP_PRIVATE option. The private memory map is synchronized
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with the current data in the file. Any reference subsequent to the execution of
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the <strong>msync()</strong> function that invalidates a page will result in a
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reference to the current value of the file. The first modification of a page
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after the privately mapped page is invalidated results in the creation of a
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fresh private copy of that page. Subsequent modifications of this page prior to
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the next execution of an msync that invalidates the page will result in
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modifications to the same private copy of the page.</p>
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<p>The MS_INVALIDATE value is ignored if the memory map was created with the
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MAP_SHARED option.</p>
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</dd>
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</dl>
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<br>
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<h3>Authorities</h3>
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<p>No authorization is required.</p>
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<br>
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<h3>Return Value</h3>
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<p>Upon successful completion, the <strong>msync()</strong> function returns
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0.</p>
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<br>
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<h3>Error Conditions</h3>
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<p>When the <strong>msync()</strong> function fails, it returns -1 and sets
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errno as follows.</p>
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<table cellpadding="5">
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<!-- cols="25 75" -->
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<tr>
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<th align="left" valign="bottom">Error condition</th>
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<th align="left" valign="bottom">Additional information</th>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EINVAL">EINVAL</a>]</em></td>
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<td align="left" valign="top">
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<p>For example, the value of the <em>len</em> parameter may be zero. Or,
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the value of the <em>addr</em> may not be a multiple of the page size or is
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out of the allowed range.</p>
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</td></tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#ENOTAVAIL">ENOTAVAIL</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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<tr>
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<td align="left" valign="top">
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<em>[<a href="unix14.htm#EUNKNOWN">EUNKNOWN</a>]</em></td>
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<td align="left" valign="top">
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</td>
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</tr>
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</table>
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<br>
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<br>
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<h3>Error Messages</h3>
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<p>The following messages may be sent from this function.</p>
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<table width="100%">
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<tr>
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<th align="left" valign="top">Message ID</th>
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<th align="left" valign="top">Error Message Text</th>
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</tr>
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<tr>
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<td width="15%" valign="top">CPE3418 E</td>
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<td width="85%" valign="top">Possible APAR condition or hardware failure.</td>
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</tr>
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<tr>
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<td valign="top">CPFA0D4 E</td>
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<td valign="top">File system error occurred. Error number &1.</td>
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</tr>
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<tr>
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<td valign="top">CPF3CF2 E</td>
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<td valign="top">Error(s) occurred during running of &1 API.</td>
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</tr>
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<tr>
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<td valign="top">CPF9872 E</td>
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<td valign="top">Program or service program &1 in library &2 ended.
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Reason code &3.</td>
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</tr>
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</table>
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<br>
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<h3>Usage Notes</h3>
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<ol>
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<li>The <strong>msync()</strong> function must be used to write changed pages
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of a shared mapping to disk. If a system crash occurs before the <strong>
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msync()</strong> function completes, some data may not be preserved.
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<p>Process termination does not automatically write changed pages to disk. Some
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or all pages may be eventually written by the paging subsystem, but no
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guarantee is given. Therefore, if the data must be preserved the <strong>
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msync()</strong> function must be used to ensure changes made through a shared
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memory map are written to disk.</p>
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</li>
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<li>The address pointer that was returned by <strong>mmap()</strong> can only
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be used with the V4R4M0 or later versions of the following languages:
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<ul>
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<li>ILE COBOL</li>
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<li>ILE RPG</li>
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<li>ILE C if the TERASPACE parameter is used when compiling the program.</li>
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</ul>
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<br>
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</li>
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<li>
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<img src="delta.gif" alt="Start of change">
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The application cannot write or store any data via the memory mapping which
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includes any tagged (16-byte) pointers because the pointer attribute will be lost.
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Some examples of tagged pointers include space pointers, system pointers,
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invocation pointers etc..
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<p>If the DTAMDL(*LLP64) parameter is used when compiling an ILE C program,
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this limitation does not apply as the pointers will be 8 byte pointers, and their
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pointer attribute will be preserved.
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<img src="deltaend.gif" alt="End of change">
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</li>
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</ol>
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<br>
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<h3>Related Information</h3>
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<ul>
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<li><a href="open.htm">open()</a>--Open File
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</li>
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<li><a href="open64.htm">open64()</a>--Open File (Large File Enabled)
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</li>
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<li><a href="mmap.htm">mmap()</a>--Memory Map a Stream File
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</li>
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<li><a href="munmap.htm">munmap()</a>--Remove Memory Mapping
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</li>
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<li><a href="mprotect.htm">mprotect()</a>--Change Access Protection for Memory
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Mapping</li>
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</ul>
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<br>
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<h3>Example</h3>
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<p>See <a href="../apiref/aboutapis.htm#codedisclaimer">Code disclaimer information</a>
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for information pertaining to code examples.</p>
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<p>The following example creates a file, creates a memory map, stores data into
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the file, and writes the data to disk using the <strong>msync()</strong>
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function.</p>
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<pre>
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#include <errno.h >
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#include <fcntl.h >
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#include <unistd.h >
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#include <stdio.h >
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#include <stdlib.h >
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#include <string.h >
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#include <sys/types.h >
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#include <sys/mman.h >
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main(void) {
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size_t bytesWritten =0;
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int fd;
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int PageSize;
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const char textÝ = "This is a test";
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if ( (PageSize = sysconf(_SC_PAGE_SIZE)) < 0) {
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perror("sysconf() Error=");
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return -1;
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}
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fd = open("/tmp/mmsyncTest",
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(O_CREAT | O_TRUNC | O_RDWR),
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(S_IRWXU | S_IRWXG | S_IRWXO) );
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if ( fd < 0 ) {
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perror("open() error");
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return fd;
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}
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off_t lastoffset = lseek( fd, PageSize, SEEK_SET);
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bytesWritten = write(fd, " ", 1 );
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if (bytesWritten != 1 ) {
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perror("write error. ");
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return -1;
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}
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/* mmap the file. */
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void *address;
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int len;
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off_t my_offset = 0;
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len = PageSize; /* Map one page */
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address =
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mmap(NULL, len, PROT_WRITE, MAP_SHARED, fd, my_offset);
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if ( address == MAP_FAILED ) {
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perror("mmap error. " );
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return -1;
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}
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/* Move some data into the file using memory map. */
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(void) strcpy( (char*) address, text);
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/* use msync to write changes to disk. */
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if ( msync( address, PageSize , MS_SYNC ) < 0 ) {
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perror("msync failed with error:");
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return -1;
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}
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else (void) printf("%s","msync completed successfully.");
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close(fd);
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unlink("/tmp/msyncTest");
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}
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</pre>
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<p><strong>Output:</strong></p>
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<pre>
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This is a test.
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</pre>
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<hr>
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API introduced: V5R1
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<hr>
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<center>
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||
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<table cellpadding="2" cellspacing="2">
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||
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<tr align="center">
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||
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<td valign="middle" align="center"><a href="#Top_Of_Page">Top</a> | <a href=
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||
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"unix.htm">UNIX-Type APIs</a> | <a href="aplist.htm">APIs by category</a></td>
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</tr>
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</table>
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||
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</center>
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</body>
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</html>
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