87 lines
6.7 KiB
HTML
87 lines
6.7 KiB
HTML
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<meta name="DC.Title" content="Open files using temporary file descriptions" />
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<meta name="abstract" content="Temporary changes can provide greater flexibility to the application. The server makes temporary changes when the program is first establishing a path to the file by opening the file." />
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<title>Open files using temporary file descriptions</title>
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</head>
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<body id="rbal3opentemp"><a name="rbal3opentemp"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Open files using temporary file descriptions</h1>
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<div><p>Temporary changes can provide greater flexibility to the application.
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The server makes temporary changes when the program is first establishing
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a path to the file by opening the file.</p>
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<div class="p">Temporary changes can be made in one of two ways: <ul><li>By information that is specified within the program itself, and which
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is passed as parameters on the open operation.</li>
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<li>By using override CL commands in the input stream that is used to set
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up the run-time environment for the application</li>
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</ul>
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</div>
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<p>The ability to use the first way depends very much on which programming
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language you used to write the program. Some programming languages do not
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allow you to control the open process to any great extent. These languages
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do the open process more or less automatically and control what information
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gets passed. Other languages allow you to have greater control over the open
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process.</p>
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<p>You can use the second option regardless of which programming language
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you use. The <span class="keyword">iSeries™ server</span> provides
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override CL commands for each file type. By including override commands with
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the application, you can temporarily change the file description in a file
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that the program wants to use.</p>
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<div class="p">You can use both options together. Information that is contained in the
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application can change some parameters; an override command can change others. Both can change the same parameter. The operating system follows this order
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when making temporary changes to a file: <ol><li>The file description provides a base of information.</li>
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<li>Change information received from the application during the open process
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is applied first to the base information.</li>
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<li>Change information found in the override command is applied last. If
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both the change information from the application and the override change the
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same information, the override has precedence.</li>
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</ol>
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</div>
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<p>Only the application that makes the changes can see the temporary changes. The file, as seen by another application, remains unchanged. In fact, two
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applications can use the same file at the same time, and each can change it
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temporarily according to its needs. Neither application is aware the other
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has made a temporary change. <a href="#rbal3opentemp__rslh143">Figure 1</a> and <a href="#rbal3opentemp__rslh196">Figure 2</a> illustrate the permanent and temporary
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change processes.</p>
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<div class="fignone" id="rbal3opentemp__rslh143"><a name="rbal3opentemp__rslh143"><!-- --></a><span class="figcap">Figure 1. Permanently changing a File</span><br /><img src="rslh143.gif" alt="The process for permanently
changing a file." /><br /></div>
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<div class="fignone" id="rbal3opentemp__rslh196"><a name="rbal3opentemp__rslh196"><!-- --></a><span class="figcap">Figure 2. Temporarily changing a File</span><br /><img src="rslh196.gif" alt="The process for temporarily
changing a file." /><br /></div>
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<p>Once an application establishes a connection between itself and the file
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by opening the file, it can then proceed to use the file for either input
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or output operations. In the case of a database file, the open process establishes
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a path between the application and the actual database file. For device files,
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a path is established between the application and the actual device, or to
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a spooled file if the spooling attribute is active for the device file. In
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all cases, the application connects to what it wants to use, and those connections
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determine what input or output operations are valid. Not all operations are
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valid with all file types. The application must be aware of what file types
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it uses and then use only those operations which are valid for those types.</p>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbal3open.htm" title="When you want an application to use a file, you do so by referring to that file by name. The file description for that file will then control how the program and the server will interact.">Open files</a></div>
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</div>
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</div>
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