71 lines
4.6 KiB
HTML
71 lines
4.6 KiB
HTML
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<meta name="DC.Title" content="Database record I/O and thread safety" />
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<meta name="abstract" content="You can use the I/O feedback area of a database file to communicate results of I/O operations to the record I/O user. A typical application might have problems with thread safety because of the nature of the feedback area." />
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<title>Database record I/O and thread safety</title>
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</head>
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<body id="rzahwdio-dioco"><a name="rzahwdio-dioco"><!-- --></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">Database record I/O and thread safety</h1>
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<div><p>You can use the I/O feedback area of a database file to communicate
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results of I/O operations to the record I/O user. A typical application might
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have problems with thread safety because of the nature of the feedback area.</p>
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<div class="section"><p>When you perform an I/O operation, the support of the run time
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and of the database ensures that the I/O operation is threadsafe. When the
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I/O is complete, any locks that the run time holds are released
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and control is given back to the application.</p>
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</div>
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<div class="section"><p>It is the application's responsibility to serialize access when
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examining the I/O feedback area so that another thread does not use it simultaneously.
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COBOL and RPG use the feedback area on every I/O operation, so if you do I/O
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to the same shared file with different languages, you should serialize access
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to the file even when you are not apparently using the feedback area.</p>
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</div>
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<div class="section"><p>For example, Thread 1 is using file A for record
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I/O operations. The system stores information about those operations in the
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feedback area for file A. After the I/O operation for Thread 1 completes,
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Thread 1 manipulates the feedback area by examining fields in the feedback
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area. If it does not protect the feedback area when Thread 2 uses file A for
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record I/O operations, the system uses the same feedback area. Thread 1 may
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get inconsistent results because Thread 2 is using the feedback area at the
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same time. This occurs when sharing a file is conducted by the use of the
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file pointer or file descriptor number. It is not a problem if the file is
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opened in each thread.</p>
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</div>
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<div class="section"><p>You should use a synchronization technique to protect the feedback
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area so that other application code or the system does not change it when
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it is being accessed.</p>
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</div>
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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="rzahwcomco.htm" title="Several programming errors often occur when writing multithreaded applications.">Common multithreaded programming errors</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="rzahwdbcco.htm" title="You should consider these items when using databases in multithreaded programs.">Database considerations for multithreaded programming</a></div>
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</div>
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</div>
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</body>
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