87 lines
5.2 KiB
HTML
87 lines
5.2 KiB
HTML
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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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="DC.Type" content="reference" />
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<meta name="DC.Title" content="Storage usage and threaded applications" />
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<meta name="abstract" content="When writing a threaded application, it is important to understand the visibility and scope of various classes of storage." />
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<meta name="description" content="When writing a threaded application, it is important to understand the visibility and scope of various classes of storage." />
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<meta name="copyright" content="(C) Copyright IBM Corporation 1998, 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 1998, 2006" />
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<meta name="DC.Format" content="XHTML" />
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<meta name="DC.Identifier" content="rzahwsto-stoco" />
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<meta name="DC.Language" content="en-us" />
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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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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Storage usage and threaded applications</title>
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</head>
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<body id="rzahwsto-stoco"><a name="rzahwsto-stoco"><!-- --></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">Storage usage and threaded applications</h1>
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<div><p>When writing a threaded application, it is important to understand
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the visibility and scope of various classes of storage.</p>
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<div class="section"><div class="p">When your application declares variables, multiple threads might
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be able to access or use them. The visibility and scope of the storage it
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uses often affect your application.<ul><li>Global storage:<p>Global storage that you declare in one of your application
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source files is visible to all other source files or modules in your application.
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All threads that run code in your application share the same global storage.
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This unintended sharing of storage is a common safety problem.</p>
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</li>
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<li>Static storage:<p>Static storage is global storage with visibility that
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is restricted to the source file, module, or function in which the storage
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or variable was declared. All threads that run code from that module share
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the same static storage. </p>
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</li>
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<li>Heap storage:<p>Heap storage is allocated and deallocated dynamically
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by your application (for example, by <span class="cmdname">malloc()</span> and <span class="cmdname">free()</span> in
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C, or new and delete in Java™ or C++). All threads that run code
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in your application share the same heap. </p>
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<p>If you give another thread
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a pointer to the heap storage that has been allocated (by storing the pointer
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in static or global storage, or by otherwise passing the pointer to another
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thread) that thread can use or deallocate the storage. This unintended sharing
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of storage is a common thread safety problem. </p>
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</li>
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<li>Automatic storage:<p>Automatic or local storage is allocated automatically
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by your language when you declare a variable that is private to a function,
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method, or subroutine. Local storage is not visible to other threads in the
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process. Each time those threads call the function, method, or subroutine,
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the thread allocates new versions of the automatic variables. Each thread
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gets its own automatic storage. A thread should not access automatic storage
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from another thread because of the complex serialization and synchronization
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issues involved.</p>
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</li>
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</ul>
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</div>
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</div>
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<div class="section"><p>The operating system further restricts
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the scope of global, static, and heap storage to the activation group that
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your application code is running in. This means that application code or threads
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running in different activation groups but using the same global or static
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variables access different versions of those variables and their storage.
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Similarly, heap storage allocated in one activation group might not be de-allocated
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in a different activation group although it can be used by different activation
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groups. </p>
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</div>
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<div class="section"><p>Some languages support only global or static storage classes for
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variables. These languages are not easy to use in threaded applications. You
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should not use threads in these languages.</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="rzahwsafco.htm" title="A function is threadsafe if you can start it simultaneously in multiple threads within the same process. A function is threadsafe if and only if all the functions it calls are also threadsafe.">Thread safety</a></div>
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</div>
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</div>
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</body>
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</html>
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