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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"><html><head><meta name="generator" content="HTML Tidy, see www.w3.org"><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"><link type="text/css" rel="stylesheet" href="style.css"><!-- Generated by The Open Group's rhtm tool v1.2.1 --><!-- Copyright (c) 2001-2004 IEEE and The Open Group, All Rights Reserved --><title>pthread_attr_destroy</title></head><body bgcolor="white"><script type="text/javascript" language="JavaScript" src="../jscript/codes.js"></script><basefont size="3"> <a name="pthread_attr_destroy"></a> <a name="tag_03_487"></a><!-- pthread_attr_destroy --> <!--header start--><center><font size="2">The Open Group Base Specifications Issue 6<br>IEEE Std 1003.1, 2004 Edition<br>Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.</font></center><!--header end--><hr size="2" noshade><h4><a name="tag_03_487_01"></a>NAME</h4><blockquote>pthread_attr_destroy, pthread_attr_init - destroy and initialize the thread attributes object</blockquote><h4><a name="tag_03_487_02"></a>SYNOPSIS</h4><blockquote class="synopsis"><div class="box"><code><tt><sup>[<a href="javascript:open_code('THR')">THR</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0"> #include <<a href="../basedefs/pthread.h.html">pthread.h</a>><br><br> int pthread_attr_destroy(pthread_attr_t *</tt><i>attr</i><tt>);<br> int pthread_attr_init(pthread_attr_t *</tt><i>attr</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]" border="0"></tt></code></div><tt><br></tt></blockquote><h4><a name="tag_03_487_03"></a>DESCRIPTION</h4><blockquote><p>The <i>pthread_attr_destroy</i>() function shall destroy a thread attributes object. An implementation may cause<i>pthread_attr_destroy</i>() to set <i>attr</i> to an implementation-defined invalid value. A destroyed <i>attr</i> attributesobject can be reinitialized using <i>pthread_attr_init</i>(); the results of otherwise referencing the object after it has beendestroyed are undefined.</p><p>The <i>pthread_attr_init</i>() function shall initialize a thread attributes object <i>attr</i> with the default value for allof the individual attributes used by a given implementation.</p><p>The resulting attributes object (possibly modified by setting individual attribute values) when used by <a href="../functions/pthread_create.html"><i>pthread_create</i>()</a> defines the attributes of the thread created. A single attributesobject can be used in multiple simultaneous calls to <a href="../functions/pthread_create.html"><i>pthread_create</i>()</a>.Results are undefined if <i>pthread_attr_init</i>() is called specifying an already initialized <i>attr</i> attributes object.</p></blockquote><h4><a name="tag_03_487_04"></a>RETURN VALUE</h4><blockquote><p>Upon successful completion, <i>pthread_attr_destroy</i>() and <i>pthread_attr_init</i>() shall return a value of 0; otherwise,an error number shall be returned to indicate the error.</p></blockquote><h4><a name="tag_03_487_05"></a>ERRORS</h4><blockquote><p>The <i>pthread_attr_init</i>() function shall fail if:</p><dl compact><dt>[ENOMEM]</dt><dd>Insufficient memory exists to initialize the thread attributes object.</dd></dl><p>The <i>pthread_attr_destroy</i>() function may fail if:</p><dl compact><dt>[EINVAL]</dt><dd>The value specified by <i>attr</i> does not refer to an initialized thread attribute object.</dd></dl><p>The <i>pthread_attr_init</i>() function may fail if:</p><dl compact><dt>[EBUSY]</dt><dd>The implementation has detected an attempt to reinitialize the thread attribute referenced by <i>attr</i>, a previouslyinitialized, but not yet destroyed, thread attribute.</dd></dl><p>These functions shall not return an error code of [EINTR].</p></blockquote><hr><div class="box"><em>The following sections are informative.</em></div><h4><a name="tag_03_487_06"></a>EXAMPLES</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_487_07"></a>APPLICATION USAGE</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_487_08"></a>RATIONALE</h4><blockquote><p>Attributes objects are provided for threads, mutexes, and condition variables as a mechanism to support probable futurestandardization in these areas without requiring that the function itself be changed.</p><p>Attributes objects provide clean isolation of the configurable aspects of threads. For example, "stack size" is an importantattribute of a thread, but it cannot be expressed portably. When porting a threaded program, stack sizes often need to be adjusted.The use of attributes objects can help by allowing the changes to be isolated in a single place, rather than being spread acrossevery instance of thread creation.</p><p>Attributes objects can be used to set up "classes' of threads with similar attributes; for example, "threads with large stacksand high priority" or "threads with minimal stacks". These classes can be defined in a single place and then referenced whereverthreads need to be created. Changes to "class" decisions become straightforward, and detailed analysis of each <a href="../functions/pthread_create.html"><i>pthread_create</i>()</a> call is not required.</p><p>The attributes objects are defined as opaque types as an aid to extensibility. If these objects had been specified asstructures, adding new attributes would force recompilation of all multi-threaded programs when the attributes objects areextended; this might not be possible if different program components were supplied by different vendors.</p><p>Additionally, opaque attributes objects present opportunities for improving performance. Argument validity can be checked oncewhen attributes are set, rather than each time a thread is created. Implementations often need to cache kernel objects that areexpensive to create. Opaque attributes objects provide an efficient mechanism to detect when cached objects become invalid due toattribute changes.</p><p>Since assignment is not necessarily defined on a given opaque type, implementation-defined default values cannot be defined in aportable way. The solution to this problem is to allow attributes objects to be initialized dynamically by attributes objectinitialization functions, so that default values can be supplied automatically by the implementation.</p><p>The following proposal was provided as a suggested alternative to the supplied attributes:</p><ol><li><p>Maintain the style of passing a parameter formed by the bitwise-inclusive OR of flags to the initialization routines ( <a href="../functions/pthread_create.html"><i>pthread_create</i>()</a>, <a href="../functions/pthread_mutex_init.html"><i>pthread_mutex_init</i>()</a>, <a href="../functions/pthread_cond_init.html"><i>pthread_cond_init</i>()</a>). The parameter containing the flags should be an opaque typefor extensibility. If no flags are set in the parameter, then the objects are created with default characteristics. Animplementation may specify implementation-defined flag values and associated behavior.</p></li><li><p>If further specialization of mutexes and condition variables is necessary, implementations may specify additional proceduresthat operate on the <b>pthread_mutex_t</b> and <b>pthread_cond_t</b> objects (instead of on attributes objects).</p></li></ol><p>The difficulties with this solution are:</p><ol><li><p>A bitmask is not opaque if bits have to be set into bitvector attributes objects using explicitly-coded bitwise-inclusive ORoperations. If the set of options exceeds an <b>int</b>, application programmers need to know the location of each bit. If bits areset or read by encapsulation (that is, get and set functions), then the bitmask is merely an implementation of attributes objectsas currently defined and should not be exposed to the programmer.</p></li><li><p>Many attributes are not Boolean or very small integral values. For example, scheduling policy may be placed in 3-bit or 4-bit,but priority requires 5-bit or more, thereby taking up at least 8 bits out of a possible 16 bits on machines with 16-bit integers.Because of this, the bitmask can only reasonably control whether particular attributes are set or not, and it cannot serve as therepository of the value itself. The value needs to be specified as a function parameter (which is non-extensible), or by setting astructure field (which is non-opaque), or by get and set functions (making the bitmask a redundant addition to the attributesobjects).</p></li></ol><p>Stack size is defined as an optional attribute because the very notion of a stack is inherently machine-dependent. Someimplementations may not be able to change the size of the stack, for example, and others may not need to because stack pages may bediscontiguous and can be allocated and released on demand.</p><p>The attribute mechanism has been designed in large measure for extensibility. Future extensions to the attribute mechanism or toany attributes object defined in this volume of IEEE Std 1003.1-2001 has to be done with care so as not to affectbinary-compatibility.</p><p>Attributes objects, even if allocated by means of dynamic allocation functions such as <a href="../functions/malloc.html"><i>malloc</i>()</a>, may have their size fixed at compile time. This means, for example, a <a href="../functions/pthread_create.html"><i>pthread_create</i>()</a> in an implementation with extensions to <b>pthread_attr_t</b> cannotlook beyond the area that the binary application assumes is valid. This suggests that implementations should maintain a size fieldin the attributes object, as well as possibly version information, if extensions in different directions (possibly by differentvendors) are to be accommodated.</p></blockquote><h4><a name="tag_03_487_09"></a>FUTURE DIRECTIONS</h4><blockquote><p>None.</p></blockquote><h4><a name="tag_03_487_10"></a>SEE ALSO</h4><blockquote><p><a href="pthread_attr_getstackaddr.html"><i>pthread_attr_getstackaddr</i>()</a>, <a href="pthread_attr_getstacksize.html"><i>pthread_attr_getstacksize</i>()</a>, <a href="pthread_attr_getdetachstate.html"><i>pthread_attr_getdetachstate</i>()</a>, <a href="pthread_create.html"><i>pthread_create</i>()</a>, the Base Definitions volume of IEEE Std 1003.1-2001, <a href="../basedefs/pthread.h.html"><i><pthread.h></i></a></p></blockquote><h4><a name="tag_03_487_11"></a>CHANGE HISTORY</h4><blockquote><p>First released in Issue 5. Included for alignment with the POSIX Threads Extension.</p></blockquote><h4><a name="tag_03_487_12"></a>Issue 6</h4><blockquote><p>The <i>pthread_attr_destroy</i>() and <i>pthread_attr_init</i>() functions are marked as part of the Threads option.</p><p>IEEE PASC Interpretation 1003.1 #107 is applied, noting that the effect of initializing an already initialized thread attributesobject is undefined.</p><p>IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/71 is applied, updating the ERRORS section to add the optional[EINVAL] error for the <i>pthread_attr_destroy</i>() function, and the optional [EBUSY] error for the <i>pthread_attr_init</i>()function.</p></blockquote><div class="box"><em>End of informative text.</em></div><hr size="2" noshade><center><font size="2"><!--footer start-->UNIX ® is a registered Trademark of The Open Group.<br>POSIX ® is a registered Trademark of The IEEE.<br>[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href="../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>]</font></center><!--footer end--><hr size="2" noshade></body></html>⌨️ 快捷键说明
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