222 lines
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222 lines
11 KiB
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<title>pthread_attr_destroy</title>
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<basefont size="3"> <a name="pthread_attr_destroy"></a> <a name="tag_03_487"></a><!-- pthread_attr_destroy -->
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<!--header start-->
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<center><font size="2">The Open Group Base Specifications Issue 6<br>
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IEEE Std 1003.1-2001<br>
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Copyright © 2001 The IEEE and The Open Group, All Rights reserved.</font></center>
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<!--header end-->
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<hr size="2" noshade>
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<h4><a name="tag_03_487_01"></a>NAME</h4>
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<blockquote>pthread_attr_destroy, pthread_attr_init - destroy and initialize the thread attributes object</blockquote>
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<h4><a name="tag_03_487_02"></a>SYNOPSIS</h4>
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<blockquote class="synopsis">
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<div class="box"><code><tt><sup>[<a href="javascript:open_code('THR')">THR</a>]</sup> <img src="../images/opt-start.gif" alt=
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"[Option Start]" border="0"> #include <<a href="../basedefs/pthread.h.html">pthread.h</a>><br>
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<br>
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int pthread_attr_destroy(pthread_attr_t *</tt><i>attr</i><tt>);<br>
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int pthread_attr_init(pthread_attr_t *</tt><i>attr</i><tt>); <img src="../images/opt-end.gif" alt="[Option End]" border=
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"0"></tt></code></div>
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<tt><br>
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</tt></blockquote>
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<h4><a name="tag_03_487_03"></a>DESCRIPTION</h4>
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<blockquote>
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<p>The <i>pthread_attr_destroy</i>() function shall destroy a thread attributes object. An implementation may cause
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<i>pthread_attr_destroy</i>() to set <i>attr</i> to an implementation-defined invalid value. A destroyed <i>attr</i> attributes
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object can be reinitialized using <i>pthread_attr_init</i>(); the results of otherwise referencing the object after it has been
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destroyed are undefined.</p>
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<p>The <i>pthread_attr_init</i>() function shall initialize a thread attributes object <i>attr</i> with the default value for all
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of the individual attributes used by a given implementation.</p>
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<p>The resulting attributes object (possibly modified by setting individual attribute values) when used by <a href=
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"../functions/pthread_create.html"><i>pthread_create</i>()</a> defines the attributes of the thread created. A single attributes
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object can be used in multiple simultaneous calls to <a href="../functions/pthread_create.html"><i>pthread_create</i>()</a>.
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Results are undefined if <i>pthread_attr_init</i>() is called specifying an already initialized <i>attr</i> attributes object.</p>
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</blockquote>
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<h4><a name="tag_03_487_04"></a>RETURN VALUE</h4>
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<blockquote>
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<p>Upon successful completion, <i>pthread_attr_destroy</i>() and <i>pthread_attr_init</i>() shall return a value of 0; otherwise,
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an error number shall be returned to indicate the error.</p>
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</blockquote>
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<h4><a name="tag_03_487_05"></a>ERRORS</h4>
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<blockquote>
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<p>The <i>pthread_attr_init</i>() function shall fail if:</p>
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<dl compact>
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<dt>[ENOMEM]</dt>
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<dd>Insufficient memory exists to initialize the thread attributes object.</dd>
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</dl>
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<p>These functions shall not return an error code of [EINTR].</p>
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</blockquote>
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<hr>
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<div class="box"><em>The following sections are informative.</em></div>
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<h4><a name="tag_03_487_06"></a>EXAMPLES</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_487_07"></a>APPLICATION USAGE</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_487_08"></a>RATIONALE</h4>
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<blockquote>
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<p>Attributes objects are provided for threads, mutexes, and condition variables as a mechanism to support probable future
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standardization in these areas without requiring that the function itself be changed.</p>
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<p>Attributes objects provide clean isolation of the configurable aspects of threads. For example, "stack size" is an important
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attribute of a thread, but it cannot be expressed portably. When porting a threaded program, stack sizes often need to be adjusted.
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The use of attributes objects can help by allowing the changes to be isolated in a single place, rather than being spread across
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every instance of thread creation.</p>
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<p>Attributes objects can be used to set up "classes' of threads with similar attributes; for example, "threads with large stacks
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and high priority" or "threads with minimal stacks". These classes can be defined in a single place and then referenced wherever
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threads need to be created. Changes to "class" decisions become straightforward, and detailed analysis of each <a href=
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"../functions/pthread_create.html"><i>pthread_create</i>()</a> call is not required.</p>
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<p>The attributes objects are defined as opaque types as an aid to extensibility. If these objects had been specified as
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structures, adding new attributes would force recompilation of all multi-threaded programs when the attributes objects are
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extended; this might not be possible if different program components were supplied by different vendors.</p>
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<p>Additionally, opaque attributes objects present opportunities for improving performance. Argument validity can be checked once
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when attributes are set, rather than each time a thread is created. Implementations often need to cache kernel objects that are
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expensive to create. Opaque attributes objects provide an efficient mechanism to detect when cached objects become invalid due to
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attribute changes.</p>
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<p>Since assignment is not necessarily defined on a given opaque type, implementation-defined default values cannot be defined in a
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portable way. The solution to this problem is to allow attributes objects to be initialized dynamically by attributes object
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initialization functions, so that default values can be supplied automatically by the implementation.</p>
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<p>The following proposal was provided as a suggested alternative to the supplied attributes:</p>
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<ol>
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<li>
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<p>Maintain the style of passing a parameter formed by the bitwise-inclusive OR of flags to the initialization routines ( <a href=
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"../functions/pthread_create.html"><i>pthread_create</i>()</a>, <a href=
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"../functions/pthread_mutex_init.html"><i>pthread_mutex_init</i>()</a>, <a href=
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"../functions/pthread_cond_init.html"><i>pthread_cond_init</i>()</a>). The parameter containing the flags should be an opaque type
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for extensibility. If no flags are set in the parameter, then the objects are created with default characteristics. An
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implementation may specify implementation-defined flag values and associated behavior.</p>
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</li>
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<li>
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<p>If further specialization of mutexes and condition variables is necessary, implementations may specify additional procedures
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that operate on the <b>pthread_mutex_t</b> and <b>pthread_cond_t</b> objects (instead of on attributes objects).</p>
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</li>
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</ol>
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<p>The difficulties with this solution are:</p>
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<ol>
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<li>
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<p>A bitmask is not opaque if bits have to be set into bitvector attributes objects using explicitly-coded bitwise-inclusive OR
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operations. If the set of options exceeds an <b>int</b>, application programmers need to know the location of each bit. If bits are
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set or read by encapsulation (that is, get and set functions), then the bitmask is merely an implementation of attributes objects
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as currently defined and should not be exposed to the programmer.</p>
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</li>
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<li>
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<p>Many attributes are not Boolean or very small integral values. For example, scheduling policy may be placed in 3-bit or 4-bit,
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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.
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Because of this, the bitmask can only reasonably control whether particular attributes are set or not, and it cannot serve as the
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repository of the value itself. The value needs to be specified as a function parameter (which is non-extensible), or by setting a
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structure field (which is non-opaque), or by get and set functions (making the bitmask a redundant addition to the attributes
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objects).</p>
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</li>
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</ol>
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<p>Stack size is defined as an optional attribute because the very notion of a stack is inherently machine-dependent. Some
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implementations may not be able to change the size of the stack, for example, and others may not need to because stack pages may be
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discontiguous and can be allocated and released on demand.</p>
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<p>The attribute mechanism has been designed in large measure for extensibility. Future extensions to the attribute mechanism or to
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any attributes object defined in this volume of IEEE Std 1003.1-2001 has to be done with care so as not to affect
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binary-compatibility.</p>
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<p>Attributes objects, even if allocated by means of dynamic allocation functions such as <a href=
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"../functions/malloc.html"><i>malloc</i>()</a>, may have their size fixed at compile time. This means, for example, a <a href=
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"../functions/pthread_create.html"><i>pthread_create</i>()</a> in an implementation with extensions to <b>pthread_attr_t</b> cannot
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look beyond the area that the binary application assumes is valid. This suggests that implementations should maintain a size field
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in the attributes object, as well as possibly version information, if extensions in different directions (possibly by different
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vendors) are to be accommodated.</p>
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</blockquote>
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<h4><a name="tag_03_487_09"></a>FUTURE DIRECTIONS</h4>
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<blockquote>
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<p>None.</p>
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</blockquote>
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<h4><a name="tag_03_487_10"></a>SEE ALSO</h4>
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<blockquote>
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<p><a href="pthread_attr_getstackaddr.html"><i>pthread_attr_getstackaddr</i>()</a> , <a href=
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"pthread_attr_getstacksize.html"><i>pthread_attr_getstacksize</i>()</a> , <a href=
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"pthread_attr_getdetachstate.html"><i>pthread_attr_getdetachstate</i>()</a> , <a href=
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"pthread_create.html"><i>pthread_create</i>()</a> , the Base Definitions volume of IEEE Std 1003.1-2001, <a href=
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"../basedefs/pthread.h.html"><i><pthread.h></i></a></p>
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</blockquote>
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<h4><a name="tag_03_487_11"></a>CHANGE HISTORY</h4>
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<blockquote>
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<p>First released in Issue 5. Included for alignment with the POSIX Threads Extension.</p>
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</blockquote>
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<h4><a name="tag_03_487_12"></a>Issue 6</h4>
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<blockquote>
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<p>The <i>pthread_attr_destroy</i>() and <i>pthread_attr_init</i>() functions are marked as part of the Threads option.</p>
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<p>IEEE PASC Interpretation 1003.1 #107 is applied, noting that the effect of initializing an already initialized thread attributes
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object is undefined.</p>
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</blockquote>
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<div class="box"><em>End of informative text.</em></div>
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