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<h1 id="firstHeading" class="firstHeading">C attribute: unsequenced, reproducible <span class="t-mark-rev t-since-c23">(since C23)</span>
</h1> <p>Provides the compiler with information about the access of objects by a function such that certain properties of function calls can be deduced.</p>
<h3 id="Syntax"> Syntax</h3> <table class="t-sdsc-begin"> <tr class="t-sdsc"> <td> <code>[[</code> <code>unsequenced</code> <code>]]</code><br><code>[[</code> <code>__unsequenced__</code> <code>]]</code> </td> <td> (1) </td> <td class="t-sdsc-nopad"> </td>
</tr> <tr class="t-sdsc"> <td> <code>[[</code> <code>reproducible</code> <code>]]</code><br><code>[[</code> <code>__reproducible__</code> <code>]]</code> </td> <td> (2) </td> <td class="t-sdsc-nopad"> </td>
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<span class="t-li">1)</span> Indicates that a function is effectless, idempotent, stateless, and independent</div> <div class="t-li1">
<span class="t-li">2)</span> Indicates that a function is effectless and idempotent</div> <h3 id="Explanation"> Explanation</h3> <p>These attributes apply to a function declarator or to a type specifier that has a function type. The corresponding attribute is a property of the function type.</p>
<h4 id="Effectless"> Effectless</h4> <p>An evaluation of a function call is effectless if any store operation that is sequenced during the call is the modification of an object that synchronizes with the call; if additionally the operation is observable, all access to the object must be based on a unique pointer parameter of the function.</p>
<h4 id="Idempotent"> Idempotent</h4> <p>An evaluation <span class="t-spar">E</span> is idempotent if a second evaluation of <span class="t-spar">E</span> can be sequenced immediately after the original one without changing the resulting value, if any, or the observable state of the execution.</p>
<h4 id="Stateless"> Stateless</h4> <p>A function <span class="t-spar">F</span> is stateless if any definition of an object of static or thread <a href="../storage_duration" title="c/language/storage duration">storage duration</a> in <span class="t-spar">F</span> or in a function that is called by <span class="t-spar">F</span> is <code>const</code> but not <code>volatile</code> qualified.</p>
<h4 id="Independent"> Independent</h4> <p>A function <span class="t-spar">F</span> is independent if for any object <span class="t-spar">X</span> that is observed by a call to <span class="t-spar">F</span> through an lvalue that is not based on a parameter of the call, all accesses to <span class="t-spar">X</span> in all calls to <span class="t-spar">F</span> during the same program execution observe the same value; otherwise if the access is based on a pointer parameter, there shall be a unique such pointer parameter <span class="t-spar">P</span> such that any access to <span class="t-spar">X</span> shall be to an lvalue that is based on <span class="t-spar">P</span>.</p>
<p>An object <span class="t-spar">X</span> is observed by a function call if both synchronize, if <span class="t-spar">X</span> is not local to the call, if <span class="t-spar">X</span> has a lifetime that starts before the function call, and if an access of <span class="t-spar">X</span> is sequenced during the call; the last value of <span class="t-spar">X</span>, if any, that is stored before the call is said to be the value of <span class="t-spar">X</span> that is observed by the call.</p>
<h3 id="Notes"> Notes</h3> <p>These attributes exist for the purpose of compiler optimization.</p>
<p>If a function is reproducible, multiple subsequent calls can be treated as a single call.</p>
<p>If a function is unsequenced, multiple subsequent calls can be treated as a single call, and the calls can be parallelized and reordered arbitrarily.</p>
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