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<ol class="chapter"><li class="affix"><a href="introduction.html">Introduction</a></li><li><a href="notation.html"><strong aria-hidden="true">1.</strong> Notation</a></li><li><a href="lexical-structure.html"><strong aria-hidden="true">2.</strong> Lexical structure</a></li><li><ol class="section"><li><a href="input-format.html"><strong aria-hidden="true">2.1.</strong> Input format</a></li><li><a href="keywords.html"><strong aria-hidden="true">2.2.</strong> Keywords</a></li><li><a href="identifiers.html"><strong aria-hidden="true">2.3.</strong> Identifiers</a></li><li><a href="comments.html"><strong aria-hidden="true">2.4.</strong> Comments</a></li><li><a href="whitespace.html"><strong aria-hidden="true">2.5.</strong> Whitespace</a></li><li><a href="tokens.html"><strong aria-hidden="true">2.6.</strong> Tokens</a></li><li><a href="paths.html"><strong aria-hidden="true">2.7.</strong> Paths</a></li></ol></li><li><a href="macros.html"><strong aria-hidden="true">3.</strong> 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aria-hidden="true">11.3.</strong> Behavior considered undefined</a></li><li><a href="behavior-not-considered-unsafe.html"><strong aria-hidden="true">11.4.</strong> Behavior not considered unsafe</a></li></ol></li><li><a href="influences.html">Appendix: Influences</a></li><li class="affix"><a href="undocumented.html">Appendix: As-yet-undocumented Features</a></li><li class="affix"><a href="glossary.html">Appendix: Glossary</a></li></ol>
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<a class="header" href="behavior-considered-undefined.html#behavior-considered-undefined" id="behavior-considered-undefined"><h2>Behavior considered undefined</h2></a>
<p>Rust code, including within <code>unsafe</code> blocks and <code>unsafe</code> functions is incorrect
if it exhibits any of the behaviors in the following list. It is the
programmer's responsibility when writing <code>unsafe</code> code that it is not possible
to let <code>safe</code> code exhibit these behaviors.</p>
<ul>
<li>Data races.</li>
<li>Dereferencing a null or dangling raw pointer.</li>
<li>Unaligned pointer reading and writing outside of <a href="https://doc.rust-lang.org/std/ptr/fn.read_unaligned.html"><code>read_unaligned</code></a>
and <a href="https://doc.rust-lang.org/std/ptr/fn.write_unaligned.html"><code>write_unaligned</code></a>.</li>
<li>Reads of <a href="http://llvm.org/docs/LangRef.html#undefined-values">undef</a> (uninitialized) memory.</li>
<li>Breaking the <a href="http://llvm.org/docs/LangRef.html#pointer-aliasing-rules">pointer aliasing rules</a> on accesses through raw pointers;
a subset of the rules used by C.</li>
<li><code>&mut T</code> and <code>&T</code> follow LLVM’s scoped <a href="http://llvm.org/docs/LangRef.html#noalias">noalias</a> model, except if the <code>&T</code>
contains an <a href="https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html"><code>UnsafeCell<U></code></a>.</li>
<li>Mutating non-mutable data — that is, data reached through a shared
reference or data owned by a <code>let</code> binding), unless that data is contained
within an <a href="https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html"><code>UnsafeCell<U></code></a>.</li>
<li>Invoking undefined behavior via compiler intrinsics:
<ul>
<li>Indexing outside of the bounds of an object with <a href="https://doc.rust-lang.org/std/primitive.pointer.html#method.offset"><code>offset</code></a> with
the exception of one byte past the end of the object.</li>
<li>Using <a href="https://doc.rust-lang.org/std/ptr/fn.copy_nonoverlapping.html"><code>std::ptr::copy_nonoverlapping_memory</code></a>, a.k.a. the <code>memcpy32</code>and
<code>memcpy64</code> intrinsics, on overlapping buffers.</li>
</ul>
</li>
<li>Invalid values in primitive types, even in private fields and locals:
<ul>
<li>Dangling or null references and boxes.</li>
<li>A value other than <code>false</code> (<code>0</code>) or <code>true</code> (<code>1</code>) in a <code>bool</code>.</li>
<li>A discriminant in an <code>enum</code> not included in the type definition.</li>
<li>A value in a <code>char</code> which is a surrogate or above <code>char::MAX</code>.</li>
<li>Non-UTF-8 byte sequences in a <code>str</code>.</li>
</ul>
</li>
</ul>
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