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<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="meet-safe-and-unsafe.html"><strong aria-hidden="true">1.</strong> Meet Safe and Unsafe</a></li><li><ol class="section"><li><a href="safe-unsafe-meaning.html"><strong aria-hidden="true">1.1.</strong> How Safe and Unsafe Interact</a></li><li><a href="what-unsafe-does.html" class="active"><strong aria-hidden="true">1.2.</strong> What Unsafe Can Do</a></li><li><a href="working-with-unsafe.html"><strong aria-hidden="true">1.3.</strong> Working with Unsafe</a></li></ol></li><li><a href="data.html"><strong aria-hidden="true">2.</strong> Data Layout</a></li><li><ol class="section"><li><a href="repr-rust.html"><strong aria-hidden="true">2.1.</strong> repr(Rust)</a></li><li><a href="exotic-sizes.html"><strong aria-hidden="true">2.2.</strong> Exotically Sized Types</a></li><li><a href="other-reprs.html"><strong aria-hidden="true">2.3.</strong> Other reprs</a></li></ol></li><li><a href="ownership.html"><strong aria-hidden="true">3.</strong> Ownership</a></li><li><ol class="section"><li><a href="references.html"><strong aria-hidden="true">3.1.</strong> References</a></li><li><a href="aliasing.html"><strong aria-hidden="true">3.2.</strong> Aliasing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.3.</strong> Lifetimes</a></li><li><a href="lifetime-mismatch.html"><strong aria-hidden="true">3.4.</strong> Limits of Lifetimes</a></li><li><a href="lifetime-elision.html"><strong aria-hidden="true">3.5.</strong> Lifetime Elision</a></li><li><a href="unbounded-lifetimes.html"><strong aria-hidden="true">3.6.</strong> Unbounded Lifetimes</a></li><li><a href="hrtb.html"><strong aria-hidden="true">3.7.</strong> Higher-Rank Trait Bounds</a></li><li><a href="subtyping.html"><strong aria-hidden="true">3.8.</strong> Subtyping and Variance</a></li><li><a href="dropck.html"><strong aria-hidden="true">3.9.</strong> Drop Check</a></li><li><a href="phantom-data.html"><strong aria-hidden="true">3.10.</strong> PhantomData</a></li><li><a href="borrow-splitting.html"><strong aria-hidden="true">3.11.</strong> Splitting Borrows</a></li></ol></li><li><a href="conversions.html"><strong aria-hidden="true">4.</strong> Type Conversions</a></li><li><ol class="section"><li><a href="coercions.html"><strong aria-hidden="true">4.1.</strong> Coercions</a></li><li><a href="dot-operator.html"><strong aria-hidden="true">4.2.</strong> The Dot Operator</a></li><li><a href="casts.html"><strong aria-hidden="true">4.3.</strong> Casts</a></li><li><a href="transmutes.html"><strong aria-hidden="true">4.4.</strong> Transmutes</a></li></ol></li><li><a href="uninitialized.html"><strong aria-hidden="true">5.</strong> Uninitialized Memory</a></li><li><ol class="section"><li><a href="checked-uninit.html"><strong aria-hidden="true">5.1.</strong> Checked</a></li><li><a href="drop-flags.html"><strong aria-hidden="true">5.2.</strong> Drop Flags</a></li><li><a href="unchecked-uninit.html"><strong aria-hidden="true">5.3.</strong> Unchecked</a></li></ol></li><li><a href="obrm.html"><strong aria-hidden="true">6.</strong> Ownership Based Resource Management</a></li><li><ol class="section"><li><a href="constructors.html"><strong aria-hidden="true">6.1.</strong> Constructors</a></li><li><a href="destructors.html"><strong aria-hidden="true">6.2.</strong> Destructors</a></li><li><a href="leaking.html"><strong aria-hidden="true">6.3.</strong> Leaking</a></li></ol></li><li><a href="unwinding.html"><strong aria-hidden="true">7.</strong> Unwinding</a></li><li><ol class="section"><li><a href="exception-safety.html"><strong aria-hidden="true">7.1.</strong> Exception Safety</a></li><li><a href="poisoning.html"><strong aria-hidden="true">7.2.</strong> Poisoning</a></li></ol></li><li><a href="concurrency.html"><strong aria-hidden="true">8.</strong> Concurrency</a></li><li><ol class="section"><li><a href="races.html"><strong aria-hidden="true">8.1.</strong> Races</a></li><li><a href="send-and-sync.html"><strong aria-hidden="true">8.2.</strong> Send and Sync</a></li><li><a href="atomics.html"><strong aria-hidden="true">8.3.</strong> Atomics</a></li></ol></li><li><a href="vec.html"><strong aria-hidden="true">9.</strong> Implementing Vec</a></li><li><ol class="section"><li><a href="vec-layout.html"><strong aria-hidden="true">9.1.</strong> Layout</a></li><li><a href="vec-alloc.html"><strong aria-hidden="true">9.2.</strong> Allocating</a></li><li><a href="vec-push-pop.html"><strong aria-hidden="true">9.3.</strong> Push and Pop</a></li><li><a href="vec-dealloc.html"><strong aria-hidden="true">9.4.</strong> Deallocating</a></li><li><a href="vec-deref.html"><strong aria-hidden="true">9.5.</strong> Deref</a></li><li><a href="vec-insert-remove.html"><strong aria-hidden="true">9.6.</strong> Insert and Remove</a></li><li><a href="vec-into-iter.html"><strong aria-hidden="true">9.7.</strong> IntoIter</a></li><li><a href="vec-raw.html"><strong aria-hidden="true">9.8.</strong> RawVec</a></li><li><a href="vec-drain.html"><strong aria-hidden="true">9.9.</strong> Drain</a></li><li><a href="vec-zsts.html"><strong aria-hidden="true">9.10.</strong> Handling Zero-Sized Types</a></li><li><a href="vec-final.html"><strong aria-hidden="true">9.11.</strong> Final Code</a></li></ol></li><li><a href="arc-and-mutex.html"><strong aria-hidden="true">10.</strong> Implementing Arc and Mutex</a></li><li><a href="ffi.html"><strong aria-hidden="true">11.</strong> FFI</a></li></ol>
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<a class="header" href="what-unsafe-does.html#what-unsafe-rust-can-do" id="what-unsafe-rust-can-do"><h1>What Unsafe Rust Can Do</h1></a>
<p>The only things that are different in Unsafe Rust are that you can:</p>
<ul>
<li>Dereference raw pointers</li>
<li>Call <code>unsafe</code> functions (including C functions, compiler intrinsics, and the raw allocator)</li>
<li>Implement <code>unsafe</code> traits</li>
<li>Mutate statics</li>
</ul>
<p>That's it. The reason these operations are relegated to Unsafe is that misusing
any of these things will cause the ever dreaded Undefined Behavior. Invoking
Undefined Behavior gives the compiler full rights to do arbitrarily bad things
to your program. You definitely <em>should not</em> invoke Undefined Behavior.</p>
<p>Unlike C, Undefined Behavior is pretty limited in scope in Rust. All the core
language cares about is preventing the following things:</p>
<ul>
<li>Dereferencing null, dangling, or unaligned pointers</li>
<li>Reading <a href="uninitialized.html">uninitialized memory</a></li>
<li>Breaking the <a href="references.html">pointer aliasing rules</a></li>
<li>Producing invalid primitive values:
<ul>
<li>dangling/null references</li>
<li>a <code>bool</code> that isn't 0 or 1</li>
<li>an undefined <code>enum</code> discriminant</li>
<li>a <code>char</code> outside the ranges [0x0, 0xD7FF] and [0xE000, 0x10FFFF]</li>
<li>A non-utf8 <code>str</code></li>
</ul>
</li>
<li>Unwinding into another language</li>
<li>Causing a <a href="races.html">data race</a></li>
</ul>
<p>That's it. That's all the causes of Undefined Behavior baked into Rust. Of
course, unsafe functions and traits are free to declare arbitrary other
constraints that a program must maintain to avoid Undefined Behavior. For
instance, the allocator APIs declare that deallocating unallocated memory is
Undefined Behavior.</p>
<p>However, violations of these constraints generally will just transitively lead to one of
the above problems. Some additional constraints may also derive from compiler
intrinsics that make special assumptions about how code can be optimized. For instance,
Vec and Box make use of intrinsics that require their pointers to be non-null at all times.</p>
<p>Rust is otherwise quite permissive with respect to other dubious operations.
Rust considers it "safe" to:</p>
<ul>
<li>Deadlock</li>
<li>Have a <a href="races.html">race condition</a></li>
<li>Leak memory</li>
<li>Fail to call destructors</li>
<li>Overflow integers</li>
<li>Abort the program</li>
<li>Delete the production database</li>
</ul>
<p>However any program that actually manages to do such a thing is <em>probably</em>
incorrect. Rust provides lots of tools to make these things rare, but
these problems are considered impractical to categorically prevent.</p>
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