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| <h1><a class="header" href="#using-c-structures-with-flexible-array-members" id="using-c-structures-with-flexible-array-members">Using C structures with Flexible Array Members</a></h1> |
| <p>Since time immemorial, C programmers have been using what was called "the struct |
| hack". This is a technique for packing a fixed-size structure and a |
| variable-sized tail within the same memory allocation. Typically this looks |
| like:</p> |
| <pre><code class="language-c">struct MyRecord { |
| time_t timestamp; |
| unsigned seq; |
| size_t len; |
| char payload[0]; |
| }; |
| </code></pre> |
| <p>Because this is so useful, it was standardized in C99 as "flexible array |
| members", using almost identical syntax:</p> |
| <pre><code class="language-c">struct MyRecord { |
| time_t timestamp; |
| unsigned seq; |
| size_t len; |
| char payload[]; // NOTE: empty [] |
| }; |
| </code></pre> |
| <p>Bindgen supports these structures in two different ways.</p> |
| <h2><a class="header" href="#__incompletearrayfield" id="__incompletearrayfield"><code>__IncompleteArrayField</code></a></h2> |
| <p>By default, bindgen will generate the corresponding Rust structure:</p> |
| <pre><code class="language-rust ignore">#[repr(C)] |
| struct MyRecord { |
| pub timestamp: time_t, |
| pub seq: ::std::os::raw::c_uint, |
| pub len: usize, |
| pub payload: __IncompleteArrayField<::std::os::raw::c_char>, |
| } |
| </code></pre> |
| <p>The <code>__IncompleteArrayField</code> type is zero-sized, so this structure represents |
| the prefix without any trailing data. In order to access that data, it provides |
| the <code>as_slice</code> unsafe method:</p> |
| <pre><code class="language-rust ignore"> // SAFETY: there's at least `len` bytes allocated and initialized after `myrecord` |
| let payload = unsafe { myrecord.payload.as_slice(myrecord.len) }; |
| </code></pre> |
| <p>There's also <code>as_mut_slice</code> which does the obvious.</p> |
| <p>These are <code>unsafe</code> simply because it's up to you to provide the right length (in |
| elements of whatever type <code>payload</code> is) as there's no way for Rust or Bindgen to |
| know. In this example, the length is a very straightforward <code>len</code> field in the |
| structure, but it could be encoded in any number of ways within the structure, |
| or come from somewhere else entirely.</p> |
| <p>One big caveat with this technique is that <code>std::mem::size_of</code> (or |
| <code>size_of_val</code>) will <em>only</em> include the size of the prefix structure. if you're |
| working out how much storage the whole structure is using, you'll need to add |
| the suffix yourself.</p> |
| <h2><a class="header" href="#using-dynamically-sized-types" id="using-dynamically-sized-types">Using Dynamically Sized Types</a></h2> |
| <p>If you invoke bindgen with the <code>--flexarray-dst</code> option, it will generate |
| something not quite like this:</p> |
| <pre><code class="language-rust ignore">#[repr(C)] |
| struct MyRecord { |
| pub timestamp: time_t, |
| pub seq: ::std::os::raw::c_uint, |
| pub len: usize, |
| pub payload: [::std::os::raw::c_char], |
| } |
| </code></pre> |
| <p>Rust has a set of types which are almost exact analogs for these Flexible Array |
| Member types: the Dynamically Sized Type ("DST").</p> |
| <p>This looks almost identical to a normal Rust structure, except that you'll note |
| the type of the <code>payload</code> field is a raw slice <code>[...]</code> rather than the usual |
| reference to slice <code>&[...]</code>.</p> |
| <p>That <code>payload: [c_char]</code> is telling Rust that it can't directly know the total |
| size of this structure - the <code>payload</code> field takes an amount of space that's |
| determined at runtime. This means you can't directly use values of this type, |
| only references: <code>&MyRecord</code>.</p> |
| <p>In practice, this is very awkward. So instead, bindgen generates:</p> |
| <pre><code class="language-rust ignore">#[repr(C)] |
| struct MyRecord<FAM: ?Sized = [::std::os::raw::c_char; 0]> { |
| pub timestamp: time_t, |
| pub seq: ::std::os::raw::c_uint, |
| pub len: usize, |
| pub payload: FAM, |
| } |
| </code></pre> |
| <p>That is:</p> |
| <ol> |
| <li>a type parameter <code>FAM</code> which represents the type of the <code>payload</code> field,</li> |
| <li>it's <code>?Sized</code> meaning it can be unsized (ie, a DST)</li> |
| <li>it has the default type of <code>[c_char; 0]</code> - that is a zero-sized array of characters</li> |
| </ol> |
| <p>This means that referencing plain <code>MyRecord</code> will be exactly like <code>MyRecord</code> |
| with <code>__IncompleteArrayField</code>: it is a fixed-sized structure which you can |
| manipulate like a normal Rust value.</p> |
| <p>But how do you get to the DST part?</p> |
| <p>Bindgen will also implement a set of helper methods for this:</p> |
| <pre><code class="language-rust ignore">// Static sized variant |
| impl MyRecord<[::std::os::raw::c_char; 0]> { |
| pub unsafe fn flex_ref(&self, len: usize) -> &MyRecord<[::std::os::raw::c_char]> { ... } |
| pub unsafe fn flex_mut_ref(&mut self, len: usize) -> &mut MyRecord<[::std::os::raw::c_char]> { ... } |
| // And some raw pointer variants |
| } |
| </code></pre> |
| <p>These will take a sized <code>MyRecord<[c_char; 0]></code> and a length in elements, and |
| return a reference to a DST <code>MyRecord<[c_char]></code> where the <code>payload</code> field is a |
| fully usable slice of <code>len</code> characters.</p> |
| <p>The magic here is that the reference is a fat pointer, which not only encodes |
| the address, but also the dynamic size of the final field, just like a reference |
| to a slice is. This means that you get full bounds checked access to the |
| <code>payload</code> field like any other Rust slice.</p> |
| <p>It also means that doing <code>mem::size_of_val(myrecord)</code> will return the <em>complete</em> |
| size of this structure, including the suffix.</p> |
| <p>You can go the other way:</p> |
| <pre><code class="language-rust ignore">// Dynamic sized variant |
| impl MyRecord<[::std::os::raw::c_char]> { |
| pub fn fixed(&self) -> (&MyRecord<[::std::os::raw::c_char; 0]>, usize) { ... } |
| pub fn fixed_mut(&mut self) -> (&mut MyRecord<[::std::os::raw::c_char; 0]>, usize) { ... } |
| pub fn layout(len: usize) -> std::alloc::Layout { ... } |
| } |
| </code></pre> |
| <p>which takes the DST variant of the structure and returns the sized variant, |
| along with the number of elements are after it. These are all completely safe |
| because all the information needed is part of the fat <code>&self</code> reference.</p> |
| <p>The <code>layout</code> function takes a length and returns the <code>Layout</code> - that is, size |
| and alignment, so that you can allocate memory for the structure (for example, |
| using <code>malloc</code> so you can pass it to a C function).</p> |
| <p>Unfortunately the language features needed to support these methods are still unstable:</p> |
| <ul> |
| <li><a href="https://doc.rust-lang.org/beta/unstable-book/library-features/ptr-metadata.html">ptr_metadata</a>, |
| which enables all the fixed<->DST conversions, and</li> |
| <li><a href="https://doc.rust-lang.org/beta/unstable-book/library-features/layout-for-ptr.html">layout_for_ptr</a>, |
| which allows he <code>layout</code> method</li> |
| </ul> |
| <p>As a result, if you don't specify <code>--rust-target nightly</code> you'll just get the |
| bare type definitions, but no real way to use them. It's often convenient to add |
| the</p> |
| <pre><code class="language-bash">--raw-line '#![feature(ptr_metadata,layout_for_ptr)]' |
| </code></pre> |
| <p>option if you're generating Rust as a stand-alone crate. Otherwise you'll need |
| to add the feature line to your containing crate.</p> |
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