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| </head> |
| <body class="article"> |
| <div id="header"> |
| <h1>Unambiguous types</h1> |
| </div> |
| <div id="content"> |
| <div id="preamble"> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Most of these mappings are obvious, but there are some nuances and gotchas with |
| Rust FFI (Foreign Function Interface).</p> |
| </div> |
| <div class="paragraph"> |
| <p>This document defines clear, one-to-one mappings between primitive types in C, |
| Rust (and possible other languages in the future). Its purpose is to eliminate |
| ambiguity in type widths, signedness, and binary representation across |
| platforms and languages.</p> |
| </div> |
| <div class="paragraph"> |
| <p>For Git, the only header required to use these unambiguous types in C is |
| <code>git-compat-util.h</code>.</p> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_boolean_types">Boolean types</h2> |
| <div class="sectionbody"> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 50%;"/> |
| <col style="width: 50%;"/> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">C Type</th> |
| <th class="tableblock halign-left valign-top">Rust Type</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">bool<sup>1</sup></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">bool</p></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_integer_types">Integer types</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>In C, <stdint.<code>h</code>> (or an equivalent) must be included.</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 50%;"/> |
| <col style="width: 50%;"/> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">C Type</th> |
| <th class="tableblock halign-left valign-top">Rust Type</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">uint8_t</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">u8</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">uint16_t</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">u16</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">uint32_t</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">u32</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">uint64_t</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">u64</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">int8_t</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">i8</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">int16_t</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">i16</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">int32_t</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">i32</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">int64_t</p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">i64</p></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_floating_point_types">Floating-point types</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Rust requires IEEE-754 semantics. |
| In C, that is typically true, but not guaranteed by the standard.</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 50%;"/> |
| <col style="width: 50%;"/> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">C Type</th> |
| <th class="tableblock halign-left valign-top">Rust Type</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">float<sup>2</sup></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">f32</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">double<sup>2</sup></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">f64</p></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_size_types">Size types</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>These types represent pointer-sized integers and are typically defined in |
| <stddef.<code>h</code>> or an equivalent header.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Size types should be used any time pointer arithmetic is performed e.g. |
| indexing an array, describing the number of elements in memory, etc…​</p> |
| </div> |
| <table class="tableblock frame-all grid-all stretch"> |
| <colgroup> |
| <col style="width: 50%;"/> |
| <col style="width: 50%;"/> |
| </colgroup> |
| <thead> |
| <tr> |
| <th class="tableblock halign-left valign-top">C Type</th> |
| <th class="tableblock halign-left valign-top">Rust Type</th> |
| </tr> |
| </thead> |
| <tbody> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">size_t<sup>3</sup></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">usize</p></td> |
| </tr> |
| <tr> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">ptrdiff_t<sup>3</sup></p></td> |
| <td class="tableblock halign-left valign-top"><p class="tableblock">isize</p></td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_character_types">Character types</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>This is where C and Rust don’t have a clean one-to-one mapping.</p> |
| </div> |
| <div class="paragraph"> |
| <p>A C <code>char</code> and a Rust <code>u8</code> share the same bit width, so any C struct containing |
| a <code>char</code> will have the same size as the corresponding Rust struct using <code>u8</code>. |
| In that sense, such structs are safe to pass over the FFI boundary, because |
| their fields will be laid out identically. However, beyond bit width, C <code>char</code> |
| has additional semantics and platform-dependent behavior that can cause |
| problems, as discussed below.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The C language leaves the signedness of <code>char</code> implementation defined. Because |
| our developer build enables -Wsign-compare, comparison of a value of <code>char</code> |
| type with either signed or unsigned integers may trigger warnings from the |
| compiler.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Note: Rust’s <code>char</code> type is an unsigned 32-bit integer that is used to describe |
| Unicode code points.</p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_notes">Notes</h3> |
| <div class="paragraph"> |
| <p><sup>1</sup> This is only true if stdbool.h (or equivalent) is used.<br/> |
| <sup>2</sup> C does not enforce IEEE-754 compatibility, but Rust expects it. If the |
| platform/arch for C does not follow IEEE-754 then this equivalence does not |
| hold. Also, it’s assumed that <code>float</code> is 32 bits and <code>double</code> is 64, but |
| there may be a strange platform/arch where even this isn’t true.<br/> |
| <sup>3</sup> C also defines uintptr_t, ssize_t and intptr_t, but these types are |
| discouraged for FFI purposes. For functions like <code>read</code>() and <code>write</code>() ssize_t |
| should be cast to a different, and unambiguous, type before being passed over |
| the FFI boundary.<br/></p> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_problems_with_stdffic_types_in_rust">Problems with std::ffi::c_* types in Rust</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>TL;DR: In practice, Rust’s <code>c_*</code> types aren’t guaranteed to match C types for |
| all possible C compilers, platforms, or architectures, because Rust only |
| ensures correctness of C types on officially supported targets. These |
| definitions have changed over time to match more targets which means that the |
| c_* definitions will differ based on which Rust version Git chooses to use.</p> |
| </div> |
| <div class="paragraph"> |
| <p>Current list of safe, Rust side, FFI types in Git:<br/></p> |
| </div> |
| <div class="ulist"> |
| <ul> |
| <li> |
| <p><code>c_void</code></p> |
| </li> |
| <li> |
| <p><code>CStr</code></p> |
| </li> |
| <li> |
| <p><code>CString</code></p> |
| </li> |
| </ul> |
| </div> |
| <div class="paragraph"> |
| <p>Even then, they should be used sparingly, and only where the semantics match |
| exactly.</p> |
| </div> |
| <div class="paragraph"> |
| <p>The std::os::raw::c_* directly inherits the problems of core::ffi, which |
| changes over time and seems to make a best guess at the correct definition for |
| a given platform/target. This probably isn’t a problem for all other platforms |
| that Rust supports currently, but can anyone say that Rust got it right for all |
| C compilers of all platforms/targets?</p> |
| </div> |
| <div class="paragraph"> |
| <p>To give an example: c_long is defined in |
| <sup class="footnote">[<a id="_footnoteref_1" class="footnote" href="#_footnotedef_1" title="View footnote.">1</a>]</sup> |
| <sup class="footnote">[<a id="_footnoteref_2" class="footnote" href="#_footnotedef_2" title="View footnote.">2</a>]</sup></p> |
| </div> |
| <div class="sect2"> |
| <h3 id="_rust_version_1_63_0">Rust version 1.63.0</h3> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code>mod c_long_definition { |
| cfg_if! { |
| if #[cfg(all(target_pointer_width = "64", not(windows)))] { |
| pub type c_long = i64; |
| pub type NonZero_c_long = crate::num::NonZeroI64; |
| pub type c_ulong = u64; |
| pub type NonZero_c_ulong = crate::num::NonZeroU64; |
| } else { |
| // The minimal size of `long` in the C standard is 32 bits |
| pub type c_long = i32; |
| pub type NonZero_c_long = crate::num::NonZeroI32; |
| pub type c_ulong = u32; |
| pub type NonZero_c_ulong = crate::num::NonZeroU32; |
| } |
| } |
| }</code></pre> |
| </div> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_rust_version_1_89_0">Rust version 1.89.0</h3> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code>mod c_long_definition { |
| crate::cfg_select! { |
| any( |
| all(target_pointer_width = "64", not(windows)), |
| // wasm32 Linux ABI uses 64-bit long |
| all(target_arch = "wasm32", target_os = "linux") |
| ) => { |
| pub(super) type c_long = i64; |
| pub(super) type c_ulong = u64; |
| } |
| _ => { |
| // The minimal size of `long` in the C standard is 32 bits |
| pub(super) type c_long = i32; |
| pub(super) type c_ulong = u32; |
| } |
| } |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>Even for the cases where C types are correctly mapped to Rust types via |
| std::ffi::c_* there are still problems. Let’s take c_char for example. On some |
| platforms it’s u8 on others it’s i8.</p> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_subtraction_underflow_in_debug_mode">Subtraction underflow in debug mode</h3> |
| <div class="paragraph"> |
| <p>The following code will panic in debug on platforms that define c_char as u8, |
| but won’t if it’s an i8.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code>let mut x: std::ffi::c_char = 0; |
| x -= 1;</code></pre> |
| </div> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_inconsistent_shift_behavior">Inconsistent shift behavior</h3> |
| <div class="paragraph"> |
| <p><code>x</code> will be 0xC0 for platforms that use i8, but will be 0x40 where it’s u8.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code>let mut x: std::ffi::c_char = 0x80; |
| x >>= 1;</code></pre> |
| </div> |
| </div> |
| </div> |
| <div class="sect2"> |
| <h3 id="_equality_fails_to_compile_on_some_platforms">Equality fails to compile on some platforms</h3> |
| <div class="paragraph"> |
| <p>The following will not compile on platforms that define c_char as i8, but will |
| if it’s u8. You can cast x e.g. <code>assert_eq</code>!(<code>x</code> <code>as</code> <code>u8,</code> <code>b</code>'a');, but then you get |
| a warning on platforms that use u8 and a clean compilation where i8 is used.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code>let mut x: std::ffi::c_char = 0x61; |
| assert_eq!(x, b'a');</code></pre> |
| </div> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_enum_types">Enum types</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Rust enum types should not be used as FFI types. Rust enum types are more like |
| C union types than C enum’s. For something like:</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code>#[repr(C, u8)] |
| enum Fruit { |
| Apple, |
| Banana, |
| Cherry, |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>It’s easy enough to make sure the Rust enum matches what C would expect, but a |
| more complex type like.</p> |
| </div> |
| <div class="listingblock"> |
| <div class="content"> |
| <pre class="highlight"><code>enum HashResult { |
| SHA1([u8; 20]), |
| SHA256([u8; 32]), |
| }</code></pre> |
| </div> |
| </div> |
| <div class="paragraph"> |
| <p>The Rust compiler has to add a discriminant to the enum to distinguish between |
| the variants. The width, location, and values for that discriminant is up to |
| the Rust compiler and is not ABI stable.</p> |
| </div> |
| </div> |
| </div> |
| </div> |
| <div id="footnotes"> |
| <hr/> |
| <div class="footnote" id="_footnotedef_1"> |
| <a href="#_footnoteref_1">1</a>. <a href="https://doc.rust-lang.org/1.63.0/src/core/ffi/mod.rs.html#175-189">c_long in 1.63.0</a> |
| </div> |
| <div class="footnote" id="_footnotedef_2"> |
| <a href="#_footnoteref_2">2</a>. <a href="https://doc.rust-lang.org/1.89.0/src/core/ffi/primitives.rs.html#135-151">c_long in 1.89.0</a> |
| </div> |
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