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<ol class="chapter"><li class="chapter-item expanded "><a href="../../intro.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="../../idioms/index.html"><strong aria-hidden="true">2.</strong> Idioms</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../idioms/coercion-arguments.html"><strong aria-hidden="true">2.1.</strong> Use borrowed types for arguments</a></li><li class="chapter-item expanded "><a href="../../idioms/concat-format.html"><strong aria-hidden="true">2.2.</strong> Concatenating Strings with format!</a></li><li class="chapter-item expanded "><a href="../../idioms/ctor.html"><strong aria-hidden="true">2.3.</strong> Constructor</a></li><li class="chapter-item expanded "><a href="../../idioms/default.html"><strong aria-hidden="true">2.4.</strong> The Default Trait</a></li><li class="chapter-item expanded "><a href="../../idioms/deref.html"><strong aria-hidden="true">2.5.</strong> Collections Are Smart Pointers</a></li><li class="chapter-item expanded "><a href="../../idioms/dtor-finally.html"><strong aria-hidden="true">2.6.</strong> Finalisation in Destructors</a></li><li class="chapter-item expanded "><a href="../../idioms/mem-replace.html"><strong aria-hidden="true">2.7.</strong> mem::{take(), replace()}</a></li><li class="chapter-item expanded "><a href="../../idioms/on-stack-dyn-dispatch.html"><strong aria-hidden="true">2.8.</strong> On-Stack Dynamic Dispatch</a></li><li class="chapter-item expanded "><a href="../../idioms/ffi/intro.html"><strong aria-hidden="true">2.9.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../idioms/ffi/errors.html" class="active"><strong aria-hidden="true">2.9.1.</strong> Idiomatic Errors</a></li><li class="chapter-item expanded "><a href="../../idioms/ffi/accepting-strings.html"><strong aria-hidden="true">2.9.2.</strong> Accepting Strings</a></li><li class="chapter-item expanded "><a href="../../idioms/ffi/passing-strings.html"><strong aria-hidden="true">2.9.3.</strong> Passing Strings</a></li></ol></li><li class="chapter-item expanded "><a href="../../idioms/option-iter.html"><strong aria-hidden="true">2.10.</strong> Iterating over an Option</a></li><li class="chapter-item expanded "><a href="../../idioms/pass-var-to-closure.html"><strong aria-hidden="true">2.11.</strong> Pass Variables to Closure</a></li><li class="chapter-item expanded "><a href="../../idioms/priv-extend.html"><strong aria-hidden="true">2.12.</strong> Privacy For Extensibility</a></li><li class="chapter-item expanded "><a href="../../idioms/rustdoc-init.html"><strong aria-hidden="true">2.13.</strong> Easy doc initialization</a></li><li class="chapter-item expanded "><a href="../../idioms/temporary-mutability.html"><strong aria-hidden="true">2.14.</strong> Temporary mutability</a></li></ol></li><li class="chapter-item expanded "><a href="../../patterns/index.html"><strong aria-hidden="true">3.</strong> Design Patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../patterns/behavioural/intro.html"><strong aria-hidden="true">3.1.</strong> Behavioural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../patterns/behavioural/command.html"><strong aria-hidden="true">3.1.1.</strong> Command</a></li><li class="chapter-item expanded "><a href="../../patterns/behavioural/interpreter.html"><strong aria-hidden="true">3.1.2.</strong> Interpreter</a></li><li class="chapter-item expanded "><a href="../../patterns/behavioural/newtype.html"><strong aria-hidden="true">3.1.3.</strong> Newtype</a></li><li class="chapter-item expanded "><a href="../../patterns/behavioural/RAII.html"><strong aria-hidden="true">3.1.4.</strong> RAII Guards</a></li><li class="chapter-item expanded "><a href="../../patterns/behavioural/strategy.html"><strong aria-hidden="true">3.1.5.</strong> Strategy</a></li><li class="chapter-item expanded "><a href="../../patterns/behavioural/visitor.html"><strong aria-hidden="true">3.1.6.</strong> Visitor</a></li></ol></li><li class="chapter-item expanded "><a href="../../patterns/creational/intro.html"><strong aria-hidden="true">3.2.</strong> Creational</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../patterns/creational/builder.html"><strong aria-hidden="true">3.2.1.</strong> Builder</a></li><li class="chapter-item expanded "><a href="../../patterns/creational/fold.html"><strong aria-hidden="true">3.2.2.</strong> Fold</a></li></ol></li><li class="chapter-item expanded "><a href="../../patterns/structural/intro.html"><strong aria-hidden="true">3.3.</strong> Structural</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../patterns/structural/compose-structs.html"><strong aria-hidden="true">3.3.1.</strong> Compose Structs</a></li><li class="chapter-item expanded "><a href="../../patterns/structural/small-crates.html"><strong aria-hidden="true">3.3.2.</strong> Prefer Small Crates</a></li><li class="chapter-item expanded "><a href="../../patterns/structural/unsafe-mods.html"><strong aria-hidden="true">3.3.3.</strong> Contain unsafety in small modules</a></li></ol></li><li class="chapter-item expanded "><a href="../../patterns/ffi/intro.html"><strong aria-hidden="true">3.4.</strong> Foreign function interface (FFI)</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../patterns/ffi/export.html"><strong aria-hidden="true">3.4.1.</strong> Object-Based APIs</a></li><li class="chapter-item expanded "><a href="../../patterns/ffi/wrappers.html"><strong aria-hidden="true">3.4.2.</strong> Type Consolidation into Wrappers</a></li></ol></li></ol></li><li class="chapter-item expanded "><a href="../../anti_patterns/index.html"><strong aria-hidden="true">4.</strong> Anti-patterns</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../anti_patterns/borrow_clone.html"><strong aria-hidden="true">4.1.</strong> Clone to satisfy the borrow checker</a></li><li class="chapter-item expanded "><a href="../../anti_patterns/deny-warnings.html"><strong aria-hidden="true">4.2.</strong> #[deny(warnings)]</a></li><li class="chapter-item expanded "><a href="../../anti_patterns/deref.html"><strong aria-hidden="true">4.3.</strong> Deref Polymorphism</a></li></ol></li><li class="chapter-item expanded "><a href="../../functional/index.html"><strong aria-hidden="true">5.</strong> Functional Programming</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../functional/paradigms.html"><strong aria-hidden="true">5.1.</strong> Programming paradigms</a></li><li class="chapter-item expanded "><a href="../../functional/generics-type-classes.html"><strong aria-hidden="true">5.2.</strong> Generics as Type Classes</a></li></ol></li><li class="chapter-item expanded "><a href="../../additional_resources/index.html"><strong aria-hidden="true">6.</strong> Additional Resources</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../../additional_resources/design-principles.html"><strong aria-hidden="true">6.1.</strong> Design principles</a></li></ol></li></ol>
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<h1 id="error-handling-in-ffi"><a class="header" href="#error-handling-in-ffi">Error Handling in FFI</a></h1>
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<h2 id="description"><a class="header" href="#description">Description</a></h2>
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<p>In foreign languages like C, errors are represented by return codes.
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However, Rust's type system allows much more rich error information to be
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captured a propogated through a full type.</p>
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<p>This best practice shows different kinds of error codes, and how to expose them
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in a usable way:</p>
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<ol>
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<li>Flat Enums should be converted to integers and returned as codes.</li>
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<li>Structured Enums should be converted to an integer code with a string error
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message for detail.</li>
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<li>Custom Error Types should become "transparent", with a C representation.</li>
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</ol>
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<h2 id="code-example"><a class="header" href="#code-example">Code Example</a></h2>
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<h3 id="flat-enums"><a class="header" href="#flat-enums">Flat Enums</a></h3>
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<pre><code class="language-rust ignore">enum DatabaseError {
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IsReadOnly = 1, // user attempted a write operation
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IOError = 2, // user should read the C errno() for what it was
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FileCorrupted = 3, // user should run a repair tool to recover it
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}
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impl From<DatabaseError> for libc::c_int {
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fn from(e: DatabaseError) -> libc::c_int {
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(e as i8).into()
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}
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}
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</code></pre>
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<h3 id="structured-enums"><a class="header" href="#structured-enums">Structured Enums</a></h3>
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<pre><code class="language-rust ignore">pub mod errors {
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enum DatabaseError {
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IsReadOnly,
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IOError(std::io::Error),
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FileCorrupted(String), // message describing the issue
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}
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impl From<DatabaseError> for libc::c_int {
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fn from(e: DatabaseError) -> libc::c_int {
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match e {
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DatabaseError::IsReadOnly => 1,
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DatabaseError::IOError(_) => 2,
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DatabaseError::FileCorrupted(_) => 3,
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}
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}
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}
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}
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pub mod c_api {
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use super::errors::DatabaseError;
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#[no_mangle]
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pub extern "C" fn db_error_description(
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e: *const DatabaseError
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) -> *mut libc::c_char {
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let error: &DatabaseError = unsafe {
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// SAFETY: pointer lifetime is greater than the current stack frame
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&*e
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};
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let error_str: String = match error {
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DatabaseError::IsReadOnly => {
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format!("cannot write to read-only database");
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}
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DatabaseError::IOError(e) => {
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format!("I/O Error: {}", e);
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}
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DatabaseError::FileCorrupted(s) => {
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format!("File corrupted, run repair: {}", &s);
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}
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};
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let c_error = unsafe {
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// SAFETY: copying error_str to an allocated buffer with a NUL
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// character at the end
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let mut malloc: *mut u8 = libc::malloc(error_str.len() + 1) as *mut _;
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if malloc.is_null() {
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return std::ptr::null_mut();
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}
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let src = error_str.as_bytes().as_ptr();
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std::ptr::copy_nonoverlapping(src, malloc, error_str.len());
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std::ptr::write(malloc.add(error_str.len()), 0);
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malloc as *mut libc::c_char
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};
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c_error
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}
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}
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</code></pre>
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<h3 id="custom-error-types"><a class="header" href="#custom-error-types">Custom Error Types</a></h3>
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<pre><code class="language-rust ignore">struct ParseError {
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|
expected: char,
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line: u32,
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ch: u16
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}
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impl ParseError { /* ... */ }
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|
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/* Create a second version which is exposed as a C structure */
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|
#[repr(C)]
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|
pub struct parse_error {
|
|
pub expected: libc::c_char,
|
|
pub line: u32,
|
|
pub ch: u16
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|
}
|
|
|
|
impl From<ParseError> for parse_error {
|
|
fn from(e: ParseError) -> parse_error {
|
|
let ParseError { expected, line, ch } = e;
|
|
parse_error { expected, line, ch }
|
|
}
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|
}
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|
</code></pre>
|
|
<h2 id="advantages"><a class="header" href="#advantages">Advantages</a></h2>
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|
<p>This ensures that the foreign language has clear access to error information
|
|
while not compromising the Rust code's API at all.</p>
|
|
<h2 id="disadvantages"><a class="header" href="#disadvantages">Disadvantages</a></h2>
|
|
<p>It's a lot of typing, and some types may not be able to be converted easily
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to C.</p>
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