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< h1 id = "error-handling-in-ffi" > < a class = "header" href = "#error-handling-in-ffi" > Error Handling in FFI< / a > < / h1 >
< 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
captured a propogated through a full type.< / p >
< p > This best practice shows different kinds of error codes, and how to expose them
in a usable way:< / p >
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< ol >
< 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
message for detail.< / li >
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< li > Custom Error Types should become " transparent" , with a C representation.< / li >
< / ol >
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< h2 id = "code-example" > < a class = "header" href = "#code-example" > Code Example< / a > < / h2 >
< 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 {
IsReadOnly = 1, // user attempted a write operation
IOError = 2, // user should read the C errno() for what it was
FileCorrupted = 3, // user should run a repair tool to recover it
}
impl From< DatabaseError> for libc::c_int {
fn from(e: DatabaseError) -> libc::c_int {
(e as i8).into()
}
}
< / 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 {
enum DatabaseError {
IsReadOnly,
IOError(std::io::Error),
FileCorrupted(String), // message describing the issue
}
impl From< DatabaseError> for libc::c_int {
fn from(e: DatabaseError) -> libc::c_int {
match e {
DatabaseError::IsReadOnly => 1,
DatabaseError::IOError(_) => 2,
DatabaseError::FileCorrupted(_) => 3,
}
}
}
}
pub mod c_api {
use super::errors::DatabaseError;
#[no_mangle]
pub extern " C" fn db_error_description(
e: *const DatabaseError
) -> *mut libc::c_char {
let error: & DatabaseError = unsafe {
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// SAFETY: pointer lifetime is greater than the current stack frame
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& *e
};
let error_str: String = match error {
DatabaseError::IsReadOnly => {
format!(" cannot write to read-only database" );
}
DatabaseError::IOError(e) => {
format!(" I/O Error: {}" , e);
}
DatabaseError::FileCorrupted(s) => {
format!(" File corrupted, run repair: {}" , & s);
}
};
let c_error = unsafe {
// SAFETY: copying error_str to an allocated buffer with a NUL
// character at the end
let mut malloc: *mut u8 = libc::malloc(error_str.len() + 1) as *mut _;
if malloc.is_null() {
return std::ptr::null_mut();
}
let src = error_str.as_bytes().as_ptr();
std::ptr::copy_nonoverlapping(src, malloc, error_str.len());
std::ptr::write(malloc.add(error_str.len()), 0);
malloc as *mut libc::c_char
};
c_error
}
}
< / 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 {
expected: char,
line: u32,
ch: u16
}
impl ParseError { /* ... */ }
/* Create a second version which is exposed as a C structure */
#[repr(C)]
pub struct parse_error {
pub expected: libc::c_char,
pub line: u32,
pub ch: u16
}
impl From< ParseError> for parse_error {
fn from(e: ParseError) -> parse_error {
let ParseError { expected, line, ch } = e;
parse_error { expected, line, ch }
}
}
< / code > < / pre >
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< 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 >
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< h2 id = "disadvantages" > < a class = "header" href = "#disadvantages" > Disadvantages< / a > < / h2 >
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< p > It's a lot of typing, and some types may not be able to be converted easily
to C.< / p >
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