Understanding Rust Error E0633: Malformed Unwind Attribute

Rust Error E0633: Malformed Unwind Attribute

The Rust error E0633 occurs when the #[unwind] attribute is malformed. Before we dive into the details of this error, it is crucial to understand the purpose of the #[unwind] attribute in Rust.

The #[unwind] attribute is used to specify the behavior of non-Rust ABI functions when they attempt to unwind. Unwinding is a mechanism used by some programming languages to clean up resources (e.g., memory, file handlers) and propagate errors when an exception occurs. It is essential to manage unwinding carefully to avoid Undefined Behavior (UB) issues.

Using the Unwind Attribute Correctly

There are two ways to use the #[unwind] attribute in Rust: 1. #[unwind(aborts)] 2. #[unwind(allowed)]

Here is an explanation for each:

#[unwind(aborts)]

This option specifies that a non-Rust ABI function should abort the process if it attempts to unwind. It is considered to be the safer and preferred option to use.

Example of #[unwind(aborts)]:

#![feature(unwind_attributes)]

#[unwind(aborts)]
pub extern "C" fn safe_function() {}

fn main() {}

#[unwind(allowed)]

This option specifies that a non-Rust ABI function should be allowed to unwind. However, this can easily result in Undefined Behavior (UB), so it should be used cautiously.

Example of #[unwind(allowed)]:

#![feature(unwind_attributes)]

#[unwind(allowed)]
pub extern "C" fn risky_function() {}

fn main() {}

It's important to note that the default behavior for non-Rust ABI functions is currently "allowed", but this is unspecified and may change in the future.

Fixing Rust Error E0633

Rust Error E0633 occurs when the #[unwind] attribute is malformed. Ensure that the attribute is written correctly with either 'allowed' or 'aborts' as its argument.

Erroneous code example:

#![feature(unwind_attributes)]

#[unwind()] // error: expected one argument
pub extern "C" fn something() {}

fn main() {}

To fix the error, simply add the appropriate argument to the #[unwind] attribute:

#![feature(unwind_attributes)]

#[unwind(aborts)]
pub extern "C" fn something() {}

fn main() {}