Fixing Rust Error E0776

Understanding Rust Error E0776

Rust Error E0776 occurs when a function marked with the #[cmse_nonsecure_entry] attribute does not have a C ABI. This attribute is used for functions that serve as non-secure entry points when writing software for Arm TrustZone systems.

An Example of Erroneous Code

#![allow(unused)]
#![feature(cmse_nonsecure_entry)]

fn main() {
#[no_mangle]
#[cmse_nonsecure_entry]
pub fn entry_function(input: Vec<u32>) {}
}

In the code above, we have a function called entry_function that is marked with the #[cmse_nonsecure_entry] attribute. However, it does not specify a C ABI, thus causing the Rust Error E0776.

Correcting the Error

To fix Rust Error E0776, you need to declare the non-secure entry function using the C ABI by adding extern "C" before the function definition.

A Corrected Example

#![allow(unused)]
#![feature(cmse_nonsecure_entry)]

fn main() {
#[no_mangle]
#[cmse_nonsecure_entry]
extern "C" fn entry_function(input: Vec<u32>) {}
}

In the corrected code, we've added extern "C" before the entry_function definition. This specifies that the function uses the C ABI, resolving the Rust Error E0776.

What is the C ABI?

The 'C ABI' stands for 'C Application Binary Interface,' which is a calling convention used by the C programming language. Using the C ABI allows functions written in Rust to be compatible when interacting with other languages or libraries that depend on the C ABI.

When to use #[cmse_nonsecure_entry]

The #[cmse_nonsecure_entry] attribute should be used only when developing software for Arm TrustZone systems that require a non-secure entry function. If you're not working on an Arm TrustZone application, this attribute is irrelevant, and you won't encounter Rust Error E0776.