Understanding Rust Error E0787: Unsupported Naked Functions


This article will discuss Rust Error E0787, which occurs when a naked function definition is unsupported. We'll look at the reasons behind this error, some code examples demonstrating the issue, and clear solutions to address it.

What is a Naked Function?

A naked function is a low-level function without any function prologue or epilogue. They are directly exposed to the hardware execution with minimal abstraction. Common use-cases for naked functions include development of kernels, interrupts, and operating systems.

Understanding Rust Error E0787

Rust Error E0787 occurs when the definition of a naked function is unsupported. The naked functions must meet certain criteria to be valid in Rust, as follows:

- The naked function must be defined using a single inline assembly block.

- Execution must never fall through past the end of the assembly code, so the block must use the noreturn option.

- The asm block can use att_syntax and raw options. Other options are not allowed.

- The asm block must not contain any operands other than const and sym.

Example of Erroneous Code


fn main() {
pub extern "C" fn f() -> u32 {

In the example above, the function 'f' is marked with the #[naked] attribute but does not follow the required constraints. Using unsupported operands or missing the inline assembly block, in this case, will lead to Error E0787.

Fixing Rust Error E0787

To fix Rust Error E0787, modify the naked function definition to meet the required criteria. The corrected version of the earlier example should look like this:


pub unsafe extern "C" fn f() -> u32 {
    asm!("ret", options(noreturn));
    panic!("Execution must never fall through past asm block");

In this-fixed example, we've added the inline assembly block using the asm! macro with the 'ret' instruction and the noreturn option. This code now adheres to the requirements for naked functions, and Rust Error E0787 will not occur.


To avoid Rust Error E0787, ensure that naked function definitions follow the required criteria. By using inline assembly blocks, specifying noreturn options, and adhering to the restrictions on operands, your naked functions will be correctly implemented, and this error will be resolved.