Understanding Rust Error E0138: Multiple Functions with #[start] Attribute

Rust Error E0138 Explained

Error code E0138 occurs when the Rust compiler encounters multiple functions within a program that have been declared with the #[start] attribute. Since a Rust program must have a unique entry point, having multiple functions with the #[start] attribute results in ambiguity and an inability for the compiler to determine which function to execute first.

Illustrating the Error

An example of erroneous code that triggers the E0138 error is:

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

fn main() {
#[start]
fn foo(argc: isize, argv: *const *const u8) -> isize {}

#[start]
fn f(argc: isize, argv: *const *const u8) -> isize {}
// error: multiple 'start' functions
}

In the example above, both `foo` and `f` functions have been erroneously declared with the #[start] attribute, causing the E0138 error.

Resolving the Error

To resolve Rust Error E0138, you must ensure that only one function in the entire program is marked with the #[start] attribute, making it the unique entry point of the program. In the erroneous code example shown earlier, the solution is to remove the #[start] attribute from an incorrect or unnecessary function or merge the required logic into a single starter function.

An example of corrected code is:

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

fn main() {
#[start]
fn foo(argc: isize, argv: *const *const u8) -> isize { 0 } // ok!
}

Overall, it is crucial to maintain only one #[start] attributed function in your Rust program to avoid the E0138 error. By adhering to this practice, you ensure a unique and clear entry point, enabling the Rust compiler to successfully execute your code.