Understanding Rust Error E0617

What is Rust Error E0617?

Rust Error E0617 occurs when you attempt to pass an invalid type of variable into a variadic function. Variadic functions are functions that accept a varying number of arguments, often used in conjunction with interfaces to C libraries.

Example of Erroneous Code

#![allow(unused)]
fn main() {
use std::os::raw::{c_char, c_int};
extern "C" {
    fn printf(format: *const c_char, ...) -> c_int;
}

unsafe {
    printf("%f\n\0".as_ptr() as _, 0f32);
    // error: cannot pass an `f32` to variadic function, cast to `c_double`
}
}

Why does Rust Error E0617 occur?

Rust Error E0617 is caused by obscure Application Binary Interface (ABI) rules, as defined by the C standard, which dictate that certain Rust types must be cast before passing them to a variadic function. This error can be resolved by casting the value to the type specified in the error message, which may need to be imported from std::os::raw.

Solution to Rust Error E0617

To resolve the error, cast the value that is causing the error as the type specified in the error message. Consider the example below, where the erroneous code has been fixed by casting the `f32` value to `c_double`.

Corrected Code Example

# use std::os::raw::{c_char, c_int, c_double};
# extern "C" {
#     fn printf(format: *const c_char, ...) -> c_int;
# }

unsafe {
    printf("%f\n\0".as_ptr() as _, 0f32 as c_double); // ok!
}

Please note the sprintf function from the libc crate is also available as a safer alternative to the problematic printf function.