E0617 - Error codes index
Description: Book listing all Rust error codes

# Error code E0617

Attempted to pass an invalid type of variable into a variadic function.

Erroneous code example:

    #![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`
    }
    }

Certain Rust types must be cast before passing them to a variadic function, because of arcane ABI rules dictated by the C standard. To fix the error, cast the value to the type specified by the error message (which you may need to import from `std::os::raw`).

In this case, `c_double` has the same size as `f64` so we can use it directly:

    # 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 _, 0f64); // ok!
    }