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! }