rust/src/librustc_error_codes/error_codes/E0617.md

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

Erroneous code example:

extern {
    fn printf(c: *const i8, ...);
}

unsafe {
    printf(::std::ptr::null(), 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).