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!
}