compiler/rustc_error_codes/src/error_codes/E0617.md

   1 Attempted to pass an invalid type of variable into a variadic function.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0617
   6 extern {
   7     fn printf(c: *const i8, ...);
   8 }
   9
  10 unsafe {
  11     printf(::std::ptr::null(), 0f32);
  12     // error: cannot pass an `f32` to variadic function, cast to `c_double`
  13 }
  14 ```
  15
  16 Certain Rust types must be cast before passing them to a variadic function,
  17 because of arcane ABI rules dictated by the C standard. To fix the error,
  18 cast the value to the type specified by the error message (which you may need
  19 to import from `std::os::raw`).
  20
  21 In this case, `c_double` has the same size as `f64` so we can use it directly:
  22
  23 ```no_run
  24 # extern {
  25 #     fn printf(c: *const i8, ...);
  26 # }
  27 unsafe {
  28     printf(::std::ptr::null(), 0f64); // ok!
  29 }
  30 ```