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
).
In this case, c_double
has the same size as f64
so we can use it directly:
# extern { # fn printf(c: *const i8, ...); # } unsafe { printf(::std::ptr::null(), 0f64); // ok! }