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Attempted to pass an invalid type of variable into a variadic function.
Erroneous code example:
```compile_fail,E0617
# 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:
```no_run
# 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!
}
```
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