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External C functions are allowed to be variadic. However, a variadic function
takes a minimum number of arguments. For example, consider C's variadic `printf`
function:
```compile_fail,E0060
use std::os::raw::{c_char, c_int};
extern "C" {
fn printf(_: *const c_char, ...) -> c_int;
}
unsafe { printf(); } // error!
```
Using this declaration, it must be called with at least one argument, so
simply calling `printf()` is invalid. But the following uses are allowed:
```
# use std::os::raw::{c_char, c_int};
# #[cfg_attr(all(windows, target_env = "msvc"),
# link(name = "legacy_stdio_definitions",
# kind = "static", modifiers = "-bundle"))]
# extern "C" { fn printf(_: *const c_char, ...) -> c_int; }
# fn main() {
unsafe {
use std::ffi::CString;
let fmt = CString::new("test\n").unwrap();
printf(fmt.as_ptr());
let fmt = CString::new("number = %d\n").unwrap();
printf(fmt.as_ptr(), 3);
let fmt = CString::new("%d, %d\n").unwrap();
printf(fmt.as_ptr(), 10, 5);
}
# }
```
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