How do I convert a &str to a *const u8? [duplicate]

Question

I have a function

extern "C" {
    fn log_impl(ptr: *const u8);
}

fn log(s: &str) {
    log_impl(s.as_bytes() as *const u8);
}

This gives me the following error:

error[E0606]: casting `&[u8]` as `*const u8` is invalid
 --> src/main.rs:6:14
  |
6 |     log_impl(s.as_bytes() as *const u8);
  |              ^^^^^^^^^^^^^^^^^^^^^^^^^

The most similar question to what I'm trying to do is Converting a str to a &[u8].

Answer 1

Rust strings are not NUL-terminated like most C functions expect. You can convert a &str to *const u8 by using &s.as_bytes()[0] as *const u8 or by using s.as_ptr(), but that will not be valid to pass to any C function expecting a NUL-terminated string.

Instead, you probably need to use CString which will copy the string to a buffer and add a NUL terminator to the end. Here is an example assuming that log_impl doesn't store a pointer to the string:

fn log(s: &str) {
    unsafe {
        let c_str = CString::new(s).unwrap();
        log_impl(c_str.as_ptr() as *const u8);
    }
}