bindings.rs

extern "C" {
    pub fn otama_open(otama: *mut *mut otama_t,
                      config_path: *const ::std::os::raw::c_char)
     -> otama_status_t;
}

test.rs

#[cfg(test)]
mod tests {
    use super::*;
    use std::mem;
    use std::ffi::CString;

    #[test]
    fn test_otama_open() {
        let config = CString::new("path.yaml").unwrap();
        unsafe {
            let &mut o = &mut mem::zeroed();
            otama_open(&mut o as *mut *mut otama_t, config.as_ptr());
        }
    }
}

$ cargo test
   Compiling otama v0.1.0 (file:///Users/hattori-h/otama-rs)
error: cannot borrow immutable local variable `o` as mutable
  --> src/lib.rs:18:29
   |
17 |             let &mut o = &mut mem::zeroed();
   |                      - use `mut o` here to make mutable
18 |             otama_open(&mut o as *mut *mut otama_t, config.as_ptr());
   |                             ^ cannot borrow mutably

error: aborting due to previous error