I've a function which has prototype as below
//opaque struct
struct mosquitto;
struct mosquitto *mosquitto_new(const char *id, bool clean_session, void *obj);
In my c code, I'm calling it as below.
struct mosquitto *m = mosquitto_new(buf, true, NULL);
Now I want to call the above API in my rust code. rust-bindgen generated the following bindings
pub enum Struct_mosquitto { }
pub fn mosquitto_new(id: *const ::libc::c_char, clean_session: u8, obj: *mut ::libc::c_void) -> *mut Struct_mosquitto;
When I'm trying to call the above API, I'm getting a mismatch at 3rd argument.
let s = CString::new("ravi").unwrap();
let mqtt = mosquitto::mosquitto_new(s.as_ptr(), 1, ptr::null());
How do I pass NULL to *mut c_void?
BONUS QUESTION: How to pass a rust struct to *mut c_void ?
Answer
The ptr::null() function returns a *const T, what you want is the ptr::null_mut() function, since the argument to your function is of type *mut ::libc::c_void.
For passing an actual value, have a look at the answer to Working with c_void in an FFI