Cast c_int to *mut c_void

[mhanusek]

I have C function:

fn my_func(p_arg : *mut c_void);

I would like pass a parameter to function:

let val : c_int = 0;
let p_val = val as *mut c_void;
if p_val == ptr::null_mut() {
    println!("TEST p_val is nullptr");
}

but parameter after cast is nullptr - why?
How to fix?

[L.F]

@mhanusek

let p_val = val as *mut c_void;

Here, val is an integer, and integers are cast to pointers by treating the integer as an address (reference). That's why you end up with nullptr.

The fix is also straightforward — cast a reference to a pointer instead:

let p_val = &mut val as *mut c_void;

Apparently, you need to mark val as mut to do this.

Demonstration on the playground:

fn main() {
    let mut val = 0;
    
    println!("{:p}",      val as *mut i32); // null pointer
    println!("{:p}", &mut val as *mut i32); // the address of the variable
}

[mhanusek]

@L.F

error[E0606]: casting `&mut i32` as `*mut std::ffi::c_void` is invalid
  --> src/lib.rs:72:29
   |
72 |                 let p_val = &mut val as *mut c_void;

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=13b6373cadaddc2cd02b40f17e1d82a1

[L.F]

You need to cast the reference to a *mut c_int before casting to *mut c_void. To retrieve the value from the *mut c_void, you need to cast it to a *const c_int first (*mut c_int works too), and then dereference it (which is an unsafe operation):

use libc::c_int;
use libc::c_void;

fn main() {
    let mut val : c_int = 0;
    
    let p_val = &mut val as *mut c_int as *mut c_void;

    println!("ptr: {:p}", p_val);
    println!("value: {:}", unsafe { *(p_val as *const c_int) });
}

(playground)