Use C variable from Rust without unsafe

Question

I can expose a C function to Rust code via the FFI as follows:

use std::os::raw::c_int;

mod c {
    #[link(name="...")]
    extern "C" {
        pub fn add(a: c_int, b: c_int) -> c_int;
    }
}

pub fn add(a: c_int, b: c_int) -> c_int {
    unsafe {
        c::add(a, b)
    }
}

Now I can call add from Rust without having to wrap it in another unsafe block. But what if I want to do the same for a variable? I.e.:

use std::os::raw::c_int;

mod c {
    #[link(name="...")]
    extern "C" {
        pub static VAR: c_int;
    }
}

pub static VAR: c_int = unsafe { c::VAR };

This results in a compiler error: "cannot read from extern static". What is the correct way (if there is one) to do this?

You can still offer some safe interface around that variable through some getter/setter. Within that interface it'll be unsafe, just like the add example. — sebpuetz
– sebpuetz, Commented Apr 24, 2021 at 16:47

Alsein · Accepted Answer · 2021-04-25 08:03:38Z

2

It should be unsafe when it is indeed unsafe, although you can make a static borrow of the imported global variable.

static VAR: &i32 = unsafe { &c::VAR };

answered Apr 25, 2021 at 8:03

Alsein

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