Passing a Rust variable to a C function that expects to be able to modify it

Question

I\'m writing a safe Rust layer with which I can call functions from a C library in Rust. I\'ve generated the unsafe bindings using rust-bindgen, but I\'m getting a little co

Answer 1

but I do have a decent background in C

The moral equivalent of your Rust code is:

int *v = NULL;
imeGet(addr, v);
*v

This will have an error because the C code is likely going to dereference that v to store the value in, except you've passed in a NULL, so it's more likely to go boom.

You need to create storage for the value, then provide a reference to that storage to the function:

fn ime_get(addr: u8) -> i32 {
    let mut v = 0;
    unsafe { imeGet(addr, &mut v) };
    v
}

The solution for any pointer type uses ptr::null_mut:

unsafe { 
    let mut v = std::ptr::null_mut();
    takes_a_pointer_pointer(addr, &mut v);
    v
}

The general solution for any type uses mem::MaybeUninit:

unsafe {
    let mut v = std::mem::MaybeUninit::uninit();
    takes_a_value_pointer(addr, v.as_mut_ptr());
    v.assume_init()
}

For completeness, you should be checking the return value:

fn ime_get(addr: u8) -> Option<i32> {
    let mut v = 0;
    let success = unsafe { imeGet(addr, &mut v) };

    if success {
        Some(v)
    } else {
        None
    }
}

the differences between how Rust and C work with regards to passing pointers.

There really aren't any, at this level.