Create interface to C function pointers in Rust

Question

I might not have described my question title properly, please edit it if needed.

I'm trying to crate a Rust interface to LXC library, which is written in C.

I have successfully called simple functions like lxc_get_version or lxc_container_new but I cannot get access to functions described in struct lxc_container block.

Here is an part of my code:

#[link(name = "lxc")]
extern {
    // LXC part
    fn lxc_get_version() -> *const c_char;
    fn lxc_container_new(name: *const c_char, configpath: *const c_char) -> LxcContainer;

    // LXC container parts
    fn is_defined(container: &LxcContainer) -> bool; 
}

And here is an error:

note: test.o: In function `LxcContainer::is_defined::heb2f16a250ac7940Vba':
test.0.rs:(.text._ZN12LxcContainer10is_defined20heb2f16a250ac7940VbaE+0x3e): undefined reference to `is_defined'

EDIT: I have managed that functions inside C structs is called function pointers. I've tried to google something like "Rust C function pointer", but without luck.

Answer 1

When you see something like this (in C):

struct S {
    void (*f)(int, long)
}

it means that struct S contains a field called f which is a pointer to a function. It does not mean that the library itself exposes a function called f. For example, this is valid:

void some_function_1(int x, long y) { ... }

void some_function_2(int a, long b) { ... }

int main() {
    struct S s1; s1.f = some_function_1;
    struct S s2; s2.f = some_function_2;
}

Here struct instance s1 contains a pointer to some_function_1, and s2 contains a pointer to some_function_2.

When you're writing FFI binding in Rust for some C library, you usually define Rust counterparts for C structures. Some tools like rust-bindgen can even do this automatically. In your case you will have to write something like this:

#[repr(C)]
struct LxcContainer {
    name: *mut c_char,
    configfile: *mut c_char,
    // ...
    numthreads: c_int,
    // ...
    is_defined_f: extern fn(c: *mut LxcContainer) -> bool,
    state_f: extern fn(c: *mut LxcContainer) -> *const c_char,
    // ...
}

That is, weird-looking C function pointer types correspond to extern fn function pointer types in Rust. You could also write extern "C" fn(...) -> ..., but "C" qualifier is default so it is not required.

You will have to write something like this to call these functions:

impl LxcContainer {
    fn is_defined_f(&mut self) -> bool {
        unsafe {
            (self.is_defined_f)(self as *mut LxcContainer)
        }
    }
}

You need to cast a reference to a raw pointer and you also need to wrap self.is_defined_f in parentheses in order to disambiguate between method call and field access.

You can find more on FFI in Rust here. Function pointers are explained very briefly there, though.