rustlang dynamic library part 2

A previous post was about getting a simple rust dynamic library working. The next thing I wanted to do was try passing a more complex type from the dynamic library to the executable using it.

First, I changed dynamiclib to:


#[derive(Debug)]
pub struct Tuple (u32,);

#[derive(Debug)]
pub struct MyStruct {
    s1: &'static str,
    s2: String,
    t: Tuple,
}

#[no_mangle]
pub extern "C" fn test() -> MyStruct {
    MyStruct {
        s1: "hello",
        s2: "world".to_string(),
        t: Tuple(47),
    }
}

And dynamic to:


#[link(name="dynamiclib")]
extern {
    fn test() -> MyStruct;
}

fn main() {
    let retval = unsafe { test() };
    println!("Got: {:?}", retval);
}

And (somewhat surprisingly) it worked as expected. Although you get the following error:


warning: found struct without foreign-function-safe representation annotation in foreign module, consider adding a #[repr(C)] attribute to the type, #[warn(improper_ctypes)] on by default

At first I considered appeasing the warning, but if it's a library written in rust and called from rust, shouldn't the default

repr(Rust)

be safe/correct?

Next, I decided to try loading and re-loading the shared library using this more complex struct... and I started getting SIGSEGV the second time I attempted to use the library.

I came across libloading (crates.io) which purports to be safer wrapper for working with dyanmic libraries, and if nothing else has much better documentation.

A quick change to dynamic/src/main.rs (devoid of error-checking):


    let lib = Library::new("libdynamiclib.dylib").unwrap();
    let test: Symbol MyStruct> = unsafe {
        lib.get(b"test\0").unwrap()
    };
    println!("Got: {:?}", test());

And it works correctly even when re-loading the library. So, I was either using stale symbols, or something else. Still several things that I obviously don't fully understand. Hopefully pouring over Rustonomicon will reveal the secrets to me.

Rendered Obsolete