Hacker News Reader

Comment by PaulDavisThe1st | Hacker News Reader

Given the implementation of this little thing, I'm mildly curious about how easy it is/will be for a C/C++ project/programmer to use these Rust crates.

The other way around is relatively well defined and has been the subject of a lot of attention for several decades. Is the Rust-lib-called-from-C even feasible?

View on HN

Rust can be called from C the same way as C++ can. You need to export unmangled extern "C" symbols. Not quite frictionless but there are helper utilities for this.

If the Rust code uses the "StupidAlloc" allocator, it should work as expected. There may be some constructor/destructor code involved with its usual complications.

But it would not replace malloc/free in C code nor will it be compatible with C++ allocators for std containers.

fairly straightforward:

        #[global_allocator]
        static GLOBAL: StupidAlloc = StupidAlloc;
        
        #[no_mangle]
        pub unsafe extern "C" fn stupid_malloc(size: usize) -> *mut u8 {
            let layout = Layout::from_size_align(size, 1).unwrap();
            GLOBAL.alloc(layout)
        }
        
        #[no_mangle]
        pub unsafe extern "C" fn stupid_free(ptr: *mut u8) {
            let layout = Layout::from_size_align(1, 1).unwrap();
            GLOBAL.dealloc(ptr, layout)
        }

then build with --crate-type cdylib, giving you a .so you can link

Although this crate likely enforces a higher alignment (to the page boundary with mmap) regardless of what is requested, you should request a higher alignment because malloc is guaranteed to be aligned to the largest C type, which is usually either 8 or 16 bytes.

fixing the bugs is left as an exercise for the reader

It has been practical for ages. You define a wrapper function as no_mangle and extern "C", then compile the lib as a staticlib. When compiled, it produces a .a file you can link into your C application

I feel like Rust was designed to fully replace old languages, and is now facing the same issue as every language before it - C and C++ interop (both ways).

Zig nailed it, Rust fucked it up. Time will tell if thats big enough of an issue to warrant some bigger changes to fix it.

That is mischaracterization. Rust has been intended to be able to incrementally integrate to C/C++ codebases, of course most significantly integrating Rust/Servo components to Firefox/Gecko.

See for example this rsvg presentation[1] and gstreamer blog post[2] for examples of the interop

[1] https://viruta.org/docs/fmq-porting-c-to-rust.pdf

[2] https://coaxion.net/blog/2016/05/writing-gstreamer-plugins-a...

Reply on news.ycombinator.com