* DRAFT dylib-skel-1

 - State "DRAFT"      from              [2023-11-05 Sun 22:23]

 ** Overview

 Our core languages are [[https://www.rust-lang.org/][Rust]] and [[https://lisp-lang.org/][Lisp]] - this is the killer combo which will allow NAS-T

 to rapidly develop high-quality software. As such, it's crucial that these two very

 different languages (i.e. compilers) are able to interoperate seamlessly.

 Some interop methods are easy to accomodate via the OS - such as IPC or data sharing,

 but others are a bit more difficult.

 In this 2-part series we'll build a FFI bridge between Rust and Lisp, which is something

 that /can/ be difficult, due to some complications with Rust and because this is not the

 most popular software stack (yet ;). This is an experiment and may not make it to our

 code-base, but it's definitely something worth adding to the toolbox in case we need it.

 ** FFI

 The level of interop we're after in this case is [[https://en.wikipedia.org/wiki/Foreign_function_interface][FFI]].

 Basically, calling Rust code from Lisp and vice-versa. There's an article about calling

 Rust from Common Lisp [[https://dev.to/veer66/calling-rust-from-common-lisp-45c5][here]] which shows the basics and serves as a great starting point

 for those interested.

 *** Rust != C

 The complication(s) with Rust I mentioned early is really just that /it is not C/. =C=

 is old, i.e. well-supported with a stable ABI, making the process of creating bindings

 for a C library a breeze in many languages.

 For a Rust library we need to first appease the compiler, as explained in [[https://doc.rust-lang.org/nomicon/ffi.html#calling-rust-code-from-c][this section]]

 of the Rustonomicon. Among other things it involves changing the calling-convention of

 functions with a type signature and editing the Cargo.toml file to produce a

 C-compatible ABI binary. The Rust default ABI is unstable and can't reliably be used

 like the C ABI can.

 *** Overhead

 Using FFI involves some overhead. Check [[https://github.com/dyu/ffi-overhead][here]] for an example benchmark across a few

 languages. While building the NAS-T core, I'm very much aware of this, and will need a

 few sanity benchmarks to make sure the cost doesn't outweigh the benefit. In particular,

 I'm concerned about crossing multiple language barriers (Rust<->C<->Lisp).

 ** Rust -> C -> Lisp

 *** Setup

 For starters, I'm going to assume we all have Rust (via =rustup=) and Lisp (=sbcl= only)

 installed on our GNU/Linux system (some tweaks needed for Darwin/Windows, not covered in

 this post).

 **** Cargo

 Create a new library crate. For this example we're focusing on a 'skeleton' for

 /dynamic/ libraries only, so our experiment will be called =dylib-skel= or *dysk* for

 short.

 src_sh[:exports code]{cargo init dysk --lib && cd dysk} 

 A =src/lib.rs= will be generated for you. Go ahead and delete that. We're going to be

 making our own =lib.rs= file directly in the root directory (just to be cool).

 The next step is to edit your =Cargo.toml= file. Add these lines after the =[package]=

 section and before =[dependencies]=:

 #+begin_src conf-toml

 [lib]

 crate-type = ["cdylib","rlib"]

 path = "lib.rs"

 [[bin]]

 name="dysk-test"

 path="test.rs"

 #+end_src

 This tells Rust to generate a shared C-compatible object with a =.so= extension which we

 can open using [[https://man.archlinux.org/man/dlopen.3.en][dlopen]].

 **** cbindgen

 ***** install

 Next, we want the =cbindgen= program which we'll use to generate header files for

 C/C++. This step isn't necessary at all, we just want it for further experimentation.

 src_sh[:exports code]{cargo install --force cbindgen}

 We append the =cbindgen= crate as a /build dependency/ to our =Cargo.toml= like so:

 #+begin_src conf-toml

 [build-dependencies]

 cbindgen = "0.24"

 #+end_src

 ***** cbindgen.toml

 #+begin_src conf-toml :tangle cbindgen.toml

 language = "C"

 autogen_warning = "/* Warning, this file is autogenerated by cbindgen. Don't modify this manually. */"

 include_version = true

 namespace = "dysk"

 cpp_compat = true

 after_includes = "#define DYSK_VERSION \"0.1.0\""

 line_length = 88

 tab_width = 2

 documentation = true

 documentation_style = "c99"

 usize_is_size_t = true

 [cython]

 header = '"dysk.h"'

 #+end_src

 ***** build.rs

 #+begin_src rust :tangle build.rs

 fn main() -> Result<(), cbindgen::Error> {

   if let Ok(b) = cbindgen::generate(std::env::var("CARGO_MANIFEST_DIR").unwrap()) {

     b.write_to_file("dysk.h"); Ok(())}

   else { panic!("failed to generate dysk.h from cbindgen.toml") } }

 #+end_src

 *** lib.rs

 #+begin_src rust :tangle lib.rs

 //! lib.rs --- dysk library

 use std::ffi::{c_char, c_int, CString};

 #[no_mangle]

 pub extern "C" fn dysk_hello() -> *const c_char {

   CString::new("hello from rust").unwrap().into_raw()}

 #[no_mangle]

 pub extern "C" fn dysk_plus(a:c_int,b:c_int) -> c_int {a+b}

 #[no_mangle]

 pub extern "C" fn dysk_plus1(n:c_int) -> c_int {n+1}

 #+end_src

 *** test.rs

 #+begin_src rust :tangle test.rs

 //! test.rs --- dysk test

 fn main() { let mut i = 0u32; while i < 500000000 {i+=1; dysk::dysk_plus1(2 as core::ffi::c_int);}}

 #+end_src

 *** compile

 #+begin_src sh

 cargo build --release

 #+end_src

 *** load from SBCL

 #+begin_src lisp :tangle dysk.lisp

 (load-shared-object #P"target/release/libdysk.so")

 (define-alien-routine dysk-hello c-string)

 (define-alien-routine dysk-plus int (a int) (b int))

 (define-alien-routine dysk-plus1 int (n int))

 (dysk-hello) ;; => "hello from rust"

 #+end_src

 *** benchmark

 #+begin_src shell

 time target/release/dysk-test

 #+end_src

 #+begin_src lisp :tangle test.lisp

 (time (dotimes (_ 500000000) (dysk-plus1 2)))

 #+end_src