Using some C code in Rust

The is-c folder contains c code that we want to use in rust. The uses-c folder contains a rust library that uses the c code. Both statically and dynamically compiled libraries can be used but we focus on statically here.

How to use

1. Have a rust library aptly names `uses-c
2. Have an extern block somewhere in the rust code, linking to a static c library. For example:

use libc::size_t;

#[link(name = "verynice", kind = "static")]
extern "C" {
    pub fn give_number_pls() -> size_t;
}

As we are linking against C code it might happen that we need to use C types in our rust code :shocking:. That's why in your Cargo.toml you will need the libc crate and use the appropriate types.

3. Have a c library with the same name somewhere that contains a function with the same signature as we just specified in the rust code. For example:

#include <stddef.h>

size_t give_number_pls() {
    return 69;
}

nice.

4. Compile the C code using gcc -c verynice.c -o .verynice.o to get an object file.

5. For whatever reason we cannot use an object file directly so now, we put it into an archive! This is done using ar rcs libverynice.a verynice.o. An important step just happend: we added "lib" to the beginning of the file. And because of reasons a library file that does not start with "lib" will not be found in the linking step later. Why? I don't know.

6. With our c library now finally being done, let's help rust with finding it. In the rust directory, right next to the Cargo.toml we create a build.rs file and put this in:

fn main() {
    println!("cargo:rustc-link-search=native=./path_to/dir_dir/with_the/compiled_libraries");
}

7. Finally we can compile the rust code using cargo build and even run it using cargo run.
8. Doooone

Provided makefiles

This repo has a few makefiles. If you run make run at the repository root first the c library is built, then the rust library and finally it is also run. make clean exists to get rid of both the c and rust artifacts.