How to build C static library into a rust crate for no_std environmrnt

Embedded
1

Hi

I am trying to create a rust crate from mbedtls C library that I compiled separately for my Cortex M4. But I ran into issue for what I believe is no_std related. My steps are following.

  1. I created a cargo lib project called mbedtls-sys with .toml below:

    [package]
name = "mbedtls-sys"
version = "0.1.0"
authors = ["feplooptest"]
edition = "2018"
build = "build.rs"

[build-dependencies]
bindgen = "*"

  2. I created a binding using bindgen after looked at reference below:

    A little C with your Rust - The Embedded Rust Book

    Since I need this to be no_std, I used "--ctypes-prefix=cty" and "--use-core" flag for bindgen.

    bindgen --ctypes-prefix=cty --use-core wrapper.h > src/lib.rs

  3. Finally I created a build.rs under to build this crate by following bindgen user guide.

    Create a build.rs File - The `bindgen` User Guide

    And my build.rs is following:

    extern crate bindgen;

use std::env;
use std::path::PathBuf;

fn main() {
    println!("cargo:rustc-link-search=native=./mbedtls");
    println!("cargo:rustc-link-lib=static=mbedtls");
    println!("cargo:rustc-link-lib=static=mbedx509");
    println!("cargo:rustc-link-lib=static=mbedcrypto");

    // Tell cargo to invalidate the built crate whenever the wrapper changes
    println!("cargo:rerun-if-changed=wrapper.h");

    // The bindgen::Builder is the main entry point
    // to bindgen, and lets you build up options for
    // the resulting bindings.
    let bindings = bindgen::Builder::default()
        // The input header we would like to generate
        // bindings for.
        .header("wrapper.h")
        // Tell cargo to invalidate the built crate whenever any of the
        // included header files changed.
        .parse_callbacks(Box::new(bindgen::CargoCallbacks))
        // Finish the builder and generate the bindings.
        .generate()
        // Unwrap the Result and panic on failure.
        .expect("Unable to generate bindings");

    // Write the bindings to the $OUT_DIR/bindings.rs file.
    let out_path = PathBuf::from(env::var("OUT_DIR").unwrap());
    bindings
        .write_to_file(out_path.join("bindings.rs"))
        .expect("Couldn't write bindings!");
}

Now when I try to run cargo build it spits out a lot of error like this:

    --> src/lib.rs:3180:43
     |
3180 |     pub fn mbedtls_rsa_self_test(verbose: cty::c_int) -> cty::c_int;
     |                                           ^^^ use of undeclared type or module `cty`

Does anyone know how to resolve this issue?
Thanks

2

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3

Is your project set to use edition 2018? Have you added cty crate to the project?

4

Hi Thanks for the information.
Adding cty did get rid of that error, but now I am seeing another error when cargo build with rust project that reference that mebedtls-sys. For some reasons it still try to find std while building it. I feel that I am not setting mbedtls-sys crate correctly for no_std but I am not sure what is the correct way to do so.

error[E0463]: can't find crate for std
|
= note: the thumbv7em-none-eabi target may not be installed

error: aborting due to previous error

For more information about this error, try rustc --explain E0463.
error: could not compile mbedtls-sys.

5

Just realize that I need to add
#![no_std]
in my lib.rs to resolve that.

6

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