Interactive Tool User Guide

Introduction

You can use Bindgen and CXX to implement the interaction between Rust and C/C++. Bindgen enables Rust to call C by converting C interfaces into Rust interfaces. CXX implement interaction between C++ and Rust by generating bindings between C interfaces and Rust interfaces.

bindgen_and_cxx_tools

Using Bindgen

Procedure

The following example shows how to use Bindgen to implement invocation of C by Rust.

  1. In the header file lib.h, define two interfaces FuncAAddB and SayHello in C. The FuncAAddB interface calculates the sum of two numbers, and the SayHello interface prints strings.

    #ifndef BUILD_RUST_TESTS_BINDGEN_TEST_LIB_H_
#define BUILD_RUST_TESTS_BINDGEN_TEST_LIB_H_
#include <stdint.h>
#include "build/rust/tests/test_bindgen_test/test_for_hello_world/lib2.h"

uint32_t FuncAAddB(uint32_t a, uint32_t b);
void SayHello(const char *message);

#endif  //  BUILD_RUST_TESTS_BINDGEN_TEST_LIB_H_

  2. Add the implementation of the two interfaces to lib.c.

    #include "build/rust/tests/test_bindgen_test/test_for_hello_world/lib.h"
#include <stdint.h>
#include <stdio.h>

void SayHello(const char *message)
{
    printf("This is a test for bindgen hello world:\n");
    printf("%s\n", message);
}

uint32_t FuncAAddB(uint32_t a, uint32_t b)
{
    printf("This is a test for bindgen of a + b:\n");
    return a + b;
}

  3. Create the main.rs file to call C interfaces through c_ffi using Rust. Note that insecure interfaces called by Rust must be encapsulated by using unsafe.

    //!  bindgen test for hello world
#![allow(clippy::approx_constant)]
mod c_ffi {
    #![allow(dead_code)]
    #![allow(non_upper_case_globals)]
    #![allow(non_camel_case_types)]
    include!(env!("BINDGEN_RS_FILE"));
}
/// pub fn add_two_numbers_in_c
pub fn add_two_numbers_in_c(a: u32, b: u32) -> u32 {
    unsafe { c_ffi::FuncAAddB(a, b) }
}

use std::ffi::c_char;
use std::ffi::CString;

/// fn main()
fn main() {
    println!("{} + {} = {}", 3, 7, add_two_numbers_in_c(3, 7));
    let c_str = CString::new("This is a message from C").unwrap();
    let c_world: *const c_char = c_str.as_ptr() as *const c_char;
    unsafe {
        c_ffi::SayHello(c_world);
    }
}

  4. Create the BUILD.gn file to define the dependency of the Rust module on the C module.

    import("//build/ohos.gni")

ohos_shared_library("c_lib") {
  sources = [ "lib.c" ]
  defines = [ "COMPONENT_IMPLEMENTATION" ]
}

rust_bindgen("c_lib_bindgen") {
  header = "lib.h"
}

ohos_rust_executable("bindgen_test") {
  deps = [ ":c_lib" ]
  deps += [ ":c_lib_bindgen" ]
  sources = [ "main.rs" ]
  bindgen_output = get_target_outputs(":c_lib_bindgen")
  inputs = bindgen_output
  rustenv = [ "BINDGEN_RS_FILE=" + rebase_path(bindgen_output[0]) ]
  crate_root = "main.rs"
}

Verification

bindgen_test

Using CXX

Calling Rust Interfaces by C++

  1. In the Rust lib.rs file, add the C++ interfaces to be called in mod ffi, and add the interfaces in extern "Rust" to expose them to C++.

    //! #[cxx::bridge]
#[cxx::bridge]
mod ffi{
    #![allow(dead_code)]
    #[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord)]
    struct Shared {
        z: usize,
    }
    extern "Rust"{
        fn print_message_in_rust();
        fn r_return_primitive() -> usize;
        fn r_return_shared() -> Shared;
        fn r_return_rust_string() -> String;
        fn r_return_sum(_: usize, _: usize) -> usize;
    }
}

fn print_message_in_rust(){
    println!("Here is a test for cpp call Rust.");
}
fn r_return_shared() -> ffi::Shared {
    println!("Here is a message from Rust,test for ffi::Shared:");
    ffi::Shared { z: 1996 }
}
fn r_return_primitive() -> usize {
    println!("Here is a message from Rust,test for usize:");
    1997
}
fn r_return_rust_string() -> String {
    println!("Here is a message from Rust,test for String");
    "Hello World!".to_owned()
}
fn r_return_sum(n1: usize, n2: usize) -> usize {
    println!("Here is a message from Rust,test for {} + {} is:",n1 ,n2);
    n1 + n2
}

  2. Include lib.rs.h (converted from lib.rs by the CXX tool) in C++ code.

    #include <iostream>
#include "build/rust/tests/test_cxx/src/lib.rs.h"

int main(int argc, const char* argv[])
{
    int a = 2021;
    int b = 4;
    print_message_in_rust();
    std::cout << r_return_primitive() << std::endl;
    std::cout << r_return_shared().z << std::endl;
    std::cout << std::string(r_return_rust_string()) << std::endl;
    std::cout << r_return_sum(a, b) << std::endl;
    return 0;
}

  3. Create the BUILD.gn file. The underlying rust_cxx calls the CXX tool to convert the lib.rs file into lib.rs.h and lib.rs.cc. ohos_rust_static_ffi implements compilation of the Rust source code, and ohos_executable implements compilation of the C++ code.

    import("//build/ohos.gni")
import("//build/templates/rust/rust_cxx.gni")

rust_cxx("test_cxx_exe_gen") {
    sources = [ "src/lib.rs" ]
}

ohos_rust_static_ffi("test_cxx_examp_rust") {
    sources = [ "src/lib.rs" ]
    deps = [ "//build/rust:cxx_rustdeps" ]
}

ohos_executable("test_cxx_exe") {
    sources = [ "main.cpp" ]
    sources += get_target_outputs(":test_cxx_exe_gen")

    include_dirs = [ "${target_gen_dir}" ]
    deps = [
    ":test_cxx_examp_rust",
    ":test_cxx_exe_gen",
    "//build/rust:cxx_cppdeps",
    ]
}

Verification cpp_call_rust

Calling C++ by Rust

  1. Create the header file client_blobstore.h.

    #ifndef BUILD_RUST_TESTS_CLIENT_BLOBSTORE_H
#define BUILD_RUST_TESTS_CLIENT_BLOBSTORE_H
#include <memory>
#include "third_party/rust/cxx/include/cxx.h"

namespace nsp_org {
namespace nsp_blobstore {
struct MultiBufs;
struct Metadata_Blob;

class client_blobstore {
public:
    client_blobstore();
    uint64_t put_buf(MultiBufs &buf) const;
    void add_tag(uint64_t blobid, rust::Str add_tag) const;
    Metadata_Blob get_metadata(uint64_t blobid) const;

private:
    class impl;
    std::shared_ptr<impl> impl;
};

std::unique_ptr<client_blobstore> blobstore_client_new();
} // namespace nsp_blobstore
} // namespace nsp_org
#endif

  2. Create the client_blobstore.cpp file.

    #include <algorithm>
#include <functional>
#include <set>
#include <string>
#include <unordered_map>
#include "src/main.rs.h"
#include "build/rust/tests/test_cxx_rust/include/client_blobstore.h"

namespace nsp_org {
namespace nsp_blobstore {
// Toy implementation of an in-memory nsp_blobstore.
//
// The real implementation of client_blobstore could be a large complex C++
// library.
class client_blobstore::impl {
    friend client_blobstore;
    using Blob = struct {
        std::string data;
        std::set<std::string> tags;
    };
    std::unordered_map<uint64_t, Blob> blobs;
};

client_blobstore::client_blobstore() : impl(new class client_blobstore::impl) {}

// Upload a new blob and return a blobid that serves as a handle to the blob.
uint64_t client_blobstore::put_buf(MultiBufs &buf) const
{
    std::string contents;

    // Traverse the caller's res_chunk iterator.
    //
    // In reality there might be sophisticated batching of chunks and/or parallel
    // upload implemented by the nsp_blobstore's C++ client.
    while (true) {
        auto res_chunk = next_chunk(buf);
        if (res_chunk.size() == 0) {
        break;
        }
        contents.append(reinterpret_cast<const char *>(res_chunk.data()), res_chunk.size());
    }

    // Insert into map and provide caller the handle.
    auto res = std::hash<std::string> {} (contents);
    impl->blobs[res] = {std::move(contents), {}};
    return res;
}

// Add add_tag to an existing blob.
void client_blobstore::add_tag(uint64_t blobid, rust::Str add_tag) const
{
    impl->blobs[blobid].tags.emplace(add_tag);
}

// Retrieve get_metadata about a blob.
Metadata_Blob client_blobstore::get_metadata(uint64_t blobid) const
{
    Metadata_Blob get_metadata {};
    auto blob = impl->blobs.find(blobid);
    if (blob != impl->blobs.end()) {
        get_metadata.size = blob->second.data.size();
        std::for_each(blob->second.tags.cbegin(), blob->second.tags.cend(),
            [&](auto &t) { get_metadata.tags.emplace_back(t); });
    }
    return get_metadata;
}

std::unique_ptr<client_blobstore> blobstore_client_new()
{
    return std::make_unique<client_blobstore>();
}
} // namespace nsp_blobstore
} // namespace nsp_org

  3. In ffi of the main.rs file, use the macro includes! to import the header file client_blobstore.h. Then, the main() function of Rust can call the C++ interfaces in ffi mode.

    //! test_cxx_rust
#[cxx::bridge(namespace = "nsp_org::nsp_blobstore")]
mod ffi {
    // Shared structs with fields visible to both languages.
    struct Metadata_Blob {
        size: usize,
        tags: Vec<String>,
    }

    // Rust types and signatures exposed to C++.
    extern "Rust" {
        type MultiBufs;

        fn next_chunk(buf: &mut MultiBufs) -> &[u8];
    }

    // C++ types and signatures exposed to Rust.
    unsafe extern "C++" {
        include!("build/rust/tests/test_cxx_rust/include/client_blobstore.h");

        type client_blobstore;

        fn blobstore_client_new() -> UniquePtr<client_blobstore>;
        fn put_buf(&self, parts: &mut MultiBufs) -> u64;
        fn add_tag(&self, blobid: u64, add_tag: &str);
        fn get_metadata(&self, blobid: u64) -> Metadata_Blob;
    }
}

// An iterator over contiguous chunks of a discontiguous file object.
//
// Toy implementation uses a Vec<Vec<u8>> but in reality this might be iterating
// over some more complex Rust data structure like a rope, or maybe loading
// chunks lazily from somewhere.
/// pub struct MultiBufs
pub struct MultiBufs {
    chunks: Vec<Vec<u8>>,
    pos: usize,
}
/// pub fn next_chunk
pub fn next_chunk(buf: &mut MultiBufs) -> &[u8] {
    let next = buf.chunks.get(buf.pos);
    buf.pos += 1;
    next.map_or(&[], Vec::as_slice)
}

/// fn main()
fn main() {
    let client = ffi::blobstore_client_new();

    // Upload a blob.
    let chunks = vec![b"fearless".to_vec(), b"concurrency".to_vec()];
    let mut buf = MultiBufs { chunks, pos: 0 };
    let blobid = client.put_buf(&mut buf);
    println!("This is a test for Rust call cpp:");
    println!("blobid = {}", blobid);

    // Add a add_tag.
    client.add_tag(blobid, "rust");

    // Read back the tags.
    let get_metadata = client.get_metadata(blobid);
    println!("tags = {:?}", get_metadata.tags);
}

  4. Create the BUILD.gn file. Use CXX to convert main.rs into lib.rs.h and lib.rs.cc, which are used as the source code of test_cxx_rust_staticlib. Compile Rust main.rs, and add the dependency test_cxx_rust_staticlib.

    import("//build/ohos.gni")

rust_cxx("test_cxx_rust_gen") {
  sources = [ "src/main.rs" ]
}

ohos_static_library("test_cxx_rust_staticlib") {
  sources = [ "src/client_blobstore.cpp" ]
  sources += get_target_outputs(":test_cxx_rust_gen")
  include_dirs = [
    "${target_gen_dir}",
    "//third_party/rust/cxx/v1/crate/include",
    "include",
  ]
  deps = [
    ":test_cxx_rust_gen",
    "//build/rust:cxx_cppdeps",
  ]
}

ohos_rust_executable("test_cxx_rust") {
  sources = [ "src/main.rs" ]
  deps = [
    ":test_cxx_rust_staticlib",
    "//build/rust:cxx_rustdeps",
  ]
}

Verification rust_call_cpp