CXX — Rust和C++之间的安全FFI

引入背景

CXX工具提供了一种安全的互相调用机制，可以实现rust和C++的互相调用。

CXX通过FFI（Foreign Function Interface）和函数签名的形式来实现接口和类型声明，并对类型和函数签名进行静态分析，以维护Rust和C++的不变量和要求。

CXX工具在OH上的使用指导

C++调用Rust接口

在Rust侧文件lib.rs里mod ffi写清楚需要调用的C++接口，并将接口包含在extern "Rust"里面，暴露给C++侧使用。

//! #[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
}

C++侧将cxx工具转换出来的lib.rs.h包含进来，就可以使用C++侧的接口。

#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;
}

添加构建文件BUILD.gn。rust_cxx底层调用CXX工具将lib.rs文件转换成lib.rs.h和lib.rs.cc文件，ohos_rust_static_ffi实现Rust侧源码的编译，ohos_executable实现C++侧代码的编译。

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",
    ]
}

调测验证 cpp_call_rust

Rust调用C++

添加头文件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

添加cpp文件client_blobstore.cpp。

#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.
//
// In reality the 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

main.rs文件，在main.rs文件的ffi里面，通过宏include!将头文件client_blobstore.h引入进来，从而在Rust的main函数里面就可以通过ffi的方式调用C++的接口。

//! 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);
}

添加构建文件BUILD.gn。使用CXX将main.rs转换成lib.rs.h和lib.rs.cc，同时将产物作为test_cxx_rust_staticlib的源码，编译Rust源码main.rs并将test_cxx_rust_staticlib依赖进来。

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",
  ]
}

调测验证 rust_call_cpp

与bindgen的对比

bindgen主要用来实现rust代码对c接口的单向调用；CXX工具可以实现rust和C++的互相调用。

基于cargo的构建

对于由Cargo的构建，需要使用一个构建脚本来运行CXX的C++代码生成器。

典型的构建脚本如下：

# Cargo.toml

[build-dependencies]
CXX-build = "1.0"

// build.rs

fn main() {
    CXX_build::bridge("src/main.rs")  // returns a cc::Build
        .file("src/demo.cc")
        .flag_if_supported("-std=C++11")
        .compile("cxxbridge-demo");

    println!("cargo:rerun-if-changed=src/main.rs");
    println!("cargo:rerun-if-changed=src/demo.cc");
    println!("cargo:rerun-if-changed=include/demo.h");
}

基于非cargo的构建

对于在非Cargo构建中的使用，如Bazel或Buck，CXX提供了另一种方式产生C++侧的头文件和源代码文件，作为一个独立的命令行工具使用。

$ cargo install cxxbridge-cmd
$ cxxbridge src/main.rs --header > path/to/mybridge.h
$ cxxbridge src/main.rs > path/to/mybridge.cc

内置类型

除了所有的原生类型（i32 <=> int32_t）之外，还有以下常见类型可用于共享结构的字段以及函数的参数和返回值。

name in Rust	name in C++	restrictions
String	rust::String
&str	rust::Str
&[T]	rust::Slice<const T>	^{cannot hold opaque C++ type}
&mut [T]	rust::Slice<T>	^{cannot hold opaque C++ type}
CXXString	std::string	^{cannot be passed by value}
Box<T>	rust::Box<T>	^{cannot hold opaque C++ type}
UniquePtr<T>	std::unique_ptr<T>	^{cannot hold opaque Rust type}
SharedPtr<T>	std::shared_ptr<T>	^{cannot hold opaque Rust type}
[T; N]	std::array<T, N>	^{cannot hold opaque C++ type}
Vec<T>	rust::Vec<T>	^{cannot hold opaque C++ type}
CXXVector<T>	std::vector<T>	^{cannot be passed by value, cannot hold opaque Rust type}
mut T, const T	T, const T	^{fn with a raw pointer argument must be declared unsafe to call}
fn(T, U) -> V	rust::Fn<V(T, U)>	^{only passing from Rust to C++ is implemented so far}
Result<T>	throw/catch	^{allowed as return type only}

rust命名空间的C++ API是由include/CXX.h文件定义的。使用这些类型时种类的时候，需要C++代码中包含这个头文件。

以下类型很快被支持，只是还没有实现。

name in Rust	name in C++
BTreeMap<K, V>	^tbd
HashMap<K, V>	^tbd
Arc<T>	^tbd
Option<T>	^tbd
^tbd	std::map<K, V>
^tbd	std::unordered_map<K, V>

开发者贡献

当前CXX工具还没有达到普遍使用阶段，在使用该工具的过程中有任何问题欢迎开发者在社区issue中反馈。

OpenHarmony / third_party_rust_cxx

CXX — Rust和C++之间的安全FFI

引入背景

CXX工具在OH上的使用指导

C++调用Rust接口

Rust调用C++

与bindgen的对比

基于cargo的构建

基于非cargo的构建

内置类型

开发者贡献

简介

发行版

贡献者

近期动态

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CXX — Rust和C++之间的安全FFI

引入背景

CXX工具在OH上的使用指导

C++调用Rust接口

Rust调用C++

与bindgen的对比

基于cargo的构建

基于非cargo的构建

内置类型

开发者贡献

简介

发行版

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