Using cxx to mix in Rust-code with a C++ application

Overview

Minimal application mixing C++ and Rust

This example uses cxx to generate bindings between C++ and Rust, and integrates the two parts through CMake.

It is basically an inverted version of cxx's demo code, using C++ for the entry point and a MultiBuf class, while implementing a simple blobstore-library in Rust.

How it works

In lib.rs we add bridge declarations for our Rust types:

#[cxx::bridge(namespace = "org::blobstore")]
mod ffi {
    // Rust types and signatures exposed to C++.
    extern "Rust" {
        type BlobstoreClient;
        fn new_blobstore_client() -> Box<BlobstoreClient>;
        fn put(&mut self, parts: Pin<&mut MultiBuf>) -> u64;
        ...
    }
}

fn new_blobstore_client() -> Box<BlobstoreClient> {
    Box::new(BlobstoreClient { blobs: HashMap::new() })
}

struct BlobstoreClient {
    blobs: HashMap<u64, Blob>,
}

impl BlobstoreClient {
    fn put(&mut self, mut parts: Pin<&mut MultiBuf>) -> u64 {
        ...
    }
}

In build.rs we add logic to generate C++ bridging code from the declarations:

fn main() {
    cxx_build::bridge("src/lib.rs");
    println!("cargo:rerun-if-changed=src/lib.rs");
}

In CMakeLists.txt we add a custom command to trigger the Rust build:

add_custom_command(
        OUTPUT ${BLOBSTORE_BRIDGE_CPP} ${BLOBSTORE_LIB}
        COMMAND cargo build --manifest-path ${BLOBSTORE_CARGO_MANIFEST}
        ...
)

In main.cpp we include the generated C++ header, construct Rust types, and call their methods:

#include "lib.rs.h"

int main() {
    auto client = org::blobstore::new_blobstore_client();
    ...
    const auto blobid = client->put(buf);
}

The application also consumes C++ types from Rust (MultiBuf), and leverages shared types between the two languages (BlobMetadata).

To learn more about the bridging layer, check out cxx's documentation.

Building and running the code

  git clone git@github.com:paandahl/cpp-with-rust.git
  mkdir cpp-with-rust/build
  cd cpp-with-rust/build

  cmake ..
  cmake --build .
  ./cpp_with_rust

NOTE: If you are using Windows, run these commands in the Developer PowerShell for VS.

Technical notes

  • As opposed to the original cxx demo, build.rs only generates the C++ bridging code, without compiling it. Instead, we pass it in to the CMake build by referencing it in add_executable().
  • For simplicity, this example always builds the Rust code in debug mode. See here for suggested changes to adhere to the specified CMAKE_BUILD_TYPE, and moving the cargo output to within the CMake build tree.

License

The code is available under the MIT license.

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Comments
  • Fix: cmake clean issue (lib.rs.cc not found after cleaning the build directory)

    Fix: cmake clean issue (lib.rs.cc not found after cleaning the build directory)

    Closes #1

    This issue can be triggered by the cmake clean target after a successful build. Any other build attempt after that will trigger the reported error. It doesn't matter if the build tool is GNU Make or Ninja. For instance:

    cd cpp-with-rust
    cmake -S . -B build
    # (configuration succeeds)
    cmake --build build/
    # (build succeeds)
    cmake --build build/ --target clean
    # (clean removes two files)
    cmake --build build/
    # (build fails!)
    

    Another available workaround is running cargo clean before building. It fully removes the target/ directory.

    The problem is that the cmake clean target removes the OUTPUT files generated by add_custom_command(), but cargo doesn't know how to regenerate them when the target directory is already populated by the last cargo execution.

    The COMMENT message added to add_custom_command() proves that cmake tries to run cargo whenever the ${BLOBSTORE_BRIDGE_CPP} file is missing. The message is printed no matter if the build fails.

    My first attempt to fix this issue was adding BYPRODUCTS ${CARGO_TARGET_DIR} to add_custom_command(), which works fine if the CMake generator is GNU Make, but fails for Ninja. The alternative is to set the ADDITIONAL_CLEAN_FILES property on the main target, which emulates the cargo clean effects.

    opened by pedrolcl 0
  • Not finding lib.rs.cc on macos when running cmake build

    Not finding lib.rs.cc on macos when running cmake build

    Thanks for your project! I get the following issue when running cmake --build . on macos:

    clang: error: no such file or directory: '/Users/{USER}/cpp-with-rust/target/cxxbridge/blobstore/src/lib.rs.cc'
    clang: error: no input files
    make[2]: *** [CMakeFiles/cpp_with_rust.dir/target/cxxbridge/blobstore/src/lib.rs.cc.o] Error 1
    make[1]: *** [CMakeFiles/cpp_with_rust.dir/all] Error 2
    make: *** [all] Error 2
    
    opened by stexa 2
Owner
Preben Aandahl
Preben Aandahl
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