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Rust Crates that bind to C++

Rust crates may bind to C++ code to enable the crate to run time-tested C++ code from Rust and for projects such as BLST that implement core algorithms in C++ and create bindings to work across many languages. The RISC Zero supports these crates by providing a toolchain to build C++ code.

How to Install the C++ toolchain​

The C++ toolchain is installed alongside the Rust toolchain. Installation instructions can be found here.

How to Build and Run these crates​

RISC Zero has a GCC-based C++ toolchain to enable these crates to build and run on the RISC Zero zkVM. The toolchain can be installed by running cargo risczero install. This command will download the guest Rust toolchain as well as a C++ toolchain that compiles C++ to riscv32im. After installing the toolchain, these crates can be built and run on the zkVM like any other Rust crate. Under the hood, the risc0-build crate will use the downloaded toolchain while building the guest crate. The C++ toolchain is typically used by the build.rs file of crates that use C++ code.

Overriding the C++ toolchain​

The guest C++ toolchain can be overridden by setting the CC_riscv32im_risc0_zkvm_elf and CFLAGS_riscv32im_risc0_zkvm_elf environment variables when running the cargo build command like so:

CC_riscv32im_risc0_zkvm_elf="[path to C++ compiler]" CFLAGS_riscv32im_risc0_zkvm_elf="[desired cflags]" cargo build

When overriding the C++ toolchain for experimentation purposes, the C++ compiler must be configured to generate riscv32im instructions. This can be done by setting CFLAGS_riscv32im_risc0_zkvm_elf="-target riscv32-unknown-elf -march=rv32im" in the cargo build command for clang compilers.

Issues Building Rust Crates that Bind to C​

Not all C code is guaranteed to build or run on the RISC Zero zkVM out of the box. Be sure to consult the crate's features and configure them in a way that's suitable for the zkVM. As an example, the BLST crate defines a no_threads feature. The RISC Zero zkVM executes on a single thread, so using this feature of the BLST crate allows it to build and run successfully. Troubleshooting these crates requires an understanding of the characteristics of the RISC Zero zkVM. If there is a crate that does not work, file an issue here.

Notes on versioning​

At this time, there is only one version of the C++ toolchain and it is intended to work with any guest rust toolchain. We will provide more information on versioning and compatibility in the future.