Hermit for C

August 13, 2025 ยท View on GitHub

This repository provides information about using the Hermit kernel with C along with some test applications.

For building C code for Hermit you need a corresponding cross compiler. We provide a Docker image containing a GCC-based cross compiler in our hermit-gcc repository. With this cross compiler you can compile most C code for Hermit.

When linking an application, you need to explicitly link against a Hermit kernel (libhermit.a) that you can build from our kernel repository.

GCC

You can compile your C applications manually with Hermit's cross compiler by linking against the Hermit kernel. A complete GCC invocation might look like this:

x86_64-hermit-gcc -o main main.c -L/absolute/path/to/libhermit.a-directory

Meson

This repository uses the Meson build system to maintain a set of C-based test applications. If your project uses Meson, you should be able to use the cross files from this repository. You can also use this project's hermit target definition (meson.build) for automatically building a Hermit kernel to link against.

To build the test applications from this repository, first set up a build directory:

meson setup --cross-file cross/x86_64-hermit.ini build-x86_64-hermit-debug

Then, compile the project:

meson compile -C build-x86_64-hermit-debug -v

You can then run the resulting Hermit applications using the Hermit loader or in Uhyve, for example:

qemu-system-x86_64 \
    -enable-kvm \
    -cpu host \
    -smp 1 \
    -m 1024M \
    -device isa-debug-exit,iobase=0xf4,iosize=0x04 \
    -display none -serial stdio \
    -kernel hermit-loader-x86_64 \
    -initrd build-x86_64-hermit-debug/hello_world

CMake

When using CMake, you can use this toolchain file:

# x86_64-hermit.cmake

set(CMAKE_SYSTEM_NAME Hermit)
set(CMAKE_SYSTEM_PROCESSOR x86_64)

set(CMAKE_C_COMPILER x86_64-hermit-gcc)
set(CMAKE_CXX_COMPILER x86_64-hermit-g++)

# Needed to pass CMake's compiler test during build system generation
set(CMAKE_EXE_LINKER_FLAGS_INIT "-L/absolute/path/to/libhermit.a-directory")

The toolchain file can be supplied to CMake during configuration:

cmake -G Ninja -B build-x86_64-hermit-debug -S . --toolchain x86_64-hermit.cmake

You can then build as usual with CMake:

cmake --build build-x86_64-hermit-debug

Autoconf

When using Autoconf, the build directory can be set up for Hermit:

First create and enter the build directory:

mkdir build-hermit
cd build-hermit

Then, configure the build directory for Hermit:

../configure --host=x86_64-hermit \
    LDFLAGS="-L/absolute/path/to/libhermit-directory"

Finally, build the project:

make

Rust applications with C code

When building a Rust application with C code that uses the C standard library, enable the hermit crate's libc feature to link against the C standard library.

cc-rs

For Rust applications with cc-rs, set the following environment variables to use Hermit's cross compiler:

export CC_x86_64_unknown_hermit=x86_64-hermit-gcc
export CXX_x86_64_unknown_hermit=x86_64-hermit-g++
export AR_x86_64_unknown_hermit=x86_64-hermit-gcc-ar

cmake-rs

For Rust applications with cmake-rs, provide the toolchain file via this environment variable:

export CMAKE_TOOLCHAIN_FILE_x86_64_unknown_hermit="$PWD/hermit-toolchain.cmake"

rust-bindgen

When building a Rust application that uses bindgen, you currently need to manually specify the Hermit's system search path. First, find Hermit's search directory list for Hermit's C preprocessor (cpp):

x86_64-hermit-cpp -v /dev/null -o /dev/null

Then, provide these directories to bindgen's Clang by adding -isystem arguments to this environment variable:

export BINDGEN_EXTRA_CLANG_ARGS_x86_64_unknown_hermit="-isystem /opt/hermit/lib/gcc/x86_64-hermit/7.5.0/include -isystem /opt/hermit/lib/gcc/x86_64-hermit/7.5.0/include-fixed -isystem /opt/hermit/lib/gcc/x86_64-hermit/7.5.0/../../../../x86_64-hermit/include"

License

Licensed under either of

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.