The Faux C Compiler

This is a reference implementation for the book Writing a C Compiler, a hands-on guide to writing your own compiler for a substantial subset of the C programming language.

This implementation is a work in progress, so please fork with caution!

Prerequisites

You’ll need:

• Rust ≥ 1.79.0 (latest stable recommended)
• Cargo, installed with Rustup
• A C toolchain (GCC or Clang) to assemble and link the output programs
• (Optional) llvm-tools-preview if you plan to emit LLVM IR or link through llc

If you’re new to Rust, install the toolchain using rustup:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Confirm installation:

rustc --version
cargo --version

Features

• Ch 1–3: Lexer, parser, expression handling
• Ch 4–5: Statement parsing, functions
• Ch 6–7: Locals, stack management
• Ch 8–10: Control flow, conditionals, loops
• Ch 11–12: Pointers, arrays, structs
• Ch 13+: Codegen optimizations, calling conventions, ABI compliance

Building FCC

This puts the executable at target/release/fcc

git clone https://github.com/princemuel/fcc.git # check out the repo
cd fcc
cargo build --release

For a faster iteration

cargo run -- path/to/source.c

Running the Tests

FCC uses Rust’s built-in test framework for unit and integration tests. There are a handful of unit tests, can be run with this command

cargo test

For compiler verification against the official Writing a C Compiler Test Suite

git clone https://github.com/nlsandler/writing-a-c-compiler-tests.git
cd writing-a-c-compiler-tests

./test_compiler --check-setup # make sure you meet all the system requirements

./test_compiler ~/path/to/fcc/target/release -- --chapter 4

Usage

Assume we have this source file at ~/hello.c

int puts(char *c);

int main(void) {
    puts("Hello, world!");
}

To compile and run it

$ target/release/fcc ~/hello.c -o ~/hello
$ ~/hello
Hello, world!

Common Tasks

cargo run -- examples/return_42.c

If you use the LLVM backend

cargo run --release --backend=llvm examples/hello.c

To emit LLVM IR (if enabled) instead of machine code

cargo run -- --emit-llvm examples/fib.c

License

MIT License © 2025

Based on the book Writing a C Compiler by Nora Sandler.