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Writing a C Compiler from Scratch: A Step‑by‑Step Guide to Building a Real C-to-x86-64 Language

ByInnoVirtuoso

Compilers feel like magic until you write one. That’s the hook: once you’ve built even a small compiler, you’ll see how source code becomes executable instructions, and the mystery melts away. If you’ve ever wondered how C turns into blazing-fast binaries, or you’ve wanted to deepen your systems understanding, building a C compiler is one of the most rewarding projects you can tackle.

This practical guide walks you through writing your own compiler for a significant subset of C—from lexing and parsing to code generation and optimization. You don’t need prior compiler or assembly experience. You’ll get a clear mental model, concrete steps, and the confidence to implement it in the language you already use. Along the way, we’ll keep the tone human and the concepts friendly—because compilers don’t have to be terrifying.

What You’ll Build: A Real Subset of C

We’ll target a meaningful slice of C—the kind of features you’d actually use in small programs:

  • Variables, types, and expressions
  • If/else, while/for loops, and blocks
  • Functions with parameters and return values
  • Pointers and arrays (enough to feel like real C)
  • Basic standard library interop (e.g., calling puts)

The goal is to compile this subset to x86-64 assembly you can assemble and run on a modern machine (Linux or macOS). This isn’t a toy language that looks like C—it is C, narrowed to the essentials so you can ship a working compiler quickly and expand from there.

How a Compiler Works (In Plain English)

Let’s demystify the pipeline most compilers follow. Once you get this mental model, everything else falls into place.

  • Lexing (tokenization): You scan characters and group them into tokens: identifiers, numbers, keywords, operators, punctuation. A token is a single meaningful unit like int, +, or ).
  • Parsing: You transform the token stream into a tree (an Abstract Syntax Tree, or AST) that represents the program’s structure. Typically you use a recursive descent parser for a C subset because it’s easy to implement and debug.
  • Semantic analysis: You check the AST for correctness. Do variables exist? Do types line up? Are we returning the right type? You also build symbol tables and resolve scopes.
  • Intermediate representation (optional): Some compilers build an IR to simplify analysis and optimization. For a first version, you can go straight from AST to assembly.
  • Code generation: You turn the AST (or IR) into machine code or assembly. We’ll aim for x86-64 assembly that follows the System V ABI (the common calling convention on Unix-like systems).
  • Optimization: You improve performance and reduce code size with techniques like constant folding, dead code elimination, and register allocation.

If you want a great overview, the “Compiler” entry on Wikipedia gives helpful context and terminology.

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Step 1: Lexing C

The lexer reads raw characters and produces tokens. Think of it as turning “int x = 42;” into [INT_KEYWORD, IDENT(x), EQUALS, NUMBER(42), SEMICOLON].

Key tips for building a lexer:

  • Treat whitespace as separators, not tokens.
  • Handle comments (// and / … /) by skipping them.
  • Recognize multi-character operators first (>=, <=, ==, !=) before single-character ones (=, <, >, !).
  • Support integer literals (start with decimal; you can add hex 0x later).
  • Keep track of line/column for good error messages.

A simple approach is a single pass over the input with a few helper functions: peek(), advance(), match(). Don’t overcomplicate it. You can always add string literals and char constants once you compile basic programs.

For reference on C’s lexical rules, the C11 draft is freely available: N1570 C11 draft. It’s long, but it’s useful when you’re unsure about edge cases.

Step 2: Parsing with Recursive Descent

Parsing turns the token list into an AST. With recursive descent, you write small parsing functions that mirror your grammar rules:

  • program → top_level_decl*
  • top_level_decl → function | global_var
  • function → type ident “(” params? “)” block
  • stmt → if_stmt | while_stmt | for_stmt | return_stmt | block | expr_stmt
  • expr → assignment → equality → comparison → term → factor → unary → primary

Handling operator precedence is easier if you structure functions from lowest precedence to highest (assignment lowest, primary highest). This hierarchy ensures you parse 1 + 2 * 3 as 1 + (2 * 3), not (1 + 2) * 3.

If you enjoy formalism, you can write your grammar in EBNF. But don’t get stuck in theory. A readable, well-structured parser is more valuable than a perfectly formal grammar on paper.

Error handling matters. If you encounter an unexpected token, emit an error with line/column and a hint: “Expected ‘)’ after parameters.” Good messages save hours later.

Step 3: Building the AST and Symbol Tables

Your AST should capture the program’s structure without extra fluff. Common node types:

  • Program, Function, Block
  • If, While, For, Return
  • BinaryOp, UnaryOp, Assign
  • VarDecl, Identifier, NumberLiteral, Call

Symbol tables map names to declarations. You’ll want a stack of scopes: enter a scope for each block, leave when the block ends. Each identifier should resolve to the closest declaration in scope.

Why this matters: once you have a clean AST and clear symbol resolution, semantic checks become simple and code generation becomes predictable. That’s the foundation you’ll build on.

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Step 4: Semantic Analysis and Type Checking

At this stage, you verify that the program makes sense:

  • Type rules: int + int → int; pointer arithmetic rules; casts if you support them.
  • Declarations: no use-before-declare (unless you choose to allow function forward declarations).
  • Function calls: argument count and types must match.
  • Control flow: ensure returns in non-void functions. You can start simple: allow missing returns and warn, then tighten later.
  • Lvalues and rvalues: ensure only assignable expressions appear on the left side of an assignment.

Also consider undefined behavior. C leaves many things undefined, but your compiler can detect some obvious issues early (e.g., returning a pointer to a local variable). Keep it friendly: warn when possible.

Step 5: Code Generation to x86-64 (System V ABI)

Here’s the part that felt like magic as a beginner—and turned out to be learnable. You’ll emit assembly that your system assembler (e.g., GNU as or NASM) turns into machine code.

Core pieces:

  • Calling convention: On System V AMD64 (Linux/macOS), the first six integer/pointer args go in RDI, RSI, RDX, RCX, R8, R9; return value in RAX. Callee must preserve RBX, RBP, R12–R15.
  • Stack frames: Prologue saves RBP and sets RBP = RSP; space for locals; epilogue restores and returns.
  • Expressions: Use registers for intermediate values; spill to the stack when you run out.
  • Control flow: Generate labels for basic blocks; use CMP/TEST + conditional jumps (JE, JNE, JL, etc.).
  • Memory: Local variables at negative offsets from RBP; globals in data sections.

A concise ABI reference: System V AMD64 ABI. For assembler docs, see GNU as or NASM’s manual: NASM docs.

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Example Strategy for Expressions

  • Post-order traversal works well: generate code for children, then combine.
  • Keep a virtual “eval stack” in registers. When short on registers, push to the real stack.
  • For binary ops:
  • Evaluate RHS into RAX, push.
  • Evaluate LHS into RBX.
  • Pop RAX, then emit the instruction (e.g., ADD RBX, RAX) and move result where you want it.

Function Calls

  • Evaluate args right-to-left, place them in the appropriate registers.
  • Align the stack to 16 bytes before calling (System V requirement).
  • Call the function symbol; result will be in RAX.

It’s okay if your first code generator is naive. Correctness first, speed later.

Step 6: Optimization Essentials (Keep It Simple)

Early optimizations deliver big wins without complexity:

  • Constant folding: Compute 2 + 3 at compile time.
  • Algebraic simplifications: x + 0 → x, x * 1 → x.
  • Dead store elimination: If you assign to a local and never read it, remove it.
  • Common subexpression elimination (CSE): If the same expression occurs repeatedly in a straight-line block, compute once and reuse.
  • Peephole optimizations: Clean up adjacent instructions in codegen (e.g., MOV RAX, RAX is useless).

Curious to go deeper? Read about Static Single Assignment (SSA) and the LLVM design docs. You don’t need SSA for a first compiler, but it teaches patterns you’ll use if you ever build a more advanced backend.

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Testing, Debugging, and Tooling

Write tests as you build features. You’ll thank yourself later.

  • Unit tests: For the lexer and parser. Feed tiny snippets, assert on token streams and AST structures.
  • Compiler tests: For end-to-end behaviors. Compile small programs and assert on exit codes or printed output.
  • Assembly inspection: Use objdump or otool to disassemble and confirm what you emitted.
  • Debugging: If you crash, inspect your stack frame and prologue/epilogue logic. 9/10 times, misaligned stack or clobbered callee-saved registers are the culprits.
  • Sanitizers: When you compile your runtime support (if any), enable AddressSanitizer or UBSan to catch memory issues early.

Here’s a helpful mindset: every bug is either “parser misunderstood,” “state lost,” or “ABI violated.” Classify it quickly and you’ll narrow the search.

Project Structure, Build System, and Language Choice

You can implement your compiler in whatever language you like. Choose the one you can write quickly and safely:

  • C or C++: Maximum control; easy interop with assembly; more footguns.
  • Rust: Safety and pattern matching help writing parsers and analyzers; great for reliability.
  • Go: Fast iteration; straightforward tooling; simple concurrency for multi-pass optimizations.
  • Python: Excellent for a prototype or teaching; slower, but fine if your programs are small.

Suggested structure:

  • src/lexer., src/parser., src/ast., src/semantics., src/codegen.*
  • test/ with input .c files and expected outputs
  • Makefile or build script for “compile → assemble → link → run tests”

Tooling checklist:

  • Assembler/linker: part of your system toolchain (binutils on Linux, Xcode CLI tools on macOS)
  • Disassembler: objdump/otool
  • Formatter/linter: keeps diffs small and reviews fast
  • CI: run your tests on every push

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From “Hello, world” to Functions, Control Flow, and Pointers

Here’s a practical implementation roadmap. Each step compiles and runs before you add the next.

1) Expressions and return – Start with a single function main that returns an int literal: return 0;. – Add arithmetic: +, -, *, / on integers. – Generate a minimal prologue/epilogue so it runs as a real program.

2) Variables and assignments – Add local variable declarations: int x; x = 42; return x;. – Manage stack slots and offsets.

3) If/else and comparisons – Parse ==, !=, <, <=, >, >= and emit conditional branches. – Implement if (…) { … } else { … }.

4) While/for loops – Emit labels for loop heads and exits. – Ensure break and continue work (optional at first).

5) Functions – Parse parameters and handle call argument registers. – Implement return values and stack alignment. – Add function-level symbol tables.

6) Pointers and arrays – Support & and * operators; pointer arithmetic (e.g., p + 1 moves by sizeof(p)). – Handle array indexing a[i] as (a + i).

7) Minimal standard library linkage – Allow calling extern functions like puts. You’ll declare them and rely on the system linker to resolve.

Pro tip: log every stage to a file (tokens, AST, IR/assembly). Your future self will use those logs to crush bugs in minutes instead of hours.

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Common Pitfalls and How to Avoid Them

  • Stack alignment before calls
  • On System V, RSP must be 16-byte aligned at call. Push/pop tracking prevents mysterious crashes.
  • Off-by-one in local offsets
  • Standardize how you allocate stack slots and stick to it. A layout diagram in comments helps.
  • Precedence bugs
  • Add tests specifically for operator precedence and associativity. Parse 1 + 2 * 3, 4 – 2 – 1, etc.
  • Dangling identifiers
  • Ensure every Identifier node either resolves to a VarDecl or a Function. Save that link in the AST.
  • Shadowing errors
  • When entering a new scope, push a new symbol table; pop on exit. Shadow by inserting into the top-most scope only.
  • Missing callee-saved restores
  • If you push RBX or R12–R15, restore them before returning—every time.
  • Incorrect calling convention on macOS vs. Linux
  • System V is shared, but small differences can bite. Keep a per-platform shim if needed.

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Learn More: Books, Papers, and Open Source Repos

FAQ: People Also Ask

Q: Can I write a C compiler without knowing assembly? A: Yes. You’ll learn just enough assembly to generate function prologues/epilogues, move data, and branch. Start with a few instructions and expand as features demand.

Q: What’s the easiest language to write a compiler in? A: The one you’re productive in. Many beginners pick C, Rust, or Go. Rust’s pattern matching makes AST work pleasant; C keeps you close to the metal; Go is fast to iterate.

Q: Do I need to implement the entire C standard? A: No. Start with a subset that compiles real programs: ints, control flow, functions, and pointers. You can add features incrementally.

Q: Should I generate machine code directly or use an assembler? A: Use an assembler. Generate human-readable x86-64 assembly, assemble it with GNU as or NASM, and link normally. It’s simpler to debug and reason about.

Q: How do I test a compiler? A: Layered testing. Unit tests for lexing/parsing, golden tests for AST/IR, and end-to-end tests that compile a program and check output or exit code. Automate everything.

Q: How long does it take to build a working subset? A: Expect a weekend to get “return 0;” compiling and a few weeks of evenings to reach functions, loops, and pointers. Pace varies with prior experience and how polished you want it.

Q: What about Windows support? A: Start on Linux or macOS with the System V ABI. Windows uses a different x64 calling convention. Port later once your core is stable.

Q: Is optimization necessary for a first version? A: No. Correctness first. Add constant folding and a few peephole passes once you trust your codegen. Sophisticated optimization can wait.

Q: What’s the difference between an interpreter and a compiler? A: An interpreter executes code directly, often line by line or from an intermediate representation. A compiler translates code to another form (like machine code) ahead of time.

The Takeaway

Writing a C compiler is the most direct path to understanding how software becomes something your CPU can execute. You’ll learn to parse real syntax, enforce semantics, and speak the language of your machine. Start small, make it work, then make it fast. If you enjoyed this, explore the resources above and consider subscribing for more deep, practical systems guides.

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InnoVirtuoso

Hello there! I'm passionate content writer navigating the digital landscape. With a deep love for words and an insatiable curiosity about technology.

With 10 years of experience in IT field, I write mostly about emerging tech, sharing news, informational articles and covering other topics I find interesting.

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