clang-format-inc 0.2.0

Incrementally format only changed C/C++ lines with clang-format — works in both local and CI pre-commit contexts

clang-format-inc

Incremental C/C++ formatting as a pre-commit hook — format only the lines you changed.

---

The problem

Tool	Issue
mirrors-clang-format	Reformats entire files — breaks diffs in large codebases
git clang-format --staged	Works locally, but staging area is empty in CI when pre-commit uses --from-ref/--to-ref

clang-format-inc solves both: it reads the PRE_COMMIT_FROM_REF / PRE_COMMIT_TO_REF environment variables that pre-commit sets in CI mode and falls back to --cached (staged changes) locally.

Inspired by darker, which does the same for Python/Black.

---

How it works

pre-commit (CI, --from-ref/--to-ref)
  └─ sets PRE_COMMIT_FROM_REF, PRE_COMMIT_TO_REF
      └─ clang-format-inc
          └─ git diff -U0 $FROM_REF $TO_REF -- <files>
              └─ clang-format --lines=<start>:<end> -i <file>  ← only changed lines
pre-commit (local)
  └─ clang-format-inc
      └─ git diff -U0 --cached -- <files>
          └─ clang-format --lines=<start>:<end> -i <file>

The diff is parsed to extract added-line ranges per file. clang-format is called with --lines=start:end for each changed hunk — only touched lines are reformatted.

---

Requirements

• Python 3.8+
• clang-format — installed automatically by pre-commit via additional_dependencies (see below), or specify a custom binary with --binary

---

Usage as a pre-commit hook

Add to your .pre-commit-config.yaml:

repos:
  - repo: https://github.com/goyaladitya05/clang-format-inc
    rev: v0.2.0
    hooks:
      - id: clang-format-inc

clang-format is installed automatically into the hook's virtualenv — no system-wide install needed.

Pin a specific clang-format version

hooks:
  - id: clang-format-inc
    additional_dependencies: ['clang-format==18.1.0']

Check mode (report without fixing)

hooks:
  - id: clang-format-inc
    args: [--check]

With --check, the hook exits non-zero if any changed line would be reformatted, but never modifies files. Useful in CI pipelines where you want to report issues without auto-applying fixes.

Run locally

pre-commit run clang-format-inc --all-files

Run in CI (GitHub Actions example)

- name: Run pre-commit
  run: |
    pre-commit run clang-format-inc \
      --from-ref ${{ github.event.pull_request.base.sha }} \
      --to-ref   ${{ github.event.pull_request.head.sha }}

---

CLI reference

clang-format-inc [options] [files ...]

Options:
  --binary PATH          Path to clang-format binary (default: clang-format)
  --style STYLE          Formatting style: file, LLVM, Google, etc. (default: file)
  --fallback-style STYLE Style to use when --style=file but no .clang-format found
  --sort-includes        Pass --sort-includes to clang-format
  --check                Don't modify files; exit non-zero if any file would be reformatted
  -p NUM                 Strip NUM leading path components from diff filenames (default: 1)

---

Install from PyPI

pip install clang-format-inc

---

Environment variables (set automatically by pre-commit)

Variable	When set	Value
PRE_COMMIT_FROM_REF	CI mode (--from-ref)	Base commit SHA
PRE_COMMIT_TO_REF	CI mode (--to-ref)	Head commit SHA

When both are set, clang-format-inc diffs those two commits. Otherwise it diffs the index (--cached).

---

License

MIT © Aditya Goyal