clang-format-inc 1.0.0

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.

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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.

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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.

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Requirements

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

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Usage as a pre-commit hook

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

repos:
  - repo: https://github.com/goyaladitya05/clang-format-inc
    rev: v1.0.0
    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.

Diff mode (show what would change)

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

Like --check, but also prints a unified diff of every change that would be applied. Useful for PR comment bots or debugging. --check and --diff are mutually exclusive.

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 }}

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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
  -p NUM                 Strip NUM leading path components from diff filenames (default: 1)
  --include REGEX        Only process files whose path matches this regex
  --exclude REGEX        Skip files whose path matches this regex
  --workers N            Number of parallel clang-format processes (default: 1)

Modes (mutually exclusive):
  --check                Don't modify files; exit non-zero if any file would be reformatted
  --diff                 Don't modify files; print a unified diff of what would change

Notes

Untracked files (not yet added to git) have no history to diff against, so they are formatted in their entirety rather than incrementally. Stage the file with git add first for incremental behaviour.

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Install from PyPI

pip install clang-format-inc

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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).

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License

MIT © Aditya Goyal