clang-format CLI Usage

clang-format CLI Usage

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Command-line usage, tools, and integrations for clang-format.

Documentation Version: Updated for Clang v22.0.0

Command-Line Options

Basic Usage

# Format from stdin, output to stdout
cat file.cpp | clang-format

# Format file(s) and output to stdout
clang-format file.cpp

# Format file(s) in-place
clang-format -i file.cpp file.h

# Format multiple files
clang-format -i src/*.cpp include/*.h

Common Flags

-i - In-Place Editing

Modify files directly instead of outputting to stdout:

clang-format -i file.cpp

--style=<style> - Set Coding Style

Specify the formatting style:

# Use a predefined style
clang-format --style=google file.cpp

# Use .clang-format file (default)
clang-format --style=file file.cpp

# Specify explicit config file path
clang-format --style=file:/path/to/.clang-format file.cpp

# Inline style configuration
clang-format --style="{BasedOnStyle: llvm, IndentWidth: 8}" file.cpp

Available Predefined Styles:

• LLVM, GNU, Google, Chromium, Microsoft, Mozilla, WebKit, InheritParentConfig

Note: InheritParentConfig is not a real style, but allows using the .clang-format file from the parent directory. If no parent file is found, it falls back to the fallback style.

--dry-run / -n - Check Without Modifying

Check what would change without making modifications:

clang-format --dry-run file.cpp
clang-format -n file.cpp

--fallback-style=<style> - Fallback Style

Style to use if .clang-format file cannot be found:

clang-format --style=file --fallback-style=google file.cpp

# Skip formatting if no config found
clang-format --style=file --fallback-style=none file.cpp

--dump-config - Show Effective Configuration

Display the configuration that will be used:

# Dump LLVM style
clang-format --style=llvm --dump-config

# Dump effective config for a file
clang-format --dump-config file.cpp

# Create .clang-format from a style
clang-format --style=google --dump-config > .clang-format

Advanced Options

--lines=<start>:<end> - Format Specific Lines

Format only specified line ranges:

# Format lines 10-20
clang-format --lines=10:20 file.cpp

# Format multiple ranges
clang-format --lines=10:20 --lines=50:60 file.cpp

--offset=<bytes> and --length=<bytes> - Format by Byte Range

Format specific byte ranges:

clang-format --offset=100 --length=500 file.cpp

# Format from offset to end of file
clang-format --offset=100 file.cpp

--assume-filename=<name> - Set Language for stdin

Specify filename for language detection when reading from stdin:

cat source.txt | clang-format --assume-filename=file.cpp

Supported Languages (as of Clang v22):

• C
• C++ (Cpp)
• C# (CSharp): .cs
• Java: .java
• JavaScript: .js, .mjs, .cjs, .ts
• JSON (Json): .json, .ipynb
• Objective-C (ObjC): .m, .mm
• Protocol Buffers (Proto): .proto, .protodevel
• TableGen: .td
• Text Protocol Buffers (TextProto): .txtpb, .textpb, .pb.txt, .textproto, .asciipb
• Verilog: .sv, .svh, .v, .vh

For .h files, you can specify the language by adding // clang-format Language: Cpp (or C, ObjC) before the first non-comment line.

--files=<filename> - Process File List

Read list of files to process from a file:

# Create file list
find src -name '*.cpp' > files.txt

# Format all files in list
clang-format -i --files=files.txt

--verbose - Show Processed Files

Display the list of files being processed:

clang-format -i --verbose src/*.cpp

Error Handling

--Werror - Treat Warnings as Errors

Convert formatting warnings to errors:

clang-format --Werror --dry-run file.cpp

--Wno-error=<type> - Ignore Specific Warnings

# Allow unknown format options
clang-format --Wno-error=unknown -i file.cpp

--ferror-limit=<n> - Limit Error Count

Set maximum number of errors before stopping:

clang-format --dry-run --ferror-limit=10 file.cpp

--fail-on-incomplete-format - Fail on Incomplete Formatting

Exit with code 1 if formatting is incomplete:

clang-format --fail-on-incomplete-format file.cpp

.clang-format-ignore File

Create a .clang-format-ignore file to exclude files from formatting.

Format

# Comments start with #
# Blank lines are ignored

# Ignore third-party code
third_party/**
external/**

# Ignore generated files
*.pb.cc
*.pb.h
*_generated.cpp

# Ignore specific directories
build/**
node_modules/**

# Negate patterns with !
# Format everything except test data
tests/**
!tests/data/**

# Ignore specific files
legacy/old_code.cpp
vendor/library.h

Pattern Rules

The following rules use characters that are also special in Markdown (#, **, !, /), so they are listed in a code block to avoid ambiguity:

Blank lines are skipped
Leading/trailing spaces are trimmed
# prefix indicates a comment
/ separator for directories
Patterns are relative to the .clang-format-ignore file location
Absolute patterns start with /
Bash globstar ** is supported
! prefix negates the pattern

Multiple .clang-format-ignore Files

Similar to .clang-format files, you can have multiple .clang-format-ignore files at different directory levels. Lower-level files override higher-level ones.

Git Integration

git clang-format Command

Format only the lines that have changed in git commits:

# Format staged changes
git clang-format

# Format everything since HEAD~1
git clang-format HEAD~1

# Format everything since main branch
git clang-format main

# Show diff instead of applying
git clang-format --diff

# Show diffstat
git clang-format --diffstat

# Format specific files only
git clang-format main -- src/*.cpp

# Interactive hunk selection
git clang-format -p

Options

git clang-format [OPTIONS] [<commit>] [<commit>|--staged] [--] [<file>...]

Common Options:

• --binary <path> - Path to clang-format binary
• --style <style> - Formatting style to use
• --diff - Print diff instead of applying
• --diffstat - Print diffstat instead of applying
• -f, --force - Allow changes to unstaged files
• -p, --patch - Select hunks interactively
• --staged, --cached - Format lines in stage instead of working dir
• -q, --quiet - Print less information
• -v, --verbose - Print extra information

Git Config

Configure default options in git config:

git config clangFormat.binary /usr/local/bin/clang-format
git config clangFormat.style file
git config clangFormat.extensions "h,cpp,cc,c"

Pre-Commit Hook Example

Create .git/hooks/pre-commit:

#!/bin/bash
# Format staged changes before commit
git clang-format --staged --quiet

Script Tools

clang-format-diff.py

Format only changed lines from a unified diff:

# Format git diff
git diff -U0 --no-color --relative HEAD^ | clang-format-diff.py -p1 -i

# Format svn diff
svn diff --diff-cmd=diff -x-U0 | clang-format-diff.py -i

# Format mercurial diff
hg diff -U0 --color=never | clang-format-diff.py -i -p1

Options:

• -i - Apply edits to files instead of displaying diff
• -p <num> - Strip N leading directories from paths
• -regex <pattern> - Custom pattern for file paths (case sensitive)
• -iregex <pattern> - Custom pattern for file paths (case insensitive)
• -style <style> - Formatting style
• -fallback-style <style> - Fallback style
• -binary <path> - Path to clang-format binary
• -v, --verbose - Be more verbose

Example with filters:

# Only format .cpp files
git diff -U0 | clang-format-diff.py -i -regex '.*\.cpp'

# Format C++ and header files
git diff -U0 | clang-format-diff.py -i -iregex '.*\.(cpp|h)'

Editor Integration

Visual Studio Code

Install the "Clang-Format" extension from the marketplace.

Configuration (settings.json):

{
  "editor.defaultFormatter": "xaver.clang-format",
  "editor.formatOnSave": true,
  "[cpp]": {
    "editor.defaultFormatter": "xaver.clang-format"
  },
  "clang-format.executable": "/usr/bin/clang-format",
  "clang-format.style": "file"
}

Default Keybinding: Alt+Shift+F

CLion

CLion has built-in clang-format support.

Enable:

1. Settings → Editor → Code Style
2. Enable "Enable ClangFormat support"
3. Place .clang-format in project root

Features:

• Automatic formatting on type
• Respects .clang-format file
• Can generate .clang-format from IDE settings

Vim

Add to .vimrc:

" Python 3 support
if has('python3')
  map <C-K> :py3f /path/to/clang-format.py<cr>
  imap <C-K> <c-o>:py3f /path/to/clang-format.py<cr>
endif

" Python 2 support
if has('python')
  map <C-K> :pyf /path/to/clang-format.py<cr>
  imap <C-K> <c-o>:pyf /path/to/clang-format.py<cr>
endif

Format on save:

function! Formatonsave()
  let l:formatdiff = 1
  py3f /path/to/clang-format.py
endfunction
autocmd BufWritePre *.h,*.cc,*.cpp call Formatonsave()

Emacs

Add to .emacs:

(load "/path/to/clang-format.el")
(global-set-key [C-M-tab] 'clang-format-region)

BBEdit

1. Copy clang-format-bbedit.applescript to ~/Library/Application Support/BBEdit/Scripts/
2. Edit the script to point to your clang-format binary
3. Access from Script menu or assign keyboard shortcut

Visual Studio

Download the extension from the LLVM builds site.

Default Keybinding: Ctrl+R, Ctrl+F

Common Workflows

Format Entire Project

# Find and format all C++ files
find . -name '*.cpp' -o -name '*.h' | xargs clang-format -i

# Using git to find tracked files
git ls-files '*.cpp' '*.h' | xargs clang-format -i

Check Formatting in CI

# Check if any files need formatting (exit non-zero if changes needed)
clang-format --dry-run --Werror src/**/*.{cpp,h}

# Generate diff for review
clang-format --dry-run src/**/*.{cpp,h} > format-diff.txt

Format Only Modified Files

# Format files modified in last commit
git diff --name-only HEAD~1 | grep -E '\.(cpp|h)$' | xargs clang-format -i

# Format uncommitted changes
git diff --name-only | grep -E '\.(cpp|h)$' | xargs clang-format -i

Parallel Formatting

# Format files in parallel (requires GNU parallel)
find src -name '*.cpp' | parallel clang-format -i {}

Troubleshooting

Configuration File Not Found

clang-format searches for .clang-format or _clang-format starting from the source file's directory up to the filesystem root.

Solutions:

# Specify config file explicitly
clang-format -style=file:/path/to/.clang-format file.cpp

# Check which config file will be used
clang-format -dump-config file.cpp

Unknown Format Options

Occurs when your config file has options not supported by your clang-format version.

Solutions:

# Warn instead of error
clang-format --Wno-error=unknown -i file.cpp

# Check your clang-format version
clang-format --version

# Regenerate config with your version
clang-format -style=llvm -dump-config > .clang-format

Performance Issues

For large files or projects:

# Format specific line ranges only
clang-format --lines=100:200 large_file.cpp

# Use parallel processing
find src -name '*.cpp' | parallel -j8 clang-format -i {}

# Format only changed lines
git clang-format

What's New in Clang v22

Clang-format v22 introduces several new style configuration options:

• AllowBreakBeforeQtProperty: Control line breaks before Qt Q_Property keywords
• NumericLiteralCase: Configure capitalization style for numeric literals
• SpaceInEmptyBraces: More granular control over spacing in empty braces

For a complete list of v22 style options, refer to the ClangFormatStyleOptions_v22.md documentation.

Reference

For complete command-line documentation, see Full CLI Reference.

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