I've been wanting to use clang-format since a while already. It turns out that installation instructions are kind outdated. I had to read various commits in the clang repository to realize clang-format command it was moved from clang to llvm repository.

2015/01/18 Update: Use brew!

Now clang-format is on brew!. Just do:
brew install clang-format

2014/08/06 Update: Use prebuilds!

Now is even simpler, just download the prebuilds for your system, decompress and use them!
# Download
$ curl -O http://llvm.org/releases/3.4.2/clang+llvm-3.4.2-x86_64-apple-darwin10.9.xz
# Decompress
$ tar xvfJ clang+llvm-3.4.2-x86_64-apple-darwin10.9.xz 
# Locate clang-format
$ ./clang+llvm-3.4.2-x86_64-apple-darwin10.9/bin/clang-format --help
Just one thing. I couldn't find the git hook python script available. Still, it is in the source and it does not need to be compiled, just copy th
# Download only (not the entire llvm project) the repository that contains the hook script
$ svn co http://llvm.org/svn/llvm-project/clang-tools-extra/trunk extra
# Voila!
$ find . name git-clang-format
# Now just copy it somewhere into your $PATH

Old approach: Build llvm

We need to compile LLVM! I wish brew provides something like --with-tools or something like --with-clang-format. For now we have to do the entire thing by ourselves. Some instructions were taken from Clang's getting started page:

Getting LLVM + Clang

$ svn co http://llvm.org/svn/llvm-project/llvm/trunk llvm
$ cd llvm/tools
$ svn co http://llvm.org/svn/llvm-project/cfe/trunk clang
$ cd ../..
$ cd llvm/tools/clang/tools
$ svn co http://llvm.org/svn/llvm-project/clang-tools-extra/trunk extra
$ cd ../../../..
$ cd llvm/projects
$ svn co http://llvm.org/svn/llvm-project/compiler-rt/trunk compiler-rt
$ cd ../..

Building LLVM + Clang

$ ./llvm/configure
$ make
In my case, make didn't succeed, it takes forever and it fails somewhere after building clang-format but that is ok. clang-format was built and I wasn't planning to install llvm/clang from source either, I am fine with Xcode/brew stuff :p

Installing clang-format

If it is not clear where makeput all compiled stuff we can always use find:
$ find . -name "clang-format"
We need to copy clang-format command from above location to somewhere in our PATH or make an alias. Also we want to install git-clang-format and maybe other scripts.
# copy clang-format command to your $PATH
$ cp ./Release+Asserts/bin/clang-format /somewhere/in/your/PATH

# install (copy) git-clang-format subcommand to your $PATH
$ cd ./tools/clang/tools/clang-format
$ cp git-clang-format ~/.bin

Using clang-format command

Now, lets say we are inside a directory which contains c/cpp/objc sources which we want to reformat. We simply create a .clang-format file which should be of the form:
Key: Value
Key: Value
Key: Value
Key: Value
And then we would run clang-format file.c Some commands to know more about the command and keys available:
# dump all configuration keys
clang-format -dump-config 
clang-format -dump-config  style=webkit

# help
clang-format --help
clang-format --help-list-hidden

# format a file (it will read .clang-format file if available)
clang-format filename.c

# format a file with a explicit style
clang-format -style=google filename.c
Check clang's official style documentation page for more info.

Using it with git

Since we installed git-clang-format file to our PATH, if we are in a git repository, we can do:
git clang-format
And it will reformat all staged code, pretty awesome!

Are you wondering where did clang-format subcommand come from?! Answer: custom git subcommands are simply executables in the PATH and its name should have the "git-" prefix and should not have extension.

Hope this info is helpful to somebody :)