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Jack Nagel edited this page Jul 11, 2013 · 23 revisions

Formula Cookbook

Making a formula is easy. Just brew create URL and then brew install FOO (perhaps with --debug --verbose). Basically, a formula is a Ruby file. You can place it anywhere you want (local or remote) and install it by pointing to the file or URL.

We want your formula to be awesome, and the cookbook will tell you how.

Terminology - Homebrew speak

Formula The package description /usr/local/Library/Formula/foo.rb
Keg The installation prefix of a Formula /usr/local/Cellar/foo/0.1
Cellar All kegs are installed in here /usr/local/Cellar
Tap An optional repository (git) of Formulae /usr/local/Library/Taps

More general: brew --prefix and brew --repository instead of /usr/local but let's KISS.

An Introduction

Did you see /usr/local/.git? Homebrew is built on Git (as long as you did brew update at least once). This means you can just do your work in /usr/local and merge in upstream updates as you go.

Homebrew installs to the Cellar, it then symlinks some of the installation into /usr/local so that other programs can see what's going on. We suggest you brew ls a few of the kegs in your Cellar to see how it is all arranged.

Packages are installed according to their formulae, which live in $(brew --repository)/Library/Formula. Check some out. You can view any formula at anytime; e.g. brew edit wget.

Basic Instructions

Ensure you’ve run brew update at least once even before you start. This turns your Homebrew prefix into a Git repository. (Homebrew is built on top of Git. You did know that, right?)

Before contributing, make sure your package:

  • isn't already waiting to be merged (check the issue tracker)
  • isn't already in Homebrew (check brew search foo)
  • isn't in another Homebrew tap.

Make sure you search thoroughly (all aliases!) We don’t want you to waste your time.

Will we merge your formula?

Probably. But we have rules to keep the quality and goals of Homebrew intact: Please read Acceptable Formulae.

Some Quick Examples Before You Get Started

Formulae aren’t that complicated. etl and browser are as simple as it gets.

And then Git and flac show more advanced functionality.

Grab the URL

All you need to make a formula is a URL to the tarball.

brew create http://example.com/foo-0.1.tar.gz

This creates:

/usr/local/Library/Formula/foo.rb

And opens it in your $EDITOR. It'll look like:

require 'formula'

class Foo < Formula
  url 'http://example.com/foo-0.1.tar.gz'
  homepage ''
  sha1 '1234567890ABCDEF1234567890ABCDEF'

  # depends_on 'cmake' => :build

  def install
    system "./configure", "--prefix=#{prefix}", "--disable-debug", "--disable-dependency-tracking"
#   system "cmake", ".", *std_cmake_args
    system "make install"
  end
end

Note: If brew said Warning: Version cannot be determined from URL when doing the create step, you’ll need to explicitly add the correct version to the formula with version 'foo' and then save the formula. brew install should then proceed without any trouble.

Note: If brew said No formula found for "php54-timezonedb". Searching open pull requests... and you are writing a Tap, you should run brew tap --repair.

Fill in the Homepage

We don’t accept formulae without homepages!

Homebrew doesn’t have a description field because the homepage is always up to date, and Homebrew is not. That’s way less maintenance for us. Try brew home qt.

Check the build system

brew install -i foo

You’re now at new prompt with the tarball extracted to a temporary sandbox.

Check the package’s README. Does the package install with autotools, cmake, or something else? Delete the commented out cmake lines if the package uses autotools (i.e. if it has a configure script).

Check for dependencies

The README probably tells you about dependencies. Homebrew or OS X probably already has them. You can check for Homebrew deps with brew search. These are the common deps that OS X comes with:

  • Freetype
  • libexpat
  • libGL
  • libiconv
  • libpcap
  • libpng
  • libxml2
  • Python
  • Ruby

There are plenty of others. Check /usr/lib to see.

We try to not duplicate libraries and complicated tools in core Homebrew. We dupe some common tools though. But generally, we avoid dupes because it’s one of Homebrew’s foundations. (And it causes build and usage problems!)

However, we maintain a special tap that provides dupes.

Important: Since the introduction of superenv, brew --prefix/bin is NOT on the $PATH during formula installation. If you have dependencies at build time, you must specify them and brew will add them to the $PATH. You can test around this with --env=std.

Specifying other formulae as dependencies

class Foo < Formula
  depends_on 'jpeg'
  depends_on 'gtk+' => :optional
  depends_on 'readline' => :recommended
  depends_on 'boost' => 'with-icu'
  depends_on :x11
end

A String specifies a formula dependency.

A Symbol specifies a special conditional dependency, such as X11.

A Hash specifies a formula dependency with some additional information. Given a single string key, the value can take several forms:

  • a Symbol (currently one of :build, :optional, :recommended).

    • :build tags that dependency as a build-time only dependency, meaning it can be safely ignored when installing from a bottle and when listing missing dependencies using brew missing.
    • :optional generates an implicit with-foo option for the formula. This means that, given depends_on 'foo' => :optional, the user must pass --with-foo in order to enable the dependency.
    • :recommended generates an implicit without-foo option, meaning that the dependency is enabled by default and the user must pass --without-foo to disable this dependency. The default description can be overridden using the normal option syntax:
    depends_on 'foo' => :optional # Generated description is "Build with foo support"
option 'with-foo', 'Compile with foo bindings' # This overrides the generated description if you want to

  • a String or an Array String values are interpreted as options to be passed to the dependency. You can also pass an array of strings, or an array of symbols and strings, in which case the symbols are interpreted as described above, and the strings are passed to the dependency as options.

    depends_on 'foo' => 'with-bar'
depends_on 'foo' => %w{with-bar with-baz}
depends_on 'foo' => [:optional, 'with-bar']

Double-check for dependencies

When you already have a lot of brews installed, its easy to miss a common dependency like glib or gettext.

You can double-check which libraries a binary links to with the otool command (perhaps you need to use xcrun otool):

$ otool -L /usr/local/bin/ldapvi
/usr/local/bin/ldapvi:
  /usr/lib/libssl.0.9.8.dylib (compatibility version 0.9.8, current version 0.9.8)
  /usr/lib/libcrypto.0.9.8.dylib (compatibility version 0.9.8, current version 0.9.8)
  /usr/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.3)
  /usr/local/Cellar/glib/2.22.4/lib/libglib-2.0.0.dylib (compatibility version 2201.0.0, current version 2201.4.0)
  /usr/local/Cellar/gettext/0.17/lib/libintl.8.dylib (compatibility version 9.0.0, current version 9.2.0)
  /usr/local/Cellar/readline/6.0/lib/libreadline.6.0.dylib (compatibility version 6.0.0, current version 6.0.0)
  /usr/local/Cellar/popt/1.15/lib/libpopt.0.dylib (compatibility version 1.0.0, current version 1.0.0)
  /usr/lib/libncurses.5.4.dylib (compatibility version 5.4.0, current version 5.4.0)
  /System/Library/Frameworks/LDAP.framework/Versions/A/LDAP (compatibility version 1.0.0, current version 2.2.0)
  /usr/lib/libresolv.9.dylib (compatibility version 1.0.0, current version 38.0.0)
  /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 125.0.0)

Specifying gems, Python modules etc. as dependencies

Homebrew doesn’t package the already-packaged. If your formula needs a gem or python module, you’ll need to check for these external dependencies:

class Foo < Formula
  depends_on 'mg' => :ruby
  depends_on 'json' => :python
  # New style (but only implemented for Python):
  depends_on :python => 'json'
  depends_on :python3 => 'json'
end

Sometimes Python or Ruby package names don't match the way that the dependency should be checked. For instance, Python's PyYAML installs the package, but its existence is checked with python -c "import yaml":

class Foo < Formula
  depends_on :python => ['yaml' => 'PyYAML']
  # for other languanges we need to use the old style like so:
  depends_on LanguageModuleDependency.new :python, 'PyYAML', 'yaml'
end

Test the formula

Exit out of the interactive shell.

brew install --verbose --debug foo

Debug will ask you to open an interactive shell when the build fails so you can try to figure out what went wrong.

Check the top of the ./configure output (if applicable)! Some configure scripts do not recognize --disable-debug. If you see a warning about it, remove the option from the formula.

Manuals

Homebrew expects to find man pages in [prefix]/share/man/..., and not in [prefix]/man/....

Some software installs to man instead of share/man, so check the output and add a "--mandir=#{man}" to the ./configure line if needed.

A Quick Word on Naming

THE NAME IS VERY IMPORTANT!

Name the formula like the project markets the product. So it’s pkg-config, not pkgconfig; sdl_mixer, not sdl-mixer or sdlmixer.

The only exception is stuff like “Apache Ant”. Apache sticks “Apache” in front of everything, but we use the formula name ant. We only include the prefix in cases like GNUplot (because it’s part of the name) and GNU Go (because everyone calls it “GNU go”—nobody just calls it “Go”). The word “Go” is too common and there are too many implementations of it.

If you’re not sure about the name check the homepage, and check the Wikipedia page.

ALSO CHECK WHAT DEBIAN CALLS IT!

If you’re still not sure, just commit. I’ll apply some arbitrary rule and make a decision ;)

When importing classes, Homebrew will require the formula and then create an instance of the class. It does this by assuming the formula name can be directly converted to the class name using a regexp. The rules are simple:

  • foo-bar.rb => FooBar
  • foobar.rb => Foobar

Thus, if you change the name of the class, you must also rename the file. Filenames should be all lowercase.

Add aliases by creating symlinks in Library/Aliases.

Audit the formula

You can run brew audit to test formulae for adherence to Homebrew house style. This includes warnings for trailing whitespace, preferred URLs for certain source hosts, and a lot of other style issues. Fixing these warnings before committing will make the process a lot smoother for us.

Commit

Everything is built on Git, so contribution is easy:

brew install git # if you already have git installed, skip this command
brew update # required in more ways than you think (initializes the brew git repository if you don't already have it)
cd `brew --repository`
# Create a new git branch for your formula so your pull request is easy to
# modify if any changes come up during review.
git checkout -b <some-descriptive-name>
git add Library/Formula/foo.rb
git commit

The established standard for Git commit messages is:

  • the first line is a commit summary of 50 characters or less, then
  • two (2) newlines, then
  • explain the commit throughly

At Homebrew, we like to put the name of the formula upfront like so "foobar: 7.3 - tool to do foo". This may seem crazy short, but you’ll find that forcing yourself to summarise the commit encourages you to be atomic and concise. If you can’t summarise it in 50-80 characters, you’re probably trying to commit two commits as one. For a more thorough explanation, please read Tim Pope’s excellent blog post, A Note About Git Commit Messages.

Ensure you reference any relevant GitHub issue #12345 in the commit message. Homebrew’s history is the first thing future contributors will look to when trying to understand the current state of formulae they’re interested in.

Push

Now you just need to push back to GitHub.

If you haven’t forked Homebrew yet, go to the repo and hit the fork button.

If you have already forked Homebrew on Github, then you can manually push (just make sure you have been pulling from the mxcl/homebrew master):

git push [email protected]:myname/homebrew.git <what-you-called-your-branch>

Now, please open a Pull Request (on your github repo page) for new and updated brews.

  • One formula per commit; one commit per formula
  • Keep merge commits out of the request
  • If you have any merge or mixup commits, please squash them.

If a commit touches multiple files, or isn’t one logical bug fix, or a file is touched in multiple commits, we’ll probably ask you to rebase and squash your commits. For this reason, you should avoid pushing to your master branch. Note, after rebase and/or squash, you'll need to push with --force to your remote.

Overview of the Formula Install Process

  • The result of Formula.download_strategy is instantiated.
  • DownloadStrategy.fetch is called (downloads tarball, checks out git repository, etc.)
  • A temporary sandbox is created in /tmp/homebrew
  • DownloadStrategy.stage is called (extracts tarball to above sandbox, exports git repository to sandbox, etc.)
  • Patches are applied
  • Current directory is changed to the stage root (so when you system make, it works)
  • Formula.install is called
  • Anything installed to the keg is cleaned (see later)
  • The keg is symlinked into Homebrew’s prefix
  • Caveats are displayed

Convenience Tools

Messaging

Three commands are provided for displaying informational messages to the user:

  • ohai for general info
  • opoo for warning messages
  • onoe for error messages

In particular, when a test needs to be performed before installation use onoe to bail out gracefully. For example:

if some_test?
  system "make install"
else
  onoe 'Error! Something is wrong.'
end

bin.install('foo')

You’ll see stuff like that in other formulae. This installs the file foo into the Formula’s bin directory (/usr/local/Cellar/pkg/0.1/bin) and makes it executable (chmod +x foo).

inreplace

A convenience function that can edit files in-place. For example:

inreplace 'path', before, after

before and after can be strings or regexps. You can also use the block form:

inreplace 'path' do |s|
  s.gsub! /foo/, 'bar'
end

Make sure you modify s! This block ignores the returned value.

There are some special functions you can use inside the block that makes it easier to modify Makefiles:

inreplace 'Makefile' do |s|
  s.remove_make_var! %w[CFLAGS LDFLAGS CC LD]
  s.change_make_var! "CC", ENV.cc
end

NOTE: We really prefer that you use patches. Patches fail to apply if the code changes too much, and this is good! in replace could theoretically continue to apply long after it has subtly broken the code.

Patches

While patches should generally be avoided, sometimes they are necessary.

Patches to make the software’s build system respect Homebrew’s installation hierarchy are usually fine and don’t necessitate an upstream bug report, but if the patch is more complex (i.e. fixing header file inclusion, fixing compiler warnings, etc.), the first thing to do is check whether or not the upstream project is aware of the issue. If not, file a bug report and/or submit your patch for inclusion. We will normally still accept your patches, but by getting the ball rolling on fixing the upstream issue, you reduce the length of time we have to carry the patch around.

Always, always, always justify a patch with a comment! Otherwise, nobody will know when it is safe to remove the patch, or safe to leave it in when updating the formula. The comment should include a link to the relevant upstream issue(s).

Return a string or array from your re-implemented patches function:

def patches
  # fixes doc install on OS X 10.6
  "http://gist.github.com/623/my.patch/raw/"
end

# or ...

def patches
  # fixes multiple problems
  [
    "http://gist.github.com/623/my-1.patch/raw/",
    "http://gist.github.com/623/my-2.patch/raw/",
    "http://gist.github.com/623/my-3.patch/raw/"
  ]
end

Patches can be gzipped...or not. If the patches are not p1, return a Hash:

def patches
  # back ported fix for memory leak in drawing functions
  { :p0 => "http://gist.github.com/623/my.patch/raw/" }
end

Small patches should be put in the formula:

class Foo < Formula
  def patches
    # fixes something small
    DATA
  end
end

__END__
diff --git a/foo/showfigfonts b/foo/showfigfonts
index 643c60b..543379c 100644
--- a/foo/showfigfonts
+++ b/foo/showfigfonts
@@ -14,6 +14,7 @@
…

The magic variable HOMEBREW_PREFIX is expanded to whatever the Homebrew prefix is before the patch is applied.

Creating the diff

We use git for everything...Didn’t you know?

brew install --interactive --git foo
…
(make some edits)
…
git diff | pbcopy
brew edit foo

Now just paste into the formula after __END__. Instead of git diff | pbcopy, for some editors git diff >> path/to/your/formula/foo.rb might help you that the diff is not touched (eg. white space removal, indentation, etc.)

Advanced Formula Tricks

If anything isn’t clear, you can usually figure it out with some grep and the Library/Formula directory. Please amend this wiki page if you think it will help!

Unstable versions (HEAD, devel)

Formulae can specify alternate downloads for the upstream project’s devel release (unstable but not trunk) or HEAD (master/trunk).

HEAD

HEAD URLs (activated by passing --HEAD) build the development cutting edge. Specifying it is easy:

class Foo < Formula
  head 'https://github.com/mxcl/lastfm-cocoa.git'
end

Homebrew understands git, svn, and hg URLs, and has a way to specify cvs repositories as a URL as well. You can test whether the HEAD is being built with build.head?.

To use a specific commit, tag, or branch from a repository, specify head with the :revision, :tag, or :branch option, like so:

class Foo < Formula
  head 'https://github.com/some/package.git', :revision => '090930930295adslfknsdfsdaffnasd13'
                                         # or :branch => 'develop'
                                         # or :tag => '1_0_release'
end

devel

The 'devel' spec (activated by passing --devel) is used for a project’s unstable releases. It is specified in a block:

devel do
  url 'https://foo.com/foo-0.1.tar.gz'
  sha1 'deadbeefdeadbeefdeadbeafdeadbeefdeadbeef'
end

You can test if the 'devel' spec is in use with build.devel?.

Build toolchain selection

With a simple if/else statement, you can use a preferred toolchain if available, and if not, fall back to a secondary toolchain. For example, the following install stanza will check whether Xcode is installed and use xcodebuild if it is, falling back to autoconf if necessary.

def install
  if MacOS::Xcode.installed?
    system "xcodebuild install DSTROOT=#{prefix}"
  else
    system "autoconf"
    system "./configure", "--prefix=#{prefix}"
    system "make install"
  end
end

Compiler selection

Sometimes a package fails to build when using a certain compiler. Since recent XCode no longer includes a vanilla GCC compiler, we cannot simply force the use of GCC. Instead, the correct way to declare this is the fails_with DSL method. A properly constructed fails_with block documents the latest compiler build version known to cause compilation to fail, and the cause of the failure. For example:

fails_with :llvm do
  build 2335
  cause <<-EOS.undent
    The 'cause' field should include a short summary of the error. Include
    the URLs of any relevant information, such as upstream bug reports. Wrap
    the text at a sensible boundary (~72-80 characters), but do not break
    URLs over multiple lines.
    EOS
end

build takes a Fixnum (you can find this number in your brew --config output). cause takes a string, and the use of heredocs is encouraged to improve readability and allow for more comprehensive documentation.

fails_with declarations can be used with any of :gcc, :llvm, and :clang. Homebrew will use this information to select a working compiler (if one is available).

Specifying the Download Strategy explicitly

To use one of Homebrew’s built-in download strategies, specify the :using => flag on a url or head. For example:

class Sip < Formula
  url 'http://www.riverbankcomputing.co.uk/hg/sip/archive/4.11'
  md5 'dbafd7101a4e7caee6f529912a1356e5'
  head 'http://www.riverbankcomputing.co.uk/hg/sip', :using => :hg
  homepage 'http://www.riverbankcomputing.co.uk/software/sip'

The downloaders offered by Homebrew are:

Value of :using Corresponds To
:bzr BazaarDownloadStrategy
:curl CurlDownloadStrategy
:cvs CVSDownloadStrategy
:git GitDownloadStrategy
:hg MercurialDownloadStrategy
:nounzip NoUnzipCurlDownloadStrategy
:post CurlPostDownloadStrategy
:svn SubversionDownloadStrategy

If you need more control over the way files are downloaded and staged, you can create a custom download strategy and specify it using the Formula#download_strategy function. Return the class type. For example:

class MyDownloadStrategy < SomeHomebrewDownloadStrategy
  # Does something cool
end

class Foo < Formula
  url "something", :using => MyDownloadStrategy
end

Specifying download strategies can be useful when used with a local repo, where a plain URL would not let you specify how to access it. For example:

class Bar < Formula
  head '/users/abc/src/git.git', :using => :git
end

Just copying some files

When your code in the install function is run, the current working directory is set to the extracted tarball.

So it is easy to just copy some files:

prefix.install 'file1', 'file2'

Or everything:

prefix.install Dir['*']

Variables for directory locations

Again, in your mind, replace /usr/local by brew --prefix:

Name Default E.g.
name [formula name] foo
HOMEBREW_PREFIX /usr/local
prefix #{HOMEBREW_PREFIX}/Cellar/#{name}/#{version} /usr/local/Cellar/foo/0.1
bin #{prefix}/bin /usr/local/Cellar/foo/0.1/bin
doc #{prefix}/share/doc/foo /usr/local/Cellar/foo/0.1/share/doc/foo
include #{prefix}/include /usr/local/Cellar/foo/0.1/include
info #{prefix}/share/info /usr/local/Cellar/foo/0.1/share/info
lib #{prefix}/lib /usr/local/Cellar/foo/0.1/lib
libexec #{prefix}/libexec /usr/local/Cellar/foo/0.1/libexec
man #{prefix}/share/man /usr/local/Cellar/foo/0.1/share/man
man[1-8] #{prefix}/share/man/man[1-8] /usr/local/Cellar/foo/0.1/share/man/man[1-8]
sbin #{prefix}/sbin /usr/local/Cellar/foo/0.1/sbin
share #{prefix}/share /usr/local/Cellar/foo/0.1/share
etc #{HOMEBREW_PREFIX}/etc /usr/local/etc
var #{HOMEBREW_PREFIX}/var /usr/local/var

These can be used, for instance, in code such as

bin.install Dir['*']

to install binaries into their correct location into the cellar, and

man.mkpath

to create the directory structure to the man location.

To install man pages into specific locations, use man1.install ['foo.1'], man2.install ['foo.2'], etc.

Note that in the context of Homebrew, libexec is reserved for private use by the formula and therefore is not symlinked into HOMEBREW_PREFIX.

Installation without linking into /usr/local (keg-only)

If you only need a program for a dependency and it does not need to be linked for public use in /usr/local, specify

keg_only "This is my rationale."

in the Formula class.

Adding optional steps

If you want to add an option:

class Yourformula < Formula
  ...
  option 'with-ham', "Description of the option"
  option 'without-spam', "Another description"
  depends_on 'foo' => :optional  # will automatically add a with-foo option
  ...

And then to define the effects the options have:

if build.with? 'ham'
  # note, no 'with' in the option name (it is added by the build.with? method)
end

if build.without? 'ham'
  # works as you'd expect. True if `--without-ham` was given.
end

if build.include? 'with-ham'
  # the old style, mostly useful for options other than `with`/`without` style
end

Option names should usually be prefixed with one of the words with, without, no, or a verb in the imperative tense describing the action to be taken. For example, an option to run a test suite should be named --with-test or --with-check rather than --test, and an option to enable a shared library should be named --enable-shared rather than --shared.

See the graphviz formula for an example.

File level operations

You can use the file utilities provided by Ruby (FileUtils). These are included in the Formula class, so you do not need the FileUtils. prefix to use them.

They are documented here

Handling files that should persist over formula upgrades

For example, Ruby 1.9’s gems should be installed to var/lib/ruby/ so that gems don’t need to be reinstalled when upgrading Ruby. You can usually do this with symlink trickery, or better a configure option.

launchd plist files

Homebrew provides two Formula methods for launchd plist files. plist_name will return homebrew.mxcl.<formula>, and plist_path will return, for example, /usr/local/Cellar/foo/0.1/homebrew.mxcl.foo.plist.

Style guide

Homebrew tries to maintain a consistent ruby style across all formulae. In case of doubt, please consult existing formulae and do the same.

  • The order of methods in a formula should be consistent with other formulae (eg.: def patches goes before def install)
  • An empty line is required before the __END__ line

Troubleshooting for people writing new formulae

Version detection fails

Homebrew tries to automatically determine the version from the URL in order to save on duplication. If the tarball has a funny name though, you may have to assign the version number:

class Foobar
  version "0.7"
end

Bad Makefiles

Not all projects have makefiles that will run in parallel so try to deparallelize:

brew edit foo

Add all this to the formula (so there will already be a class line, don’t add another or change that, and there’s already an install function, don't add another one, add the lines in the install function below to the top of the problem formula’s install function).

class Foo < Formula
  skip_clean :all
  def install
    ENV.deparallelize
    ENV.no_optimization
    system "make"  # separate make and make install steps
    system "make install"
  end
end

If that fixes it, please open an issue so that we can fix it for everyone.

Still won’t work?

Check out what macports and fink do:

brew -S --macports foo

brew -S --fink foo

Superenv Notes

superenv is a "super" environment that tries to improve reliability for the general case. But it does make making formula harder.

To not use superenv, install with --env=std.

Superenv isolates builds by removing /usr/local/bin and all user-PATHs that are not determined to be essential to the build. It does this because other PATHs are full of stuff that breaks builds. (We have 15,000 tickets as testament!)

superenv tries to remove bad-flags from the commands passed to clang/gcc and injects others (for example all keg_only dependencies are added to the -I and -L flags. If superenv troubles you, try to brew install --env=std and report to us if that fixes it.

Fortran

Some software requires a Fortran compiler. This can be declared by adding depends_on :fortran to a formula. :fortran is a special dependency that does several things.

First, it looks to see if you have set the FC environment variable. If it is set, Homebrew will use this value during compilation. If it is not set, it will check to see if gfortran is found in PATH. If it is, Homebrew will use its location as the value of FC. Otherwise, the gfortran formula will be treated as a dependency and installed prior to compilation.

If you have set FC to a custom Fortran compiler, you may additionally set FCFLAGS and FFLAGS. Alternatively, you can pass --default-fortran-flags to brew install to use Homebrew's standard CFLAGS.

When using Homebrew's own gfortran compiler, the standard CFLAGS are used and user-supplied values of FCFLAGS and FFLAGS are ignored for consistency and reproducibility reasons.

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