When Emacs runs lots of subprocesses (LSP servers, file watchers, TRAMP, Magit, terminals), the default macOS descriptor limits (1024) can surface as “Too many open files.” A proven mitigation is to compile Emacs with larger descriptor set size and a Darwin-specific define: -DFD_SETSIZE=10000 -DDARWIN_UNLIMITED_SELECT [1].

This post shows how to make that build reproducible with a small, local Homebrew tap that carries a lightly patched emacs formula.

What you need to do:

• Create a local tap to host your customized emacs formula.
• Patch the formula to append CFLAGS="-DFD_SETSIZE=10000 -DDARWIN_UNLIMITED_SELECT" during build [1].
• Install from your tap now, and upgrade later with the same commands.

Prerequisites:

• Homebrew set up on macOS.
• Command Line Tools for Xcode: xcode-select --install (if you haven’t already).

Create a local tap

Initialize an empty tap repository you control:

brew tap-new $(id -u)/local

Seed your formula

brew extract emacs $(id -u)/local

This creates $(brew --repository)/Library/Taps/$(id -u)/homebrew-local/Formula/emacs.rb for you to modify.

Patch the formula to raise FD limits

Open your tap’s emacs.rb (you’ll need existing emacs installed for this to work, otherwise just find the formula it in the path above and edit it):

brew edit $(id -u)/local/emacs

Inside def install, append the CFLAGS before ./configure step (this is how it looks like as of emacs@30.2):

def install
  # ...
  ENV.append "CFLAGS", "-DFD_SETSIZE=1048576 -DDARWIN_UNLIMITED_SELECT"
  system "./configure", *args
  system "make"
  system "make", "install"
  # ...
end

Install from your tap

brew install $(id -u)/local/emacs

Verify the build behaves as expected

Launch Emacs and check the max open files:

M-! ulimit -n RET

References

[1] Fix annoying max open files for Emacs