Note: this workaround works, but you can’t just copy/paste paths blindly. Instead, look at the error installation gave you. It will say something like
To see why this extension failed to compile, please check the mkmf.log which can be found here:
/Library/Ruby/Gems/2.6.0/extensions/universal-darwin-21/2.6.0/ffi-1.15.5/mkmf.log
So open the log it mentions, and see what it can’t find. It will tell you the correct versions of MacOSX__._.sdk and universal-darwin-__ you need to link
Catalina, Big Sur, etc. (As of 2020, 2021, and 2022)
UPDATE: This workaround still works as of 2022. For the commands below, please change version string (e.g., 11.1, ruby 2.6, universal-darwin20, etc.) accordingly to your macOS version environment.
Recent versions of macOS and Xcode (e.g. Catalina 10.15 and Xcode 12.2) have this broken. Reinstalling Xcode and xcode-select --install did not help for me at all (the solutions already posted seem to be outdated). I did not want to use rvm because it may cause some annoying problems.
I did a workaround by manually making a symbolic link:
cd /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0/ruby
ln -sf ../../../../Headers/ruby/config.h
where ruby/config.h can be found at:
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/Headers/ruby/config.h.
You should change SDK versions (e.g. 11.1 in the above example) accordingly to your current xcode installation.
It is also very likely that one runs into another error where .../universal-darwin19/ruby/config.h cannot be found (Commonmarker gem cannot be installed (needed for jekyll) macos). It can be quick-fixed as follows:
cd /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.1.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/include/ruby-2.6.0
ln -sf universal-darwin20 universal-darwin19
If after using this workaround, you get a crash on pod install like:
/Library/Ruby/Gems/2.6.0/gems/ffi-1.15.5/lib/ffi/library.rb:275: [BUG] Bus Error at 0x0000000100f38000
follow this solution