Computing is rich enough and deep enough as a profession that you can spent literally over a decade at something and still learn new things. And, as frustrating as this is, I find that one of the best parts. Personally I’ve been using Ruby now since 2006 and one of the things that is absolutely bedeviling is when a gem won’t install.

Upfront Disclaimer: This problem was one of my own making but it proved to be an interesting exercise as I waited for someone to arrive for a meeting. Far better than simply surfing the web or playing on my phone.

I was trying to get the FeedBin aggregator to install so I did the usual open source Rails dance:

  • fork it
  • clone it
  • bundle install

And, WHAM, this gave me the wonderfulness below:

Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/sjohnson/.rvm/gems/ruby-2.3.1@feedbin/gems/nio4r-2.1.0/ext/nio4r
/Users/sjohnson/.rvm/rubies/ruby-2.3.1/bin/ruby -r ./siteconf20170828-91881-jc70x3.rb extconf.rb
checking for unistd.h... *** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Nothing like an abstractly referenced log file WITHOUT A FULL PATH to be completely unhelpful. I’ve been seeing references to mkmf.log about as long as Gems have been in common usage and I’ve never understood exactly where that persnickety file was located. Normally a quick Google or Stack Overflow gives me the answer but this time those just weren’t helpful. What I did discover was that the mkmf.log file is bundled with the gem where it is built. Given that I use RVM, the example was to look in a path like this:


I started with this find command:

find ~/.rvm -name mkmf.log

and that produced a huge number of results, 995 to be specific (a decade or more of hacking Ruby stuff will do that).

This took me into adding a grep to the end:

find ~/.rvm -name mkmf.log grep nio4r

which still gave me a full page of output so I added feedbin as a secondary grep:

find ~/.rvm -name mkmf.log grep nio4r grep feedbin

which finally produced:


Digging into that file revealed this error:

xcrun: error: active developer path ("/Applications/") does not exist, use `xcode-select --switch path/to/` to specify the Xcode that you wish to use for command line developer tools (or see `man xcode-select`)

So somehow XCode is missing and I’m certain that’s my fault (this isn’t my primary dev box so perhaps it got deleted at some point to save disc space).