gem eventmachine fatal error: 'openssl/ssl.h' file not found

brew link --force openssl

Then:

gem install eventmachine

It also fixes:

• gem install taks
• bundle install
• rake and rails tasks

P.S. Probably you'll need to remove and run brew install openssl again

The issue happends because Apple had removed openssl from OSX (in El Captain build)

gem pristine eventmachine -- --with-cppflags=-I/usr/local/opt/openssl/include

pristine removes the old gem and recompiles it.

The cpp flags option allows the compiler to find the openssl headers.

If you prefer to abstract away the cpp flags argument, you can do the following:

PKG_CONFIG_PATH="$(brew --prefix openssl)/lib/pkgconfig"

You can also add the version if you like:

gem pristine eventmachine -v '1.0.4' -- --with-cppflags=-I/usr/local/opt/openssl/include

$ gem install eventmachine -- --with-cppflags=-I/usr/local/opt/openssl/include                                                                                                             
Building native extensions with: '--with-cppflags=-I/usr/local/opt/openssl/include'
This could take a while...
Successfully installed eventmachine-1.0.8
1 gem installed

You can also set up bundler like this but I think that is superficial

bundle config build.eventmachine --with-cppflags=-I/usr/local/opt/openssl/include

When using bundler and homebrew:

$ bundle config build.eventmachine --with-cppflags=-I$(brew --prefix openssl)/include
$ bundle install