我的系统是mac，然后今天在安装eventmachine的时候发现的报了错误，安装命令为：

 sudo gem install eventmachine -v '1.2.7'

错误信息为：

Building native extensions.  This could take a while...
ERROR:  Error installing eventmachine:
	ERROR: Failed to build gem native extension.

    current directory: /Library/Ruby/Gems/2.3.0/gems/eventmachine-1.2.7/ext
/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/bin/ruby -r ./siteconf20190123-26894-gknmbc.rb extconf.rb
mkmf.rb can't find header files for ruby at /System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/lib/ruby/include/ruby.h

extconf failed, exit code 1

Gem files will remain installed in /Library/Ruby/Gems/2.3.0/gems/eventmachine-1.2.7 for inspection.
Results logged to /Library/Ruby/Gems/2.3.0/extensions/universal-darwin-18/2.3.0/eventmachine-1.2.7/gem_make.out

解决方式：

xcode-select --install

然后成功安装：

EtCampusDoc git:(master) sudo gem install eventmachine -v '1.2.7'
Building native extensions.  This could take a while...
Successfully installed eventmachine-1.2.7
Parsing documentation for eventmachine-1.2.7
Installing ri documentation for eventmachine-1.2.7
Done installing documentation for eventmachine after 6 seconds
1 gem installed

参考文献

[1]. Fail to Building native extensions. on Mac OSX 10.13.6. https://github.com/castwide/vscode-solargraph/issues/78