[json] Error while installing json gem 'mkmf.rb can't find header files for ruby'

For context, it on a remote server which has a firewall. I'm setting up my environment through a proxy. I have ruby 1.8.7. When I try to gem install..

sudo gem install --http-proxy <host address>:<port> json

I get the following error:

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

/usr/bin/ruby extconf.rb
mkmf.rb can't find header files for ruby at /usr/lib/ruby/ruby.h

Gem files will remain installed in /usr/lib64/ruby/gems/1.8/gems/json-1.8.1 for inspection.
Results logged to /usr/lib64/ruby/gems/1.8/gems/json-1.8.1/ext/json/ext/generator/gem_make.out

Since I was unsure what the problem is, I googled and found these

Any hints? Thanks!

This question is related to json ruby rubygems

The answer is

Modern era update, as stated by mimoralea:

In case that you are using ruby 2.0 or 2.2 (thanks @patrick-davey).

sudo apt-get install ruby2.0-dev
sudo apt-get install ruby2.2-dev
sudo apt-get install ruby2.3-dev

or, generic way:

sudo apt-get install ruby-dev


sudo apt-get install ruby`ruby -e 'puts RUBY_VERSION[/\d+\.\d+/]'`-dev

The first link you’ve posted is exactly your case: there is no ruby development environment installed. Development env is needed to compile ruby extensions, which are mostly written in C. Proxy has nothing to do with the problem: everything is downloaded fine, just compilation fails.

I would suggest you to install ruby-dev (ruby-devel for rpm-based distros) package onto you target machine.

gcc package might be needed as well.


$ sudo apt-get install ruby-dev

Or, for Redhat distro:

$ sudo yum install ruby-devel

Or, for [open]SuSE:

$ sudo zypper install ruby-devel

For Xcode 11 on macOS 10.14, this can happen even after installing Xcode and installing command-line tools and accepting the license with

sudo xcode-select --install
sudo xcodebuild -license accept

The issue is that Xcode 11 ships the macOS 10.15 SDK which includes headers for ruby2.6, but not for macOS 10.14's ruby2.3. You can verify that this is your problem by running

ruby -rrbconfig -e 'puts RbConfig::CONFIG["rubyhdrdir"]'

which on macOS 10.14 with Xcode 11 prints the non-existent path


However, Xcode 11 installs a macOS 10.14 SDK within /Library/Developer/CommandLineTools/SDKs/MacOS10.14.sdk. It isn't necessary to pollute the system directories by installing the old header files as suggested in other answers. Instead, by selecting that SDK, the appropriate ruby2.3 headers will be found:

sudo xcode-select --switch /Library/Developer/CommandLineTools
ruby -rrbconfig -e 'puts RbConfig::CONFIG["rubyhdrdir"]'

This should now correctly print


Likewise, gem install should work while that SDK is selected.

To switch back to the current Xcode SDK, use

sudo xcode-select --switch /Applications/Xcode.app

In case that you are using ruby 2.0 or 2.2 (thanks @patrick-davey) or 2.3 (thanks @juanitofatas).

sudo apt-get install ruby-dev
sudo apt-get install ruby2.0-dev
sudo apt-get install ruby2.2-dev
sudo apt-get install ruby2.3-dev

And you get the pattern here...

I also encountered this problem because I install Ruby on Ubuntu via brightbox, and I thought ruby-dev is the trunk of ruby. So I did not install. Install ruby2.3-dev fixes it:

sudo apt-get install ruby2.3-dev

For those who are getting this on Mac OS X you may need to run the following command to install the XCode command-line tools, even if you already have XCode installed:

sudo xcode-select --install

Also you must agree the terms and conditions of XCode by running the following command:

sudo xcodebuild -license

Most voted solution didn't work on my machine (linux mint 18.04). After a careful look, i found that g++ was missing. Solved with

sudo apt-get install g++

I had a similar problem using cygwin to run the following command:

$ gem install rerun

I solved it by installing the following cygwin packages:

  • ruby-devel
  • libffi-devel
  • gcc-core
  • gcc-g++
  • make
  • automake1.15

in case you use SUSE

sudo yast2 -i ruby-devel

Xcode 11 / macOS Catalina

On Xcode 11 / macOS Catalina, the header files are no longer in the old location and the old /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg file is no longer available.

Instead, the headers are now installed to the /usr/include directory of the current SDK path:


Most of this directory can be found by using the output of xcrun --show-sdk-path. And if you add this path to the CPATH environment variable, then build scripts (including those called via bundle) will generally be able to find it.

I resolved this by setting my CPATH in my .zshrc file:

export CPATH="$(xcrun --show-sdk-path)/usr/include"

After opening a new shell (or running source .zshrc), I no longer receive the error message mkmf.rb can't find header files for ruby at /usr/lib/ruby/ruby.h and the rubygems install properly.

Note on Building to Non-macOS Platforms

If you are building to non-macOS platforms, such as iOS/tvOS/watchOS, this change will attempt to include the macOS SDK in those platforms, causing build errors. To resolve, either don't set CPATH environment variable on login, or temporarily set it to blank when running xcodebuild like so:

CPATH="" xcodebuild --some-args

In Fedora 21 and up, you simply open a terminal and install the Ruby Development files as root.

dnf install ruby-devel

On Mac 10.14, the header files don't seem to be installed in the correct place. Rather than changing paths like the other fixes, I was able to just run this:

open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

Follow the instructions and it resolved this problem for me.

You may need to install gcc after install ruby-devel

Xcode -> Preferences -> Locations

change Command Line Tools to Xcode 11.2.1

BEFORE you follow the tip from Joki's answer (below) and IF :

  • you have MacOS 10.14.6

  • at /Library/Developer/CommandLineTools/SDKs/ you have folders MacOSX.sdk(symbolic), MacOSX10.14.sdk, MacOSX10.15.sdk

  • Move MacOSX10.15.sdk to anywhere (admin privileges needs)

  • Delete symbolic link (admin privileges needs)

  • At /Library/Developer/CommandLineTools/SDKs/ create another symbolic link now to MacOSX10.14.sdk folder using (admin privileges needs)

    sudo ln -s /Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk MacOSX.sdk

  • Now you can follow Joki's answer

WARNING! If you move MacOSX10.15.sdk folder to /Library/Developer/CommandLineTools/SDKs/ again, the command

ruby -rrbconfig -e 'puts RbConfig::CONFIG["rubyhdrdir"]'

will show MacOSX10.15.sdk folder like default again, nowadays I dunno how to fix it! My suggestion, compress the folder and put the original folder until fix will be available.

I faced a similar issue on Xcode 12 with macOS 10.15 and cocoapods. Just make sure that the xcode-select command points to the SDK you want to build against. It should build without issues afterwards.

You need to install the entire ruby and not just the minimum package. The correct command to use is:

sudo apt install ruby-full

The following command will also not install a complete ruby:

sudo apt-get install ruby2.3-dev

For Ubuntu 18, after checking log file mentioned while install

Results logged to /var/canvas/vendor/bundle/ruby/2.5.0/extensions/x86_64-linux/2.5.0/nio4r-2.5.2/gem_make.out


less /var/canvas/vendor/bundle/ruby/2.5.0/extensions/x86_64-linux/2.5.0/nio4r-2.5.2/gem_make.out

I noticed that make is not found. So installed make by

sudo apt-get install make

everything worked.

sudo apt-get --reinstall install ruby

try it for ubuntu 16.04

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