[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

• gem install: Failed to build gem native extension (can't find header files) - the instructions here seem to be specific to the gem being installed.

• How to install json gem - Failed to build gem native extension This seems to be slightly different error.

Any hints? Thanks!

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

with

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.

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

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

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

sudo apt-get --reinstall install ruby

try it for ubuntu 16.04

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++

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.

Xcode -> Preferences -> Locations

change Command Line Tools to Xcode 11.2.1

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:

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include

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

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.

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

/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0

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

/Library/Developer/CommandLineTools/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Ruby.framework/Versions/2.3/usr/include/ruby-2.3.0

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 you use SUSE

sudo yast2 -i 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.

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

You may need to install gcc after install ruby-devel

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

dnf install ruby-devel

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...