failed to build gem native extension. xcode

gistfile1.txt

Fetching nio4r 2.5.4
Installing nio4r 2.5.4 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/gems/2.7.0/gems/nio4r-2.5.4/ext/nio4r
/Users/ianvaughan/.asdf/installs/ruby/2.7.1/bin/ruby -I /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0 -r ./siteconf20200918-40223-cj3k1e.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.

Provided configuration options:
        --with-opt-dir
        --without-opt-dir
        --with-opt-include
        --without-opt-include=${opt-dir}/include
        --with-opt-lib
        --without-opt-lib=${opt-dir}/lib
        --with-make-prog
        --without-make-prog
        --srcdir=.
        --curdir
        --ruby=/Users/ianvaughan/.asdf/installs/ruby/2.7.1/bin/$(RUBY_BASE_NAME)
        --with-unistd-dir
        --without-unistd-dir
        --with-unistd-include
        --without-unistd-include=${unistd-dir}/include
        --with-unistd-lib
        --without-unistd-lib=${unistd-dir}/lib
/Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:471:in `try_do': The compiler failed to generate an executable file. (RuntimeError)
You have to install development tools first.
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:613:in `try_cpp'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:1124:in `block in have_header'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:971:in `block in checking_for'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:361:in `block (2 levels) in postpone'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:331:in `open'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:361:in `block in postpone'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:331:in `open'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:357:in `postpone'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:970:in `checking_for'
        from /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/2.7.0/mkmf.rb:1123:in `have_header'
        from extconf.rb:15:in `<main>'

To see why this extension failed to compile, please check the mkmf.log which can be found here:

  /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/gems/2.7.0/extensions/x86_64-darwin-19/2.7.0/nio4r-2.5.4/mkmf.log

extconf failed, exit code 1

Gem files will remain installed in /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/gems/2.7.0/gems/nio4r-2.5.4 for inspection.
Results logged to /Users/ianvaughan/.asdf/installs/ruby/2.7.1/lib/ruby/gems/2.7.0/extensions/x86_64-darwin-19/2.7.0/nio4r-2.5.4/gem_make.out

An error occurred while installing nio4r (2.5.4), and Bundler cannot continue.
Make sure that `gem install nio4r -v '2.5.4' --source 'https://rubygems.org/'` succeeds before bundling.

In Gemfile:
  active_storage_validations was resolved to 0.9.0, which depends on
    rails was resolved to 6.0.3.1, which depends on
      actioncable was resolved to 6.0.3.1, which depends on
        nio4r


> sudo xcodebuild -license
Password:
xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance


> xcode-select --print-path

/Library/Developer/CommandLineTools


> which gcc
/usr/bin/gcc
> which gcc-4.2
gcc-4.2 not found

> sudo llvm-gcc
Password:
xcrun: error: active developer path ("/Library/Developer/CommandLineTools") does not exist
Use `sudo xcode-select --switch path/to/Xcode.app` to specify the Xcode that you wish to use for command line developer tools, or use `xcode-select --install` to install the standalone command line developer tools.
See `man xcode-select` for more details.

> gcc
xcrun: error: active developer path ("/Library/Developer/CommandLineTools") does not exist
Use `sudo xcode-select --switch path/to/Xcode.app` to specify the Xcode that you wish to use for command line developer tools, or use `xcode-select --install` to install the standalone command line developer tools.
See `man xcode-select` for more details.

> sudo xcode-select --switch path/to/Xcode.app
xcode-select: error: invalid developer directory 'path/to/Xcode.app'



https://github.com/CocoaPods/CocoaPods/issues/1727



https://developer.apple.com/download/more/?name=for%20Xcode%20-





https://github.com/CocoaPods/CocoaPods/issues/1727