gem not working in Cygwin

Question

On a freshly installed Windows 7 Professional 64 Bit I installed Cygwin (64) and some of its packages, including Ruby. I have also installed Ruby using the Ruby installer, because I'll likely need it for both the default Windows shells and Cygwin.

Now when I try to execute a gem command like gem list or gem install foo, I get a weird error I failed to solve within the last few hours of searching the internet.

$ which ruby
/usr/bin/ruby

$ which gem
/usr/bin/gem

$ ruby -v
ruby 2.2.4p230 (2015-12-16 revision 53155) [x86_64-cygwin]

$ gem -v
2.4.8

$ gem list
ERROR:  Loading command: list (Fiddle::DLError)
        can't load kernel32
ERROR:  While executing gem ... (NoMethodError)
    undefined method `invoke_with_build_args' for nil:NilClass

$ gem install sass
ERROR:  Loading command: install (Fiddle::DLError)
        can't load kernel32
ERROR:  While executing gem ... (NoMethodError)
    undefined method `invoke_with_build_args' for nil:NilClass

However, with the native Windows version, from the Windows CMD, it works without any problems. I cannot however use native Windows Ruby stuff from Cygwin, because that gives me errors, but that's not the question here anyway.

With Process Monitor I figured out that Ruby tries to open C:\cygwin64\bin\kernel32.dll and fails, because that file is not there. I tried copying the kernel32.dll from C:\Windows\System32 and the one from C:\Windows\SysWOW64 into that Cygwin bin folder and still got the same error (apart from that it then said can't load kernel32.dll), although the Process Monitor didn't show a NAME NOT FOUND error anymore.

What magic is happening here? I'd really like to understand what's wrong here. I appreciate any help.

Answer 1

It seems a unexpected result of cygwin 2.5.2 release

https://www.cygwin.com/ml/cygwin/2016-06/msg00378.html

As workaround, downgrade cygwin package to 2.5.1

Answer 2

A way to fix this without changing the rvm build process is:

ln -s /cygdrive/c/Windows/System32/kernel32.dll /usr/lib/kernel32

This happens because ruby is looking for a shared library named simply kernel32. Cygwin 2.5.1 and earlier automatically added the ".dll" extension to shared library loads. But cygwin 2.5.2 introduced a patch requiring full shared library filenames. Adding a symbolic link within the library search path (/usr/lib) allows the library to be found even when loaded with the old-style name.

Answer 3

I installed ruby using rvm so the updated Cygwin ruby binaries weren't much help and I didn't really want to downgrade my Cygwin installation - how would I know when it was safe to upgrade again?

Following the information in Michael D's answer, the problem appears to be in the resolv.rb file located in ~\.rvm\rubies\ruby-<version>\lib\ruby\<version>\win32 (in my case ~\.rvm\rubies\ruby-2.1.7\lib\ruby\2.1.0\win32).

Somewhere near the top of this file, there is the code

module Kernel32
  extend Importer
  dlload "kernel32"
end

Simply changing the dlload "kernel32" line to dlload "kernel32.dll" seemed to fix it for me. Alternatively using the full path

dlload "c:/Windows/System32/kernel32.dll"

also worked, but it seems to be the extension that is the crucial bit (the full path without the extension also doesn't work).

This may have been fixed in a more recent version of rvm, but I didn't want to go through the hassle of updating and reinstalling, so this works for me. Or course this would likely need to be changed for all installed rubies.

Answer 4

The Ruby package has problem loading native libraries (at least kernel32.dll). The problem comes from a call to dns.getresource("_rubygems._tcp.#{host}", Resolv::DNS::Resource::IN::SRV) which probably make a native call to kernel32.dll hence loading the kernel32.dll library.

If you specify the full path to the library, it will work correctly.

kernel = Fiddle::Handle.new("c:/Windows/System32/kernel32.dll")

To solve the issue, try the following:

1. Require devkit when running extconf.rb as follows: ruby -rdevkit extconf.rb , or just adding require "devkit" to extconf.rb then running the script normally.
2. Run the devkitvars.bat script from the devkit to configure the PATH with the toolchain before compiling.