Issue #1277 has been updated by Dave B. Charlie, I was searching earlier for some explanation and came up with a post from July 2006. Small world, the author is you :) http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/8267 On Windows, errno is implemented by macro (in errno.h) to a function: #define errno (*_errno()) Different runtimes might have different opinions on where the real errno resides. Searching the Ruby source tree finds quite a few ref's to errno ... /\s+errno\W/ ---------------------------------------- http://redmine.ruby-lang.org/issues/show/1277 ---------------------------------------- http://redmine.ruby-lang.org