```ruby
module Vagrant
  module Util
    # This module provies a `safe_exec` method which is a drop-in
    # replacement for `Kernel.exec` which addresses a specific issue
    # which manifests on OS X 10.5 and perhaps other operating systems.
    # This issue causes `exec` to fail if there is more than one system
    # thread. In that case, `safe_exec` automatically falls back to
    # forking.
    module SafeExec
      def safe_exec(command)
        fork_instead = false
        begin
          pid = nil
          pid = fork if fork_instead
          Kernel.exec(command) if pid.nil?
          Process.wait(pid) if pid
        rescue Errno::E045
          # We retried already, raise the issue and be done
          raise if fork_instead

          # The error manifested itself, retry with a fork.
          fork_instead = true
          retry
        end
      end
    end
  end
end
```

Fix errno not defined [closes GH-465]
user
```ruby
module Vagrant
  module Util
    # This module provies a `safe_exec` method which is a drop-in
    # replacement for `Kernel.exec` which addresses a specific issue
    # which manifests on OS X 10.5 and perhaps other operating systems.
    # This issue causes `exec` to fail if there is more than one system
    # thread. In that case, `safe_exec` automatically falls back to
    # forking.
    module SafeExec
      def safe_exec(command)
        # Create a list of things to rescue from. Since this is OS
        # specific, we need to do some defined? checks here to make
        # sure they exist.
        rescue_from = []
        rescue_from << Errno::EOPNOTSUPP if defined?(Errno::EOPNOTSUPP)
        rescue_from << Errno::E045 if defined?(Errno::E045)

        fork_instead = false
        begin
          pid = nil
          pid = fork if fork_instead
          Kernel.exec(command) if pid.nil?
          Process.wait(pid) if pid
        rescue *rescue_from
          # We retried already, raise the issue and be done
          raise if fork_instead

          # The error manifested itself, retry with a fork.
          fork_instead = true
          retry
        end
      end
    end
  end
end
```
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