class INotify::Watcher

def initialize(notifier, path, *flags, &callback)
  @notifier = notifier
  @callback = callback || proc {}
  @path = path
  @flags = flags.freeze
  @id = Native.inotify_add_watch(@notifier.fd, path,
    Native::Flags.to_mask(flags))
  unless @id < 0
    @notifier.watchers[@id] = self
    return
  end
  raise SystemCallError.new(
    "Failed to watch #{path.inspect}" +
    case FFI.errno
    when Errno::EACCES::Errno; ": read access to the given file is not permitted."
    when Errno::EBADF::Errno; ": the given file descriptor is not valid."
    when Errno::EFAULT::Errno; ": path points outside of the process's accessible address space."
    when Errno::EINVAL::Errno; ": the given event mask contains no legal events; or fd is not an inotify file descriptor."
    when Errno::ENOMEM::Errno; ": insufficient kernel memory was available."
    when Errno::ENOSPC::Errno; ": The user limit on the total number of inotify watches was reached or the kernel failed to allocate a needed resource."
    else; ""
    end,
    FFI.errno)
end