FD_RENUMBER

;;; Atomically replace a file descriptor by renumbering another file descriptor.
  ;;
  ;;; Due to the strong focus on thread safety, this environment does not provide
  ;;; a mechanism to duplicate or renumber a file descriptor to an arbitrary
  ;;; number, like `dup2()`. This would be prone to race conditions, as an actual
  ;;; file descriptor with the same number could be allocated by a different
  ;;; thread at the same time.
  ;;
  ;;; This function provides a way to atomically renumber file descriptors, which
  ;;; would disappear if `dup2()` were to be removed entirely.
  (@interface func (export "fd_renumber")
    (param $fd $fd)
    ;;; The file descriptor to overwrite.
    (param $to $fd)
    (result $error (expected (error $errno)))
  )