r, w, err := pipe2(&pp, flags)
	if err == nil {
		p[0], p[1] = r, w
	}
	return err
}

//sys	extpread(fd int, p []byte, flags int, offset int64) (n int, err error)

func pread(fd int, p []byte, offset int64) (n int, err error) {
	return extpread(fd, p, 0, offset)
}

//sys	extpwrite(fd int, p []byte, flags int, offset int64) (n int, err error)

	for {
		changed := false
		for _, id := range exit.contents() {
			b := idToBlock[id]
		NextPred:
			for _, pe := range b.Preds {
				p := pe.b
				if exit.contains(p.ID) {
					continue
				}
				for _, s := range p.Succs {
					if !exit.contains(s.b.ID) {
						continue NextPred
					}
				}
				// All Succs are in exit; add p.
				exit.add(p.ID)
				changed = true
			}
		}