MPF_NOZERO
nmax_read, nmax_read, MPF_NOZERO | MPF_CALLBACK,
nmax_write, nmax_write, MPF_NOZERO | MPF_CALLBACK,
if ((mpipe->mpflags & (MPF_CACHEDATA|MPF_NOZERO)) == 0)
if ((mpflags & MPF_NOZERO) == 0)