SW_SNDZERO
if ((write_size == 0) && !(stp->sd_wput_opt & SW_SNDZERO))
stp->sd_wput_opt &= ~(SW_SIGPIPE|SW_SNDZERO);
stp->sd_wput_opt |= SW_SNDZERO;
if (stp->sd_wput_opt & SW_SNDZERO)