“防火墙”是在互联网与内部网之间建立一个安全网关，以防止非法用户的侵入，它通常由屏蔽器和代理服务器组成，其中，屏蔽路由器是一个(47)，但是它却难以防止黑客的攻击，其主要原因在于：(48)。

(84)

(85)

(86)

(83)

The　UNIX　operating　system　is　made　up　of　three　parts;　the　kernel,　the　shell　and　the　programs.　The　kernel　of　UNIX　is　the(71)of　the　operating　system:　it　allocates　time　and　memory　to　programs　and　handles　the　filestore　and　communications　in　response　to(72).　　　The　shell　acts　as　an(73)between　the　user　and　the　kernel.　When　a　user　logs　in,　the(74)program　checks　the　username　and　password,　and　then　starts　another　program　called　the　shell.　The　shell　is　a　command　line(75)(CLI).　It　interprets　the　commands　the　user　types　in　and　arranges　for　them　to　be　carded　out.　The　commands　are　themselves　programs:　when　they　terminate,　the　shell　gives　the　user　another　prompt(%　on　our　systems).

(79)

(80)

(81)

(78)

As　mentioned　above,　C　imposes　relatively　few　built　-　in　ways　of　doing　things　on　the　program-　mer.　Some　common　tasks,　such　as　manipulating(66),(67),　and　doing(68)(I/O),　are　performed　by　calling　on　library　functions.　Other　tasks　which　you　might　want　to　do,　such　as　creating　or　listing　directories,　or　interacting　with　a　mouse,　or　displaying　windows　or　other(69)elements,　or　doing　color　graphics,　are　not　defined　by　the　C　language　at　all.　You　can　do　these　things　from　a　C　program,　of　course,　but　you　will　be　calling　on　services　which　are　peculiar　to　your　programming　environment(70),　processor,　and　operating　system)and　which　are　not　defined　by　the　C　standard.　Since　this　course　is　about　portable　C　programming,　it　will　also　be　steering　clear　of　facilities　not　provided　in　all　C　environments.