在TCP/IP参考模型中，(8)的主要作用是在互联网络的源主机与目的主机对等实体之间建立用于会话的端对端连接。

(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.