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加时赛啊# Fashion - 美丽时尚

v*F2010-07-11 07:07

1 楼

1) On a traditonal UNIX system, how many times data is copied when system
read()s a file from disk and send()s it across the network?
2）Consider a singly linked list of the form
F -> A -> B -> C -> D ->E ::= | |
::= |
::= |
::= a|b|c|...|y|z
::= 0|1|2|...|9
Which of the following lexical entities can be derived from ? (Zero or more
answers may be correct.)
1. word
2. words
3. c22
4. 42
5. None of the above.
请解析一下你的答案。谢谢大家

p*e2010-07-11 07:07

2 楼

比赛结束了，这里是不是吵得更厉害了？？

r*e2010-07-11 07:07

3 楼

1）个人理解，Unix的memory space分为user space和kernel space
用户程序调用read()这个system call的时候，产生一个软中断
kernel的中断服务程序从设备读入数据到kernel space
然后调用 copy_to_user() 把数据copy到user space
同理，send()的本质和write()一样，先从user space复制到kernel space
然后再发送到network device
所以read()+send()至少要copy 4次
http://www.ibm.com/developerworks/linux/library/l-kernel-memory
http://www.makelinux.net/ldd3/chp-3-sect-7

more

【在 v******F 的大作中提到】

: 1) On a traditonal UNIX system, how many times data is copied when system
: read()s a file from disk and send()s it across the network?
: 2）Consider a singly linked list of the form
: F -> A -> B -> C -> D ->E ::= | |
: ::= |
: ::= |
: ::= a|b|c|...|y|z
: ::= 0|1|2|...|9
: Which of the following lexical entities can be derived from ? (Zero or more
: answers may be correct.)

v*F2010-07-11 07:07

4 楼

光顾看答案，忘了说谢谢
有人可以帮帮忙第二题吗？