同步和互斥的概念
2 同步概念
内核中的同步和进程之间的同步要相似之处，要介绍它首先需要了解竞争条件与临界区的概念（race condition & critical area）当两个以上的进程要读写同一个共享的数据，而且最后的执行结果依赖于进程执行的顺序的时候，我们说存在一种竞争条件。而进程中访问共享数据的那一部分代码就被称为临界区。我们可以通过保证两个进程不在同一时刻处于临界区内，从而抑制竞争条件的发生，实现这一目的的技术就是同步技术。在前面叙述竞争条件的时候，所举的例子都是进程，而更通用的说法应该是代码路径，每一个代码路径都在CPU 上独立运行，它们之间不存在明确的调用关系。在内核中，这样的代码路径称为内核控制路径。
同步
所谓同步，就是在发出一个功能调用时，在没有得到结果之前，该调用就不返回。也就是必须一件一件事做,等前一件做完了才能做下一件事.就像早上起床后,先洗涮,然后才能吃饭,不能在洗涮没有完成时,就开始吃饭.按照这个定义，其实绝大多数函数都是同步调用（例如sin,isdigit等）。但是一般而言，我们在说同步、异步的时候，特指那些需要其他部件协作或者需要一定时间完成的任务。最常见的例子就是SendMessage。该函数发送一个消息给某个窗口，在对方处理完消息之前，这个函数不返回。当对方处理完毕以后，该函数才把消息处理函数所返回的LRESULT值返回给调用者。
异步
异步的概念和同步相对。当一个异步过程调用发出后，调用者不能立刻得到结果。实际处理这个调用的部件在完成后，通过状态、通知和回调来通知调用者。以CAsycSocket类为例（注意，CSocket从CAsyncSocket派生，但是起功能已经由异步转化为同步），当一个客户端通过调用Connect函数发出一个连接请求后，调用者线程立刻可以朝下运行。当连接真正建立起来以后，socket底层会发送一个消息通知该对象。这里提到执行部件和调用者通过三种途径返回结果：状态、通知和回调。可以使用哪一种依赖于执行部件的实现，除非执行部件提供多种选择，否则不受调用者控制。如果执行部件用状态来通知，那么调用者就需要每隔一定时间检查一次，效率就很低（有些初学多线程编程的人，总喜欢用一个循环去检查某个变量的值，这其实是一种很严重的错误）。如果是使用通知的方式，效率则很高，因为执行部件几乎不需要做额外的操作。至于回调函数，其实和通知没太多区别。
新概念第一册同步测试(59—60)
姓名______________ 分数_____________ 一选择填空 1 The lady _____ some envelops. A want B wants C wanting D wanted 2 _____you want the large size or the small size? A Are B Do C Does D Be 3 The children aren’t ______ their homework at the moment. A do B does C doing D be doing 4 Where do Jim and Tom ______from? A come B is C are D go 5________come from England . A Englishmen B Frenchmen C Dutch D American 6 There are________ on the table. A some milk B some bottle of milk C some milks D some bottles of milk 7 I can’t ______my pen . A to find B finds C finding D find 8 I only have small boxes. Do you want ______? A it B this C that D one 9 Can you _____ the pen to Sandy ? A put B take C giving D have 10 He usually shaves at 7:00 o’clo ck every day, but today , he _____at 8:00 o’clock. A shaves B shave C shaving D is shaving. 11 ______are they going to school? A What B Which C How D Where 12 Can you show the picture _______ Sue. A with B to C of D from 13 Can your birds fly? Yes , ______can. A their B them C they D theirs 14 Your trousers are on the bed. His _______behind the door. A is B am C are D has. 15 Look! Jim _______ his clothes now . A puts in B is putting on C in D wearing 二语法训练：用所给词的适当形式填空。 Please look at the picture . There is a small house in it . In front of the house , there are some people . Look! The old man under the big tree is Uncle Wang . He is working . He __1__ a clock. It’s Mrs. Green’s clock . It doesn’t work .Mrs. Green ___2___ a book .The tall man is Mr Green. He __3__on his coat. There are two children behind Uncle Wang. They are Jim and Kate. They
第三章进程管理课后习题答案
进程管理习题 1现代操作系统中为什么要引入“进程”概念? 它与程序有什么区别? 答：之所以要引入进程的概念，是因为在一些可以并发的程序段之间，存在着某种相互制约的关系，每个程序段的执行不仅要受到其它程序执行的制约，而且还要动态地依赖系统资源的分配情况，因此每个可以并发执行的程序段就会因外界条件的限制而不能运行，被迫处于阻塞状态。仅用程序的概念无法表示程序的走走停停以及程序运行过程中对资源的竞争现象，因此需要采用一种动态的概念描述并发程序这种走走停停的现象，这就产生了进程的概念。 进程和程序的区别： （1）进程是程序的执行过程，是动态的过程，属于一种动态概念。程序是一组有序静态指令和数据的集合，用来指示处理机的操作，是一种静态概念。 （2）从结构上看，每个进程实体是由程序段和相应的数据段两部分构成，并且进程结构中还要包含PCB,即进程控制块。 （3）一个进程可以涉及到一个或几个程序的执行；反之，同一程序可以对应多个进程，即同一个程序段可以在不同数据集合上运行，可以构成不同的进 程。 （4）进程能真实地描述并发执行的过程，而程序仅仅是静态指令堆积的序列。 （5）进程有可创建其他进程的功能，而一般的程序不具有创建其它程序的功能。 （6）每一个程序都是在一个进程现场中运行的。 2叙述进程的并发性和制约性。 答：并发性是进程的重要特征。即多道程序中多个进程同时向前推进的过程，没个进程总是与其它进程并发地执行的。进程的制约性是指一个进程的运行受到另一进程的制约。比如有的进程可能正在等待另一进程的计算结果而无法运行，或者进程所需的资源被别的进程占有而无法运行。 3进程的含义是什么? 如何构造和描述进程? 答：进程是程序的一次执行。进程由“进程控制块+程序+数据”构成，用进程控制块描述进程。 4有三个并发进程，R负责从输入设备读入信息并传送给M，M将信息加工并传送给P，P将打印输出，写出下列条件下的并发程序。 (1) 双缓冲区，每个区大小为K。 (2) 单缓冲区，其大小为K。 答： （1）双缓冲区，每个区大小为K，信号量初值如下： mutexR=mutexP=1； emptyR=emptyP=k; fullR= fullP=0; 变量的初值如下： inR=outR=inP=outP=0； 用类Pascal编写程序如下：
新概念英语第一册课后答案
新概念英语第一册课后答案 Lesson122 A 1 This is the car which the mechanic repaired yesterday. 2 He is the man whom I invited to the party. 3 These are the things which I bought yesterday. 4 He is the man who came here last week. 5 He is the policeman who caught the thieves. 6 She is the nurse who looked after me. 7 She is the woman whom I met at the party. 8 I am the person who wrote to you. B 1 Who met him? That woman? Yes, she's the woman who met him. 2 Who sat there? That man? Yes, he's the man who sat there. 3 Who made it? That woman? Yes, she's the woman who made it. 4 Who read it? That man? Yes, he's the man who read it. 5 Who shut it? That man? Yes, he's the man who shut it. 6 Who took it? That woman? Yes, she's the woman who took it. 7 Who told me? That man? Yes, he's the man who told me. 8 Who saw me? That woman? Yes, she's the woman who saw me. C 1 Whom did you see? That man? Yes, he's the man whom I saw. 2 Whom did you telephone? That woman? Yes, she's the woman whom I telephoned. 3 Whom did you invited? That man? Yes, he's the man whom I invited. 4 Whom did you to the to the cinema? That man? Yes, he's the man whom I took to the cinema. 5 Whom did you find in the garden? That man? Yes, he's the man whom I found in the garden. 6 Whom did you drive to London? That woman? Yes, she's the woman whom I drove to London. 7 Whom did you hear? That woman? Yes, she's the woman whom I heard. 8 Whom did you remember? That man? Yes, he's the man whom I remembered. Lesson124 A 1 She is the woman I drove to London. 2 That's the film I saw. 3 That's the man I spoke to. 4 They are the thieves the police caught. 5 These are the letters I typed. 6 These are the people you asked me about. B 1 Which man? That man? Yes, that's the man I saw yesterday.
新概念英语第一册Lesson47_52课后答案
新概念英语第一册Lesson47~52课后答案新概念英语第一册Lesson47~48课后答案 A 1 The aeroplane is flying over the village. 2 The ship is going under the bridge. 3 The children are swimming across the river. 4 Two cats are running along the wall. 5 The boy is jumping off the branch. 6 The girl is sitting between her mother and her father. 7 The teacher is standing in front of the blackboard. 8 The blackboard is behind the teacher.
B 1 Do you like honey? Yes, I do. I like honey, but I don't want any. 2 Do you like bananas? Yes, I do. I like bananas, but I don't want one. 3 Do you like jam? Yes, I do. I like jam, but I don't want any. 4 Do you like oranges? Yes, I do. I like oranges, but I don't want one. 5 Do you like ice cream? Yes, I do. I like ice cream, but I don't want any. 6 Do you like whisky?
(完整版)新概念英语同步测试卷(L1-6)
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新概念英语第一册 同步习题37课左右
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新版新概念英语第一册·练习册含答案
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新概念英语第一册L49-54测试卷
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新概念第一册85-90课测试
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三．连词成句。 1. women, the, word, last, have ________________________________________________. 2. have, been, ever, to, Shanghai, you ________________________________________________ 3. have, they, watched, TV, already _______________________________________________. 4. does, how, it, cost, much ________________________________________________ 5. he, crash, just, had, a, car, has _________________________________________________. 6. it, is, worth, of, penny, it, every _________________________________________________. 7. Beijing, beautiful, a, city, is _________________________________________________. 8. I, TV, saw, it, last, week, on _________________________________________________. , lunch, yet, had, haven’t _________________________________________________. 10. this, is, house, sale, for _________________________________________________. 四．用所给词的适当形式填空。 1. I have _____ (do) my homework. 2. Can you ______(open) the door, please 3. How much ____ (do) it cost
新概念英语第一册同步试题
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概念、含义、定义和涵义的区别复习进程
概念、含义、定义和涵义的区别
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