操作系统复习笔记(二)

1.问答题:有一个文件F,有A,B两组进程共享这个文件，同组的进程可以同时读文件F,但当有A组(或B组)的进程在读文件F时就不允许B组(或A组）的进程读,

解:定义两个计数器C1,C2,分别记录A组和B组中读文件的进程数,三个信号量S1,S2,SAB,其中S1用于通知A组进程已经有B组进程在读文件F了,S2用于通知B进程已经有A进程在读文件F了,SAB用于实现对共享变量C1和C2以及临界区的互斥访问.

begin
            var S1,S2,SAB:semaphore = 1,1,1;
                 C1,C2:integer = 0,0;

                   process A-i(i=1,2)
                    begin
                       repeat
             P(SAB);
                   C1 = C1+1;
             if(C1=1) then P(S2);
             V(SAB);
              读文件F;
             P(SAB)
             C1 = C1-1;
             if(C1==0)V(S2)
             V(SAB)  
                until false
                    end      
    

                   process B-i(i=1,2)
                    begin
                       repeat
             P(SAB);
                   C2 = C2+1;
             if(C2=1) then P(S1);
             V(SAB);
              读文件F;
             P(SAB)
             C2 = C2-1;
             if(C2==0)V(S1)
             V(SAB)  
                until false
                    end        
  end 

2,应用题:get进程读数据到buffer1里，然后进程copy从buffer1里的数据复制到buffer2里，再由put进程取出buffer2里的数据去进行打印.

分析:这是两个阶段的生产-消费问题.第一阶段的生产者和消费者是get和copy,第二阶段的生产者和消费者是copy和put.为简单计,假设buffer1,buffer2 都是单缓冲区,因此只要设4个信号量empty1,full1,empty2,full2,就可以了.

 begin 
              buffer:integer
              empty1,empty2,full1,full2,:semaphore=1,1,0,0;
              cobegin

                        process Get
                            begin
                               repeat
                  读数据
                                p(empty1);
                                把数据放到buffer1里
                                 v(full1);
                                 until false
                            end

                        process Copy
                            begin
                  repeat
                                p(full1)
                                从buffer1里读出数据
                                v(empty1);
                  p(empty2)
                                把数据放到buffer2里
                  v(full2)
                                 until false
                           end

         process Put
                            begin
                  repeat
                                p(empty2)
                                从buffer2里读出数据
                                v(full2);         
                                打印数据
                                 until false
                           end
              coend
  end

3.输入进程输入数据到缓冲区buffer1中，计算进程从buffer1中读出数据进行计算,并把结果送入buffer2,然后打印进程从buffer2中读出结果进行打印.假设缓冲区大小分别为n1和n2.

 begin 
              buffer:integer
              empty1,empty2,full1,full2,mutex1,mutex2:semaphore=n1,n2,0,0,1,1;
              cobegin

                        process Input
                            begin
                               repeat
                  输入数据
                                p(empty1);
                  p(mutex1);
                                把数据放到buffer1里
                  v(mutex1);
                                 v(full1);
                                 until false
                            end

                        process Compute
                            begin
                  repeat
                                p(full1)
                  p(mutex1);
                                从buffer1里读出数据
                                v(mutex1);
                                v(empty1);

                  p(empty2)
                  p(mutex2);
                                把数据放到buffer2里
                   v(mutex2);
                  v(full2);
                                 until false
                           end

         process Print
                            begin
                  repeat
                                p(empty2)
                   p(mutex2);
                                从buffer2里读出数据
                  v(full2);
                                v(full2);         
                                打印数据
                                 until false
                           end
              coend
  end

4.过桥问题.
(1)桥每次只能有一辆车通过,
(2)不允许两车交会,但允许同方向的多辆车依次通过

解:(1)

    begin   
                     mutex:semaphore=1;
                     cobegin
                        process Scar//南边来的车
                            begin
                                come;
                                p(mutex);
                                过桥;
                                 v(mutex);
                                go;
                            end

                        process Ncar//北边来的车
                            begin
                 come;
                                p(mutex);
                                过桥;
                                 v(mutex);
                                go;
                           end
                coend
     end

(2)

 begi
         var Smutex=1,Nmutex=1,mutex=1:semaphore;
    SCarCount=0,NCarCount=0:integer;
                cobegin
                    process Scari(i=1,2)
                    begin
                        p(Smutex); 
                         if(SCarCount=0) then p(mutex);
           SCarCount = SCarCount +1;
                        v(Smutex); 
          过桥；
           p(Smutex); 
           SCarCount = SCarCount -1;
            if(SCarCount=0) then v(mutex);
                        v(Smutex); 
                    end

                process Ncarj(j=1,2)
                    begin
                        p(Nmutex); 
                         if(NCarCount=0) then p(mutex);
          NCarCount = NCarCount +1;
                        v(Nmutex); 
          过桥；
           p(Nmutex); 
          NCarCount = NCarCount -1;
            if(NCarCount=0) then v(mutex);
                        v(Nmutex); 
                    end
                coend
end

5.在管道通信机制中，用信号量描述读进程和写进程访问管道文件的过程，假设管道文件大小为10KB.
分析:在UNIX系统中，利用一个打开的共享文件来连接两个相互通信的进程，这个共享文件叫管道．作为管道输入的发送进程，以字符流的形式将信息送入管道，而作为管道输出的接收进程，从管道中获取信息．管道通信机制要提供三方面的协调能力：(1)互斥．当一个进程对管道进行读／写操作时，另一个进程必须等待．(2) 同步．当写进程把数据写入管道后便去睡眠等待，直到输出进程取走数据后唤醒它．若一次写入的数据超过缓冲区剩余空间的大小，当缓冲区满时，写进程必须阻塞，并唤醒读进程。(3)对方是否存在．只有确定对方存在时，才能够进行通信．
本题只需要考虑互斥，同步问题。由于只有一对进程访问管道，因此不需要设置互斥信号量，只要设置两个同步信号量empty,full．分别表示管道可写和可读．

 begin 
                  pipe:array[09] of kilobytes;
     ts=10,length,in=0,out=0:integer;
                   empty,full:semaphore=1,0;
                    cobegin
                        process PipeWriter
                            begin
                               repeat
        产生数据；
                                p(empty);
                        length = data length;
         while(length>0 and ts>0)
        begin
             pipe[in] = data of 1KB;
             in = (in+1) mod n;
             ts = ts-1;
             length = length - 1;
        end
                                 v(full);
                            end
                        process Consumer
                            begin
                  repeat;
                                p(full);
                                从缓冲区取出一件物品;
        out = (out+1) mod n;
        ts = ts +1;
                                v(empty);
                           end
               coend
    end