PV.rar_pv_读者写者问题

读者写者问题，共享原则如下：1、读/写互斥访问；2、写/写互斥访问；3、允许多个读者同时对文件进行访问。 （1）读者优先 semaphore rmutex=1,rwmutex=1; int count=0; Cobegin reader: writer: begin begin repeat repeat P(rmutex) P(rwmutex) if (count==0) then P(rwmutex); writing; count:=count+1; V(rwmutex) V(rmutex) until false; reading; end P(rmutex) count:=count-1; if (count==0) then V(rwmutex); V(rmutex) until false; end; Coend （2）写者优先 int readcount=0,writecount=0; semaphore rmutex=1,wmutex=1,rwmutex=1,z=1,x=1; reader writer begin begin repeat repeat P(z); P(wmutex) P(x); writecount:=writecount+1; P(rmutex) if writecount=1 then P(x); readcount:=readcount+1; V(wmutex) if (readcount==1) P(rwmutex); P(rwmutex) V(rmutex) writing; V(x) V(rwmutex) V(z) P(wmutex) reading; writecount:=writecount-1; P(rmutex) if writecount=0 then V(x); readcount:=readcount-1; V(wmutex) if (readcount==0) V(rwmutex); until false; v(rmutex) end until false; end （3）读者与写者公平竞争 int readcount=0; semaphore rmutex=1,rwmutex=1,z=1; void reader() void writer() { { while(1) while(1) { { P(z) P(z) P(rmutex) P(rwmutex) ++readcount; writing; if (readcount==1) P(rwmutex); V(rwmutex) V(rmutex) V(z) V(z) } reading; } P(rmutex) --readcount; if(readcount==0) V(rwmutex); V(rmutex) } }