讀者寫(xiě)者問(wèn)題(讀者優(yōu)先,寫(xiě)者優(yōu)先,讀寫(xiě)公平)

1、讀者寫(xiě)者問(wèn)題（讀者優(yōu)先,寫(xiě)者優(yōu)先,讀寫(xiě)公平）讀者優(yōu)先讀者優(yōu)先的解決方案：互斥信號(hào)量wrt,初值是1,代表一個(gè)共享文件，解決“讀一寫(xiě)”互斥，“寫(xiě)一寫(xiě)”互斥。個(gè)記數(shù)器，即整型變量readcount,記錄讀者數(shù)，初值是0。來(lái)一個(gè)讀者，readcount加1當(dāng)readcount=1表示是第一個(gè)讀者，則需要執(zhí)行p操作搶占文件；否則表示已有讀者在安全的讀數(shù)據(jù)。走一個(gè)讀者，readcount減1當(dāng)readcount=0表示是最后一個(gè)讀者，則需要v操作釋放資源；否則表示還有讀者在讀數(shù)據(jù)。readcount為多個(gè)讀者共享的變量，是臨界資源。用互斥信號(hào)量mutex控制，mutex初值是1。寫(xiě)者優(yōu)先解決方案：在讀者

2、優(yōu)先的基礎(chǔ)上增加信號(hào)量r,初值是1:當(dāng)至少有一個(gè)寫(xiě)進(jìn)程準(zhǔn)備訪問(wèn)數(shù)據(jù)區(qū)時(shí)，用于禁止所有的讀進(jìn)程。增加一個(gè)記數(shù)器，即整型變量writecount,記錄寫(xiě)者數(shù)，初值是0。writecount為多個(gè)寫(xiě)者共享的變量，是臨界資源。用互斥信號(hào)量mutex2控制，mutex2初值是1。增加mutex3,初值是1:在r上不允許建造長(zhǎng)隊(duì)列，否則寫(xiě)進(jìn)程將不能跳過(guò)這個(gè)隊(duì)列，因此，只允許一個(gè)讀進(jìn)程在r上排隊(duì)，而所有其他讀進(jìn)程在等待r之前，在信號(hào)量mutex3上排隊(duì)。代碼:ntreadcount=0,writecount=0;semaphoremutex1=1,mutex2=1,mutex3=1,w=1,r=1讀者進(jìn)程：

3、P(mutex3);P(r)；P(mutex1);readcount+;if(readcount=1)P(w);V(mutex1);V(r);V(mutex3);readingisperformedP(mutex1);readcount-;if(readcount=0)V(w);V(mutex1);寫(xiě)者進(jìn)程：P(mutex2);writecount+;if(writecount=1)P(r);V(mutex2);P(w);writingisperformedV(w);P(mutex2);writecount-;if(writecount=0)V(r);V(mutex2);讀寫(xiě)公平代碼:ntreadcount=0;semaphoremutex=1,rw=1w=1讀者進(jìn)程：wait(w);wait(mutex);if(readcount=0)wait(rw);readcount+;signal(mutex);signal(w);readingisperformedwait(mutex);readcount-;if(readcount=0)signal(rw);signal(mutex);寫(xiě)者進(jìn)