 回复北大必考题操作系统之PV操作
<html><body>我的解法：（以下几进程并发执行） semaphore:RedGuest,BlkGuest,ShipGuest,Check,ship,s;
int Red=0,Blk=0;
RedGuest=0; /*用于红客登船安全同步*/
BlkGuest=0; /*用于黑客登船安全同步*/
ShipGuest=4; /*用于红、黑客等船同步，即船运客未回*/
check=0; /*用于登船安全检查同步*/
ship=0; /*开船同步信号*/
s=1; /*用于红、黑客登船互斥*/
void RedGuestGetShip(){/*红客登船进程*/
L1:V(check); /*Call the SafeCheck()*/
P(ShipGuest);
P(s);
P(RedGuest);
Red=Red+1;
the RedGuest get ship;
V(check); /*Call the SafeCheck()*/
V(s);
goto L1;
} void BlkGuestGetShip(){/*黑客登船进程*/
L2:V(check); /*Call the SafeCheck()*/
P(ShipGuest);
P(s)
P(BlkGuest);
Blk=Blk+1;
the RedGuest get ship;
V(check); /*Call the SafeCheck()*/
V(s);
goto L2;
}
void SafeCheck(){ /*登船安全检查进程*/
p(check);
if(Red+Blk==4)
{
V(ship); /*start the ship*/
}
if(Red+Blk==3)
{
if(Red%2!=0) /*if Red%2!=0,so there are 3 or 1 RedGuest on ship,*/
V(RedGuest); /* then let 1 RedGuest get ship is safe.*/
else /*if Red%2==0,so there are 2 or 0 RedGuest on ship,*/
V(BlkGuest); /* then let 1 RedGuest get ship is safe.*/
}
else
{
V(RedGuest);
V(BlkGuest);
}
}
void ShipGuest(){ /*船送乘客过河进程*/
P(ship);
Ship the Guest to another bank and return;
Red=0;
Blk=0;
V(ShipGuest);
V(ShipGuest);
V(ShipGuest);
V(ShipGuest);
} </body></html>
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2006/10/14 18:50:00
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