操作系统信号量

设信号量为S，初值为1<br>PV操作如下：<br>进程P1：&nbsp; &nbsp; &nbsp; 进程P2&nbsp;<br>{&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;{<br>&nbsp; &nbsp; P(S);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;P(S);<br>&nbsp; &nbsp; 访问资源;&nbsp; &nbsp; &nbsp; 访问资源;<br>&nbsp; &nbsp; V(S);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;V(S);<br>}&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }<br>

设两个信号量：S生，S消。<br>初值为S生=1(代表仓库是否为空，只有仓库为空，生产者才能往里面放物品)<br>S消=0(代表仓库是否为满，只有仓库是满的，消费者才能从里面取物品)<br>PV操作如下：<br>生产者:&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 消费者:<br>while(true)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; while(true)<br>{&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; {<br>&nbsp; &nbsp; 生产一个产品;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; P(S消);//判断仓库是否为满<br>&nbsp; &nbsp; P(S生);//判断仓库是否为空&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;取走物品;<br>&nbsp; &nbsp; 产品送入仓库;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;V(S生);//通知生产者仓库已空<br>&nbsp; &nbsp; V(S消);//通知消费者仓库已满&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; 消费者使用物品;<br>}&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;}<br>

艾兹格·W·迪科斯彻 （Edsger Wybe Dijkstra，<br>1930年5月11日~2002年8月6日）荷兰人。 <br>计算机科学家，毕业就职于荷兰Leiden大学，<br>早年钻研物理及数学，而后转为计算学。<br>曾在1972年获得过素有计算机科学界的诺<br>贝尔奖之称的图灵奖，之后，他还获得过<br>1974年 AFIPS Harry Goode Memorial Award、<br>1989年ACM SIGCSE计算机科学教育教学杰出贡献奖、<br>以及2002年ACM PODC最具影响力论文奖。

提出“goto”有害论

提出信号量和PV原语

解决了哲学家聚餐问题

Dijkstra最短路径算法和银行家算法的创造者

第一个Algol 60编译器的设计者和实现者

THE操作系统的设计者和开发者

与D. E. Knuth并称为我们这个时代最伟大的计算机科学家的人

信号量的初值为非负的整数。当其值&gt;=0的时候，表<br>示系统中对应资源的数目，当其值&lt;0时，其绝对<br>值表示因该资源而被阻塞的的进程数目，当其值等于<br>0时，表示系统中对应资源都已经被占用，并且没有<br>因等待该资源而被阻塞的的进程。

每个信号量s除一个整数值s.value（计数）外，<br>还有一个进程等待队列s.L，其中是阻塞在该<br>信号量的各个进程的标识

信号量的初值只能为0或1