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Oracle Solaris Studio 12.3：线程分析器用户指南 Oracle Solaris Studio 12.3 Information Library (简体中文) |
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Oracle Solaris Studio 12.3：线程分析器用户指南 Oracle Solaris Studio 12.3 Information Library (简体中文) |
哲学家就餐方案是一个结构如下的经典方案。编号从 0 到 4 的五位哲学家坐在圆桌旁思考。随着时间的推移，每个人开始饥饿并决定进餐。餐桌上有一盘面条，但是每位哲学家只有一根筷子可用。为了吃到面条，他们必须共用筷子。每位哲学家右边（他们面向餐桌而坐）的筷子的编号与该哲学家的编号相同。
图 3-1 哲学家就餐
每位哲学家首先拿到与自己编号相同的筷子。他拿到分配给自己的筷子后，他将去拿分配给邻座的筷子。拿到两根筷子后，他就可以进餐了。吃完后，他将筷子放回桌上的原来位置，一边一根。该过程一直重复，直到把面条吃完。
每位哲学家都拿到自己的筷子并等待使用其邻座的筷子时，就会发生实际的死锁。
编号为 0 的哲学家拿着编号为 0 的筷子，但在等待编号为 1 的筷子
编号为 1 的哲学家拿着编号为 1 的筷子，但在等待编号为 2 的筷子
编号为 2 的哲学家拿着编号为 2 的筷子，但在等待编号为 3 的筷子
编号为 3 的哲学家拿着编号为 3 的筷子，但在等待编号为 4 的筷子
编号为 4 的哲学家拿着编号为 4 的筷子，但在等待编号为 0 的筷子
在这种情况下，没人可以吃到东西，于是哲学家们陷入了死锁之中。多次运行该程序，您将看到该程序有时会挂起，而有时又可以运行至完成。该程序挂起的情况如以下运行过程样例所示：
prompt% cc din_philo.c prompt% a.out Philosopher 0 is done thinking and now ready to eat. Philosopher 2 is done thinking and now ready to eat. Philosopher 2: got right chopstick 2 Philosopher 2: got left chopstick 3 Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 4 is done thinking and now ready to eat. Philosopher 4: got right chopstick 4 Philosopher 2: eating. Philosopher 3 is done thinking and now ready to eat. Philosopher 1 is done thinking and now ready to eat. Philosopher 0: got right chopstick 0 Philosopher 3: got right chopstick 3 Philosopher 2: got right chopstick 2 Philosopher 1: got right chopstick 1 (hang) Execution terminated by pressing CTRL-C
一种避免死锁的方法是让 1 号哲学家在拿起他的筷子之前等待一段时间。就代码而言，可让他在拿起自己的筷子之前休眠指定的一段时间 (sleep_seconds)。如果他休眠的时间足够长，该程序就可能会完成而不发生任何实际死锁。可以将他休眠的秒数指定为可执行程序的参数。如果不指定参数，该哲学家就不会休眠。
以下伪代码显示了每位哲学家的逻辑：
while (there is still food on the table)
{
if (sleep argument is specified and I am philosopher #1)
{
sleep specified amount of time
}
grab right fork
grab left fork
eat some food
put down left fork
put down right fork
}
下面列出了该程序的一次运行，在这次运行中 1 号哲学家在拿起他的筷子之前等待 30 秒。该程序运行至完成，所有五位哲学家都吃到了面条。
% a.out 30 Philosopher 0 is done thinking and now ready to eat. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 4 is done thinking and now ready to eat. Philosopher 4: got right chopstick 4 Philosopher 3 is done thinking and now ready to eat. Philosopher 3: got right chopstick 3 Philosopher 0: eating. Philosopher 2 is done thinking and now ready to eat. Philosopher 2: got right chopstick 2 Philosopher 1 is done thinking and now ready to eat. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. ... Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0: got right chopstick 0 Philosopher 0: got left chopstick 1 Philosopher 0: eating. Philosopher 0 is done eating. Philosopher 4: got left chopstick 0 Philosopher 4: eating. Philosopher 4 is done eating. Philosopher 3: got left chopstick 4 Philosopher 3: eating. Philosopher 3 is done eating. Philosopher 2: got left chopstick 3 Philosopher 2: eating. Philosopher 2 is done eating. Philosopher 1: got right chopstick 1 Philosopher 1: got left chopstick 2 Philosopher 1: eating. Philosopher 1 is done eating. % Execution terminated normally
尝试多次运行该程序，并指定不同的休眠参数。如果 1 号哲学家在拿起他的筷子之前仅等待很短的一段时间，那么发生什么情况呢？如果他等待的时间更长一些，又会怎么样？尝试为可执行程序 a.out 指定不同的休眠参数。在使用或不使用休眠参数的情况下多次重新运行该程序。该程序有时会挂起，而有时又可以运行至完成。程序是否挂起取决于线程的调度情况以及线程进行锁请求的时间。
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