3.2 哲人进餐方案

哲人进餐方案是一个结构如下的经典方案：五个哲人，其编号为零到四，坐在圆桌旁思考。随着时间的推移，不同的个人感到饿了，决定去吃面条。桌子上有一个盛有面条的大浅盘，但是每个哲人只有一根筷子可以使用。为了吃面条，他们必须共用筷子。每个哲人左侧（哲人面向餐桌而坐）的筷子具有与该哲人相同的编号。

每个哲人首先去拿带有其编号的筷子。当他拿到指定给自己的筷子后，他将去拿指定给其邻桌的筷子。拿到两根筷子后，他就可以吃了。吃完后，他将筷子放回桌子上的原始位置，一侧放置一根。重复该过程，直到将面条吃完。

3.2.1 哲人怎样发生死锁

当每个哲人都持有他自己的筷子并等待邻座的筷子变为可用时，会发生实际死锁：

在这种情况下，没有人可以吃，这些哲人处于死锁状态。多次重新运行该程序，您将看到该程序可能有时挂起，或有时运行直到完成。

运行哲人进餐程序，并查看它完成还是发生死锁。它可能挂起，如以下样例运行所示：

prompt% cc din_phil.c -mt
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

3.2.2 为哲人 1 引入休眠时间

潜在死锁的一种可能解决方案是，哲人 1 在拿他的筷子之前先等待。就代码而言，可以让他在拿自己的筷子之前休眠指定的时间 (sleep_seconds)。如果他休眠的时间足够长，则程序可能完成而不会发生实际死锁。您可以将他休眠的秒数作为可执行程序的参数进行指定。如果您不指定参数，则哲人不休眠。

下列伪代码显示了每个哲人的逻辑：

   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 
      }

以下列表说明，如果哲人 1 在拿自己的筷子前等待 30 秒，程序是如何运行的。程序运行直到完成，所有五个哲人都吃完了。

% 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

尝试运行几次该程序，指定不同的休眠参数。如果哲人在拿自己的筷子前仅等待很短的时间，会发生什么情况？如果他等待更长的时间，又会怎么样？尝试为可执行的 a.out 指定不同的休眠参数。将该程序重新运行几次，使用或不使用休眠参数。有时程序会挂起，有时则可以完成运行。程序是否挂起取决于线程的调度，以及线程请求锁定的时间。