デッドロックを回避する 1 つの方法は、哲学者 1 が自分の箸に手を伸ばす前に待機するというものです。コードの観点からは、自分の箸に手を伸ばす前に、指定した時間 (sleep_seconds)、哲学者 1 を休眠状態にすることができます。十分休眠した場合、プログラムは実デッドロックなしに終了できます。実行可能ファイルに対する引数として休眠する秒数を指定できます。引数を指定しない場合、哲学者は休眠しません。
次の擬似コードは各哲学者のロジックを示します。
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 chopstick
grab left chopstick
eat some food
put down left chopstick
put down right chopstick
}
次のリストは、哲学者 1 が自分の箸に手を伸ばすまでに 30 秒間待機するようにしたプログラムを 1 回実行した様子を示しています。プログラムの実行は完了し、5 人の哲学者全員が食事し終わります。
% 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
プログラムを数回実行して異なる休眠引数を指定してみてください。哲学者 1 が箸を取る前に短時間しか待機しないとどうなるか、あるいは長い時間待機させたらどうなるかを観察するために、実行可能ファイル a.out にいろいろな休眠引数を指定してみます。そして、休眠引数を使用した状態と使用しない状態で複数回プログラムを再実行します。プログラムがハングアップする場合も、最後まで実行する場合もあります。プログラムがハングアップするかどうかは、スレッドのスケジュールと、スレッドによるロックの要求のタイミングによって異なります。