3.2.1 How the Philosophers Can Deadlock
An actual deadlock occurs when every philosopher is holding his own chopstick and waiting for the one from his neighbor to become available:
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Philosopher zero is holding chopstick zero, but is waiting for chopstick one |
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Philosopher one is holding chopstick one, but is waiting for chopstick two |
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Philosopher two is holding chopstick two, but is waiting for chopstick three |
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Philosopher three is holding chopstick three, but is waiting for chopstick four |
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Philosopher four is holding chopstick four, but is waiting for chopstick zero |
In this situation, nobody can eat and the philosophers are in a deadlock. Rerun the program a number of times and you will see that the program may sometimes hang, or run to completion at other times.
Run the dining philosophers program and see whether it completes or deadlocks. It may hang as shown in the following sample run:
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
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