1. When grading your lab 5 submissions, we noticed that there are quite a few of you which made the following errors: (a) you did not call pthread_mutex_init or pthread_cond_init to initialize the mutex nor condition variables; (b) you lock a mutex in one thread, and unlock it in another; (c) you pass a mutex to pthread_cond_wait in one thread, that is locked by another thread.

   The pthread implementation on Linux is surprisingly forgiving about the errors, thus many of you are not aware that you made the error. We graded the lab using a pthread implementation that is more strict, and were able to catch many of these, leading to low marks for many of you (for instance, if you do not initialize the mutex, locking has no effect). We have taken a second, more careful look at these submissions, and reward appropriately if you have the right logic but your code did not work because of the errors above. However, we discovered that further errors on a few submission, which lead to a lower score than before for a few students.

2. There are also quite a number of you who did not rename your directory appropriately. We had to take off 3 points for these students. This is quite painful as you have worked hard on the lab, and loosing 3 points for not following instruction is a real pity. So, please and follow the instructions exactly and carefully for Lab 7 and 8.

In the solution we present in class, we use two mutex variables. Many of the solutions submitted use one mutex only. A question raised during discussion with a student whether any solution with one mutex is wrong.

I will post some of the solutions submitted by you, and let you take a look and discuss. I think debugging threads is not easy and is a skill that comes with experience. So hopefully this little exercise will serve as a practice for you.
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Lab 5 has been graded by Mr. Xu Yecheng and your mark is now available through IVLE Gradebook.   Many students did not do as well as expected, and did not receive full marks.  In particular, many solutions either lead to deadlock or lead to overflow/underflow of pot.  If you did not get a 10 and were not aware of bugs in your code, you should look at your code carefully to find out what is wrong (it’s quite illuminating).  If you are convinced that your code works but still did not receive a full mark, please feel free to email Mr. Xu or me.