Exam Second Spring 2009
March 25, 2009
Name:
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Question
1. True/False
2. Short questions
3.
4.
5.
6.
7.
8. Programming question
Total
Maximum
11 pts
4
4
4
4
4
4
15
50
Current
1. True/False Questions (write F or T) (11 points, 1 point each)
__ Semaphores are only initialized with a count >= 0
__ cond_wait needs to be called inside a testing loop
__ Heap-allocated memory can be shared between threads
__ The calls sema_wait and sema_post are different in that sema_post must be called
inside a critical section.
__ Mutexes must be unlocked always in the same order to avoid a deadlock
__ The read/write lock implementation discussed in class allows simultaneous lock
acquisition by different writers.
__ An iterative server passes slave sockets in round-robin fashion to a pool of threads.
__ Thread-safe code is always re-entrant.
__ The principle of least common mechanism warns against sharing resources
between services of different importance.
__ Different threads in the same process can run different algorithms.
__ Monitors are a fair synchronization mechanism.
Short Questions
2. Describe a situation in which you would use spinlocks.
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3. Which information does the OS need to keep track of when a process opens files,
and where is it stored?
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4. What is the difference between dup() and dup2() ? For each of them, give an
example of where they are used in the shell project.
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5. Comment on the idea of using a global variable to indicate error values in a multithreaded environment. Is it good or bad and why?
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6. "count++" If is only one C statement, why does it need to be protected by a mutex
to be thread-safe ?
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7. In a system with N numbered mutexes, threads need to get two consecutive mutex
locks (e.g., m[i] and m[i+1]) before doing some operations. Describe in pseudo-code
how the locking should be done to prevent deadlocks, including for corner cases.
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8. You are hired to make a simulation of a farm. The simulation needs to keep track
of eggs, hens, workers and a supply of egg boxes recycled by customers. In the space
below, write a function start_simulation that is passed the number of hens, boxes,
customers and workers as arguments, initializes any needed synchronization
mechanisms, and starts the appropriate threads. Also write the functions that will run
each thread. Your answers will be graded based on whether your implementation
enforces the following constraints:
hens lay an egg every day. (1 point)
.A customer consumes 2 eggs every day out of a box, if they managed to get one. A box
contains 12 eggs. (1 point)
.The
must queue and so are sold boxes on a first-come first-served basis. They
can only have one box at a time and come back once they have consumed the eggs in that
box (3 points)
.Workers fill the boxes one at a time. That is, there is a single filling station that only
one worker may use at a time to fill a single box. Workers queue to use the filling station.
This avoids having several partially filled boxes. (3 points)
.Obviously, a worker needs to wait inside the filling station until 12 hens have laid an
egg, as the boxes must contain 12 eggs. (3 points)
.There is a fixed, limited number of boxes in the entire simulation, and they are
recycled. There must not be a worker with a box in hand waiting for the filling station while
a worker without a box is inside the filling station. Consumers must return their empty box
before waiting in line for a full one. (3 points)
.Workers take 1 minute to make a filled box available to a consumer (e.g., in a
refrigerator). (1 point)
.Consumers
Total 15 points. Tip: use a day_sleep(int x) function, which you don't have to
define, to simulate threads waiting for x days, and sleep(60) to wait a minute.
// define any globals here
void hen(){
}
void customer() {
}
void worker() {
}
void start_simulation (int n_hens, int n_boxes, int n_customers, int n_workers) {
}