Example Homework & Test Questions

Why does "fread" take two integer arguments?  What's wrong with this from an interface design perspective, and how would you fix it?

"fread" takes two different size arguments--size and nmemb--which it multiplies together.  The idea is that "size" gives the number of bytes per element (where the definition of "element" is up to the user, and could be bytes, integers, or records), and "nmemb" gives the number of elements. 

The problem with this interface is that it's too complex.  A better design would have the caller multiply the size and element count, and pass "fread" a single integer byte count.  Google for "fread(ptr," and you'll see that something like 90% of all calls to fread pass the constant 1 for one or the other argument.

If you write a program that uses too much memory and starts paging, why do other programs (e.g., the debugger window you're trying to use to stop the runaway program) slow down?  How would you change the operating system's behavior to fix this?

When a program B asks for memory, to make room the operating system may have to swap some existing memory to disk.  If this paged-out memory contains code, data, or I/O buffers that another program C will need, then C will have to read that memory back in before it can continue execution.  This means program B's request for memory slowed down program C--and because disk I/O is so slow, the slowdown can be a factor of millions.

The fix for this is to somehow stop B's requests from paging out data belonging to C.  For example, we could limit the amount of memory B is allowed to consume (in UNIX, using "ulimit"), which would prevent it from impacting the performance of other processes.  If only process C is important, we could mark its data as locked in memory (in UNIX, using "mlock"), to prevent it from being paged out.

Example Questions I'll Never Ask

14 (a). Simulate the FIFO, Optimal, LRU, and MRU page replacement algorithms for this page reference stream.  Simulate three runs, assuming a memory size of 1, 2, and 3 pages, initially all empty.  Show which page is to be swapped out at each step.
[ This is an example of the sort of actual test question I've had to crank through by hand.  It's just tedious busywork.  It'd make a good machine problem, though. ]

26 (c). How many times does the letter 'e' appear on page 487 of the textbook?  Count both upper and lower case. (NO CHEATING: CLOSED BOOK)
[ This is satire of the irrelevant "minutia" question that we professors love to ask, because they're really easy to grade. ]

83 (b). Sort the following operating systems by the length of the facial hair of the listed key designer.  For the purposes of this question, consider a 5 o'clock shadow as having some small positive length epsilon.
[ This is satire of the "make a judgement" question (about a clearly related but useless topic). ]