CMPSC 311, Spring 2013, Midterm Exam 1, Sample questions for review

The first midterm exam will be Tuesday, Feb. 12, 6:30 - 7:45 pm, in 112 Kern Bldg.

Anyone who has a conflict with the scheduled time, especially another class or exam the same night, should have asked about a conflict exam already.  Time and place information has been sent to you by email.

The exam is closed book, closed computer, closed neighbor, no cell phones, etc.  But, you can bring one 8 1/2 x 11 sheet of paper, with your name on it, as a "cheat sheet"; you must turn this in with the exam.

Class time on Monday, Feb. 11, will be devoted to review only.  Project solutions are posted on ANGEL.


The exam will ask questions about general knowledge of C and Unix, and will require both programming and debugging.  Any material that was covered in class, as assigned reading, as background for the projects, or in the projects, could be on the exam.  In particular, don't neglect the programming examples in the Intro to Unix notes, or the exercises in the notes.  The "cheat sheet" could remind you of Unix function prototypes and C syntax, but you should be able to answer most questions without referring to it.

The questions will be
The number of points in each category might be different, but not by much.

Some of the sample questions given here are harder than are actually on the exam.


1.  True/False, circle T or F.  [1 point each]
 
  T  F  (a)  The Posix Standard describes only an interface to the operating system, as a set of functions callable from C.
 
  T  F  (b)  When a Unix process starts, the input and output streams stdin, stdout and stderr are opened.
 
  T  F  (c)  When a C or Unix library function fails, it sets the global variable errnum to indicate the reason for the failure.

  T  F  (d)  The assert() macro can be used to verify a boolean expression at runtime; if the expression evaluates to false, the program is terminated.
 
  T  F  (e)  Parameter-passing in C is always call-by-value (or, pass-by-value, which is the same thing).
 
  T  F  (f)  Reference parameters (from C++) can be approximated by pointer parameters in C.


2.  Multiple Choice  [3 points each]
 
Choose only one of the four given answers in each part, by circling 1, 2, 3, or 4.  There is at least one correct answer given in each part, and there may be other correct answers.  Partial credit is possible for some incorrect answers.
 
(a) A file descriptor is
 
    (1)  a non-negative integer obtained from a call to open().
    (2)  a non-negative integer obtained from a call to fopen().
    (3)  a pointer obtained from a call to open().
    (4)  a pointer obtained from a call to fopen().

(b) Comparing system functions and library functions, ...
 
    (1)  system functions are provided with the operating system, while library functions are provided with the programming language.
    (2)  library functions are provided with the operating system, while system functions are provided with the programming language.
    (3)  there is no difference, they are both provided with the operating system.
    (4)  there is no difference, they are both provided with the programming language.

(c) An operating system would be expected to ...

    (1)  provide methods for connecting hardware and software components.
    (2)  manage resources.
    (3)  provide abstractions of the resources.
    (4)  map virtual resources to actual resources over time.


3.  [20 points]  Design and write a function that will estimate the size of the largest single block of memory that can be successfully allocated with malloc().  Do this efficiently.  Pseudocode is acceptable as long as it's not "too pseudo".  It would help in assigning partial credit if you also give a diagram of memory allocation within a process address space, to help explain how your function works.

"Cheat sheet" info -  malloc() is given one argument, a number of bytes to allocate.  If there is enough space available in the process heap segment, malloc() returns a non-NULL pointer to that space, otherwise it returns NULL.  The function free() takes one argument, which is expected to be a return value from malloc(), and deallocates that space.  malloc() does not initialize the memory it allocates.


4.  [8 points]  Consider the swap function

static inline void int_swap(int *a, int *b)
{ int t = *a; *a = *b; *b = t; }

Explain what is wrong with each of the following uses of swap().

(a)  int m, n; int_swap(&m, &n);
(b)  int_swap(m, n);
(c)  int_swap(&m, &3);  // assign 3 to m


5.  [lots of points]  This problem has happened to several students.  The program seems to run but it produces no output.  Explain why.


6.  [10 points]  Write a simple version of the od program.

"Cheat sheet" info - See the Course Intro notes and the Project 2 and 3 descriptions for examples.  The actual test would include a short example, since od has several options, and we need to be definite about which one to use.


7.  [6 points]  Explain the errors and repair the code.

int size_intarray = ... some value from the input ... ;
int *intarray = malloc(size_intarray);
for (i = 0; i < size_intarray; i++)
  { total += intarray[i]; }


8.  [8 points]  Explain the errors and repair the code.
pid_t pid;
int status;

if ((pid = fork()) = 0)
  { /* child */
    execlp("bin/ls", "bin/ls", NULL);
  }
else
  { /* parent */
    waitpid(pid, &status, 0);
  }


9.  [8 points]  Indicate what is wrong with the following C program, and correct the program.  (The return type of foo() is intended, so don't change that.)    (Hint:  parameter passing, not malloc)

 1     #include <stdlib.h>  /* for malloc(), free() */

 2     /* This function acquires memory and returns the
 3      * address to the caller via the parameter target.
 4      * The memory can be released by calling free().
 5      */
 6     void foo(char *target)
 7     {
 8       void * buf = malloc(sizeof(char) * 10);
 9       (*target) = buf;
10     }

11     int main(void)
12     {
13       char *line;
14       foo(line);    /* allocate memory */
15       free(line);   /* deallocate memory */
16     }