Systems Programming in C and C++
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School of Computer Science
Systems Programming in C and C++
Main Summer Examinations 2021
Systems Programming in C and C++
Question 1
The question is about pointers and memory management in C.
(a) Will there be any memory leakage in the following program? Explain your answer.
1 int main() 2 –
3 int *A = (int *) malloc(sizeof(int));
4 scanf(”%d”, A);
5 int *B;
6 B = A;
7 free(B);
8 return 0; 9 ˝
[6 marks]
(b) A programmer has written the following function with the aim to return a pointer
to an array of 10 random integers (int) to a caller function. There is a serious problem with this code. Explain what is wrong, why it is a problem, and how it can be fixed. Use this to write a correct version of the function without changing the function-signature. Assume that the caller function of randomArray is responsible for freeing any memory occupied by the array.
1 int* randomArray(void) 2 –
3 int array[10], i;
4 for (i = 0; i ¡ 10; i++)
5 array[i] = rand();
6 return &array[0]; 7 ˝
[7 marks]
(c) Consider the following two C functions sum2Darray1 and sum2Darray2. Both of them compute the sum of all the elements of an input 2-dimensional matrix. Which one of them will be able to exploit memory hierarchy and thus achieve faster com- putation time? Explain your answer.
1 int sum2Darray1(int a[N][M]) 2 –
3 int i, j, sum=0;
4 for(i=0;i¡M;i++)
5 for(j=0;j¡N;j++)
6 sum =sum + a[j][i];
7 return sum; 8 ˝
1 int sum2Darray2(int a[N][M]) 2 –
3 int i, j, sum=0;
4 for(i=0;i¡N;i++)
5 for(j=0;j¡M;j++)
6 sum =sum + a[i][j];
7 return sum; 8 ˝
[7 marks]
Question 2
The question is about concurrent programming.
(a) Consider a concurrent system with three processes P1, P2 and P3.
Provide a (semaphore based) solution to synchronize P1, P2 and P3 such that the following constraints on execution order is satisfied:
● Statement B before Statement A,
● Statement A before Statement C
Provide a possible (deadlock free) trace of your solution.
[7 marks]
(b) In the following program, the main thread creates four peer threads and passes
a pointer to the loop variable to each one. Each peer thread prints a message containing the loop variable.
1 #include¡stdio.h¿
2 #include¡pthread.h¿
3
4 void *foo(void *arg)–
5 int *myid = (int *) arg;
6 printf(”Hello from thread %d“n”, *myid);
7 return NULL;
8 ˝
9
10 int main()–
11 pthread˙t tid[4];
12 int i; 13
14 for(i=0; i¡4; i++)
15 pthread˙create(&tid[i], NULL, foo, &i); 16
17 for(i=0; i¡4; i++)
18 pthread˙join(tid[i], NULL);
19
20 return 0; 21 ˝
We might expect that the program will print all the four values of i, but when the program is executed, we see the following incorrect result containing repetitions:
Hello from thread 1
Hello from thread 3
Hello from thread 3
Hello from thread 3
What causes this behavior? Explain your answer. [7 marks]
(c) [Continuation of Question 2b above] Rectify only the main() function such that the concurrent peer threads print unique values, i.e., the first thread prints 0, the second thread prints 1, the third thread prints 2 and the final thread prints 3. We don’t expect the threads will print ”in order” (we expect that they just print the correct value per thread). Explain your answer. [6 marks]
Question 3
This question is about Processes, Memory management & Scheduling.
(a) The C program shown below is compiled and run on a UNIX machine. Predict all
possible outputs that this program will print to the console and explain your answer.
1 #include ¡sys/types.h¿
2 #include ¡sys/wait.h¿
3 #include ¡stdio.h¿
4 #include ¡unistd.h¿
5 int main() –
6 int x = 1;
7 pid˙t pid = fork();
8 if (pid == 0) –
9 x = x * 2;
10 ˝ else if (pid ¿ 0) –
11 wait(NULL);
12 x = 3; 13 ˝
14 printf(”%d“n”,x); 15 ˝
[4 marks]
(b) Schedule the processes (given in Table 1) using round robin scheduling with quantum
10. Also, calculate the total turnaround time. [5 marks]
Table 1: Process table
|
ID |
Arrival Time |
Duration |
|
P0 |
3 |
20 |
|
P1 |
15 |
22 |
|
P2 |
0 |
32 |
|
P3 |
5 |
3 |
|
P4 |
49 |
29 |
(c) What are the physical memory addresses for each of the following logical addresses for the segment table (Table 2)? Note any that are invalid. The logical addresses are:
(i) 3 , 15 (ii) 0 , 512 (iii)1 , 4096
(iv) 0 , 1 [4 marks]
Table 2: Segment table
|
Segment |
Base Address |
Length |
|
0 |
16384 |
400 |
|
1 |
4096 |
4096 |
|
2 |
32768 |
810 |
|
3 |
20480 |
1024 |
(d) Consider a system with four frames of memory, and the following sequence of page accesses: 0,1,2,3,4,2,3,0,1,4,2. When do page faults occur using FIFO and LRU replacement algorithms? Briefly justify your answer. [4 marks]
(e) Consider a demand-paged computer system which was recently measured to deter-
mine the utilisation of CPU and the paging disk to decide the degree of multipro- gramming. The results are shown in the figure below. Explain what is happening in each scenario and what actions an operating system can take.
[3 marks]
2021-12-28