代写Systems Programming-C代写-C++代写

School of Computer Science

Main Summer Examinations 2021

代写Systems Programming There is a serious problemwith this Explain what is wrong, why it is a problem, and how it can be fixed.

Systems Programming in C and C++

Note

Answer ALL questions. Each question will be marked out of 20. The paper will be marked out of 60, which will be rescaled to a mark out of 100.

Question 1

The question is about pointers and memory management in C.

(a)Willthere 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 problemwith this 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++)

5array[i] =rand();

6return&array[0];

 7 }

(c)Consider the following two C functions sum2Darray1 and sum2Darray2. Both of themcompute the sum of all the elements of an input 2-dimensional Which one of them will be able to exploit memory hierarchy and thus achieve faster com- putation time? Explain your answer. 代写Systems Programming

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++)

6sum =sum +a[j][i];

7returnsum;

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++)

6sum =sum +a[i][j];

7returnsum;

8 }

[7 marks]

Question 2 代写Systems Programming

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:

StatementB before Statement A,
Statement A before StatementC

Provide a possible (deadlock free) trace of your solution. 代写Systems Programming

(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

4void *foo(void*arg){

5int *myid = (int *)arg;

6printf(”Hello  from  thread % d\n”,*myid);

7returnNULL;

8}

9

10intmain(){

11pthread˙ttid[4];

12 inti;

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

What causes this behavior? Explain your answer. [7 marks]

(c)[Continuation of Question 2b above] Rectify only the main() function such thatthe concurrent peer threads print unique values, 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 代写Systems Programming

This question is about Processes, Memory management & Scheduling.

(a)TheC program shown below is compiled and run on a UNIX 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>

5int main(){

6int x =1;

7pid˙t pid = fork();  

8 if (pid == 0)–

9 x = x * 2;

10}else if (pid > 0)–

wait(NULL);

12 x = 3;

13 }

14 "%d\n”,x);     

15  }

[4 marks]

(b)Schedulethe processes (given in Table 1) using round robin scheduling with quantum 10. Also, calculate the totalturnaroundtime. [5 marks] 代写Systems Programming

Table 1: Process table

(c)Whatare the physical memory addresses for each of the following logical addresses for the segment table (Table 2)? Note any that are 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)Considera 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 [4 marks] 代写Systems Programming

(e)Consider a demand-paged computer system which was recently measured to deter- minethe utilisation of CPU and the paging disk to decide the degree of multipro- gramming. The results are shown in the figure Explain what is happening in each scenario and what actions an operating system can take.