Good Luck! Marking Guide. APRIL 2014 Final Exam CSC 209H5S

Transcription

1 APRIL 2014 Final Exam CSC 209H5S Last Name: Student #: First Name: Signature: UNIVERSITY OF TORONTO MISSISSAUGA APRIL 2014 FINAL EXAMINATION CSC209H5S System Programming Daniel Zingaro Duration - 3 hours Aids: 1 page of double-sided Letter (8-1/2 x 11) sheet The University of Toronto Mississauga and you, as a student, share a commitment to academic integrity. You are reminded that you may be charged with an academic offence for possessing any unauthorized aids during the writing of an exam, including but not limited to any electronic devices with storage, such as cell phones, pagers, personal digital assistants (PDAs), ipods, and MP3 players. Unauthorized calculators and notes are also not permitted. Do not have any of these items in your possession in the area of your desk. Please turn the electronics off and put all unauthorized aids with your belongings at the front of the room before the examination begins. If any of these items are kept with you during the writing of your exam, you may be charged with an academic offence. A typical penalty may cause you to fail the course. Please note, you CANNOT petition to re-write an examination once the exam has begun. Marking Guide This final examination consists of 8 questions on 16 pages (including this one). When you receive the signal to start, please make sure that your copy of the examination is complete. If you need more space for one of your solutions, use the last pages of the exam and indicate clearly the part of your work that should be marked. Do not add #include lines, and do not errorcheck system calls. Good Luck! # 1: /10 # 2: /12 # 3: /14 # 4: / 8 # 5: / 6 # 6: / 7 # 7: / 6 # 8: /13 TOTAL: /76 Page 1 of 16 cont d...

2 CSC 209H5S Final Exam APRIL 2014 Question 1. [10 marks] The following code compiles. In fact, it compiles with no warnings at all, even though it has seven bugs. Note that a single line could contain multiple bugs. Assume that malloc succeeds (i.e. the fact that I m not checking the return value of malloc doesn t count as a bug). #include <stdlib.h> #include <stdio.h> #include <string.h> char **init(int n) { char **result; result = malloc(n * sizeof(char)); return result; } int main(void) { char **strs; strs = init(5); char message[] = "this is a test"; char *name = "dan"; *strs = name; strcat(strs[1], name); char *s = malloc(10); strncat(s, message, sizeof(s) - 1); printf("%s\n", strs[0]); char *where = strchr(name, a ); *where = \0 ; printf("%s\n", name); return 0; } (The questions related to this code are on the facing page.) Page 2 of 16 cont d...

3 APRIL 2014 Final Exam CSC 209H5S Part (a) [5 marks] Carefully describe five of the program s bugs. Part (b) [5 marks] Now, fix those five bugs that you found by making changes directly to the code on the previous page. Page 3 of 16 cont d...

4 CSC 209H5S Final Exam APRIL 2014 Question 2. [12 marks] C strings end with a \0 character, and this design decision leads to some inefficiency. For example, strlen is O(n) instead of O(1), because it must find the \0 in order to calculate the string length. In addition, no \0 can exist in the string without being treated as a termination character. We ll solve these problems here. In this question, we ll use the following struct that stores the string length along with the string s characters: struct wrapstr { int len; // bytes allocated, accessible using s char *s; }; Part (a) [4 marks] Write the following function to allocate and initialize a new struct wrapstr, and return a pointer to the newly-allocated struct wrapstr. struct wrapstr *empty_wrapstr(void) { Page 4 of 16 cont d...

5 APRIL 2014 Final Exam CSC 209H5S Part (b) [5 marks] w is an initialized struct wrapstr, and newchars has at least num bytes in it. Concatenate the first num bytes of newchars to the end of w. You will have to use malloc to increase the number of bytes that w can store, and you might like to use memmove to move existing characters into your new memory. struct wrapstr *wrapstr_cat(struct wrapstr *w, char *newchars, int num) { Part (c) [3 marks] One day, we might like to turn a struct wrapstr back into a regular C string, with a \0 at the end. However, our struct wrapstr might have embedded \0 characters in it. How might you solve this problem? Page 5 of 16 cont d...

6 CSC 209H5S Final Exam APRIL 2014 Question 3. [14 marks] Consider the following struct: struct packet { unsigned short block_size; unsigned char *payload; struct packet *next; }; Part (a) [3 marks] Write function new_packet that allocates and returns a struct packet based on the function s parameters. struct packet *new_packet(unsigned short block_size, unsigned char *payload) { Part (b) [5 marks] old is a linked list of packets. Write the following function that makes a copy of the entire linked list. Nothing should be shared: new structs and payloads should be allocated so that the copy you make is completely independent of old. struct packet *copy_packets(struct packet *old) { Page 6 of 16 cont d...

7 APRIL 2014 Final Exam CSC 209H5S Part (c) [3 marks] fd is a file descriptor on which data can be written. Complete the following function to write a single struct packet (not a list of packets) to the fd. Return 0 on success, -1 on failure. int write_packet(int fd, struct packet *p) { Part (d) [3 marks] Imagine we write a linked list to a file, like in Assignment 2. When we later read the linked list back into memory, can we make use of the next pointers that we stored? Explain. Page 7 of 16 cont d...

8 CSC 209H5S Final Exam APRIL 2014 Question 4. [8 marks] The questions below concern this makefile and the programs it builds. xmodemserver : xmodemserver.o crc16.o gcc -Wall -g -o xmodemserver xmodemserver.o crc16.o client1 : client1.o crc16.o gcc -Wall -g -o client1 client1.o crc16.o xmodemserver.o : xmodemserver.c crc16.h xmodemserver.h gcc -Wall -g -c xmodemserver.c crc16.o : crc16.c crc16.h gcc -Wall -g -c crc16.c client1.o : client1.c crc16.h gcc -Wall -g -c client1.c clean : rm *.o out client1 xmodemserver Part (a) [2 marks] I run make clean, and then make. List the files that are created by make. Part (b) [2 marks] In the xmodemserver rule, explain why crc16.o is listed on the right of the colon. What could go wrong if it was not included? Part (c) [2 marks] Add a rule to the makefile that updates all files, runs the xmodem server in the background, then runs the client in the foreground and connects to the xmodem server. You can name your new rule whatever you wish. Part (d) [2 marks] crc16.c has no main function. How is it possible to compile crc16.c? Page 8 of 16 cont d...

9 APRIL 2014 Final Exam CSC 209H5S Question 5. [6 marks] When you logout of your shell, the shell might send the SIGHUP signal to the processes that the shell created. By default, SIGHUP, like SIGINT, terminates a process unless the process takes special action. (This means that a background process may not survive when you logoff, which can be annoying if you re trying to keep something running!) A Unix program called nohup solves this problem. From the shell, if I run: nohup xmodemserver -p 9999 & then xmodemserver is run in the background, and I can safely disconnect without the unfortunate side effect of killing xmodemserver. nohup ignores SIGHUP, redirects standard output to nohup.out, then arranges for the program on its commandline to be run with the provided arguments. Write nohup below. int main(int argc, char *argv[]) { Page 9 of 16 cont d...

10 CSC 209H5S Final Exam APRIL 2014 Question 6. Part (a) [4 marks] [7 marks] I described two other ways to monitor multiple file descriptors: using nonblocking reads and using fork. For each technique, give one disadvantage and one possible advantage compared to select. Part (b) [3 marks] What do we mean by signals don t queue? Give a scenario showing how signal semantics complicate signal-driven I/O. Page 10 of 16 cont d...

11 APRIL 2014 Final Exam CSC 209H5S Question 7. [6 marks] Here s some code that uses pipe. #include <stdlib.h> #include <stdio.h> #include <unistd.h> #include <sys/wait.h> int child_write(int fd) { write(fd, "secret", 6); exit(fd); } int child_read(int fd) { int got; char buf[50]; while ((got = read(fd, buf, 50)) > 0) { write(stdout_fileno, buf, got); } exit(fd); } int main(void) { int status; int fds[2]; pipe(fds); if (fork() == 0) { child_write(fds[1]); } else if (fork() == 0) { child_read(fds[0]); } wait(&status); printf("%d\n", WEXITSTATUS(status)); wait(&status); printf("%d\n", WEXITSTATUS(status)); return 0; } Part (a) [4 marks] The code doesn t work. Explain the problem, and carefully show the code to be added that will fix it. Do not change any of the existing code. Note that all syscalls will succeed; the fact that I haven t error-checked is not the problem. Part (b) [2 marks] Give one possible output of the (fixed) program. Page 11 of 16 cont d...

12 CSC 209H5S Final Exam APRIL 2014 Question 8. Part (a) [5 marks] [13 marks] fds is an array where each element is -1 (signifying empty) or a file descriptor open for reading. n is the number of elements in fds. Write the below function that calls select with all of the file descriptors in fds, and then returns the lowest-numbered file descriptor on which a read will not block. On error, return -1. int simple_select(int fds[], int n) { Part (b) [8 marks] In this part, you will write the body of a server that lets clients play a game. (A terrible game.) Assume that all setup has been done and that you have a global socket file descriptor listenfd that is in the listening state. Your server should first accept three clients. Then, send the message Go! to each client, and then wait for read action from one of the clients. The first client that sends anything to the server should get the message win, while the other two clients get Lose. Once there is a winner, all three clients are to be dropped, and then the server should wait for another batch of three clients. You can assume that no client will send anything before it gets the Go! message. You are required to use simple_select from the first part of this question. simple_select operates correctly when evaluating your server. The next page is blank for your server code. We will assume that Page 12 of 16 cont d...

13 APRIL 2014 Final Exam CSC 209H5S Please write your server code on this page. //accept(listenfd, (struct sockaddr *)&r, &socklen) //make sure you set socklen = sizeof(r) before calling accept int listenfd; int main(void) { struct sockaddr_in r; socklen_t socklen; setup(); //socket(), bind(), listen() done; listenfd is listening //Declare all variables and write code Page 13 of 16 cont d...

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