McGill University School of Computer Science COMP-206A Introduction to Software Systems

Transcription

1 McGill University School of Computer Science COMP-206A Introduction to Software Systems Final Exam Wednesday, December 8, 2004 at 9:00 in GYM Instructor: Joseph Vybihal Associate Examiner: Prof. Ratzer Student Name: Student ID: Instructions No notebooks, calculators or textbooks permitted in this exam. Language translation dictionaries are permitted. You are permitted to write your answers in either English or French. Attempt all questions. Parts marks are given for all questions. Show your work and do not leave a question empty. Write all answers in the exam booklet provided. Grading Section Grade Your Mark Question 1 15 Question 2 15 Question 3 15 Question 4 10 Question 5 10 Question 6 10 Question 7 15 Question Total 100 % Instructor: Joseph Vybihal Page 1 of 9

2 Question 1: String Manipulation in C You are a C programmer jealous of Perl s string manipulation functions. You want to create your own string manipulation functions but you d also like to one-up your features as compared to Perl s. To do this you want your functions to be accessible from the Unix command-line prompt. You also want your program to manipulate either strings or text files. To do this, the program s interface will be through the Unix command-line and it will have the following syntax: $ substitution expression switch argument Where: substitution is the name of your C program (i.e. the a.out file name) expression is a quoted simple regular expression switch is either S or F. S signifies that the argument will be a quoted string. F signifies that argument will be a file name To simplify this problem a little assume that quoted arguments from the command line are read in as a unit into a single cell of the argv array. To simplify the problem a little more assume that legal regular expressions are limited to the following syntax: /search/replace/. There is no g for global nor i for ignore case. The search argument is what we are looking for and it will be replaced by the replace argument. Only the first occurrence of the match is replaced. The program is case sensitive. The only valid search code is /d which is interpreted as /d+. For example: $ substitution /bob/mary/ S this is an example for bob this is an example for mary $ substitution /bob/mary/ F letter.txt Dear mary, bla bla bla Notice in either case the substitution program displays the replaced data. Note: The program should do error checking if the arguments at the command-line are wrong. The program should do file existence error checking as well. The program has a special case syntax where the user can enter: $ substitution H That displays the syntax of the command-line invocation, it then terminates. Instructor: Joseph Vybihal Page 2 of 9

3 Question 2: PERL Programming with Regular Expressions Write a Perl program that opens a file called FILENAME.TXT and reads in each line of the file one by one. It then outputs the file with a line number at the beginning of each line and converts every long date into a short date. Note that the line number should be four digits, e.g. line 64 should look like You should get something like: 0001 Dear Mom, 0002 School is great! 0003 Can t wait to get home for the holidays. Etc A long date is considered to be something like: January 1, A short date is considered to be: 1 Jan 04. The months can be anything from January to December while the days and years are assumed to be correct (i.e. for example, you do not need to verify that the number of days obeys rules like months have at most 31 days). The program outputs the converted FILENAME.TXT into another file called CONVERTED.TXT. There is no interface to a CGI file. Question 3: HTML, CGI and PERL This question asks you to write an advanced login script. You must provide the HTML and CGI code for the web page. The web page will call a PERL script to validate the login (you must provide this code as well). If the user name and password match the user sees WELCOME otherwise it returns the user to the login page. The HTML page is called LOGIN.HTML. The PERL source file is called LOGIN.PL. The login page has two text boxes: one for the user name and one for the password. The password input field does not show the text entered by the user. The page also has a checkbox called ENCRYPTION. When checked the PERL program assumes that the password text file is encrypted using a Caesar cipher. A Caesar cipher is a simple cipher that shifts the characters by a fixed number. In our case this will be the integer number 2. For example: DOG = FQI, ZAP = BCR. The password text file is called PASSWORD.TXT. The web page must have a gray background with blue writing. At the top is a heading saying WELCOME. At the bottom of the page are links to HOTMAIL and MCGILL s main web site with text indicating this (you can make up these links if you do not know them). Centered in the middle of the screen is your CGI interface. The McGill logo (logo.gif) is displayed beside the WELCOME message. Instructor: Joseph Vybihal Page 3 of 9

4 Question 4: Profiling and Timing Look at the following program: 1. #include <stdio.h> 2. #include <stdlib.h> 3. #include <ctype.h> 4. #define WORDLEN #define WORDMAX char words[wordmax]{wordlen+1]; 7. int get_word(char *str) { 8. int ch, I =0; 9. do { 10. ch = getchar(); 11. } while ((ch!= EOF) &&!isalpha(ch)); 12. if (ch == EOF) return 1; 13. while (ch!= EOF && isalpha(ch) && I<WORDLEN) { 14. str[i++] = tolower(ch); 15. ch = getchar(); 16. } 17. str[i] = \0 ; 18. return I; 19. } 20. int compare(char *s1, char *s2) { 21. while ((*s1!= \0 ) && (*s1 == *s2)) { 22. s1++; s2++; } 23. if (*s1 == *s2) return 0; 24. if (*s1 > *s2) return 1; 25. return 1; 26. } 27. int main (void) { 28. int max = 0, length, I, j, total; 29. for(i=0; I<WORDMAX; I++) { 30. length = get_word(words[i]); 31. if (length == -1) break; 32. max = (length > max)? length : max; } 33. total = I; 34. for (I=0; I<tital-1; I++) { 35. if (words[i][0]!= \0 ) { 36. for (j=i+1; j<total; j++) { 37. if (compare(words[i],words[j]) == 0) 38. words[j][0] = \0 ; 39. }}} 40. for(i=total-1; I>=0; I--) { 41. if (words[i][0]!= \0 ) 42. printf( %s\n,words[i]); } 43. printf( %d\n, total); 44. printf( %d\n, max); 45. return EXIT_SUCCESS; } Answer the following questions: 1. What does this program do? Where is this program slow and why? 2. Suggest locations where you would put your own timing code to verify the slow areas in the above code. Also provide actual example code. Instructor: Joseph Vybihal Page 4 of 9

5 Question 5: GNU Profiling The importance of profiling becomes evident when creating critical software that is called often or is processing large amounts of data. One way to optimize your program is to look at the gprof profile, to identify functions for optimization. Below is the gprof profile generated from a fairly large program. By looking at it, how can you optimize this program and why? called/total parents index %time self descendents called+self name index called/total children <spontaneous> [1] main [1] /1 FFT3D [2] /128 my_image_write [4] /1 shuff_wedge [5] /1 contrast_handler [6] /1 gamma_correction [7] /1 find_scalar [8] /1 composite_multi [9] /74719 malloc [21] /1 histogram [26] /1 read_image [31] /128 sprintf [44] /395 fprintf [30] /33050 free [29] /1 main [1] [2] FFT3D [2] / FFT [3] /74719 malloc [21] /33050 free [29] /3 Powerof2 [70] / FFT3D [2] [3] FFT [3] /128 main [1] [4] my_image_write [4] /128 write_image [10] /128 free_image [28] /74719 malloc [21] /1 main [1] Instructor: Joseph Vybihal Page 5 of 9

6 [5] shuff_wedge [5] /395 fprintf [30] /1 main [1] [6] contrast_handler [6] /1 main [1] [7] gamma_correction [7] /1 main [1] [8] find_scalar [8] /395 fprintf [30] /1 main [1] [9] composite_multi [9] /12606 random [52] /1 srandomdev [54] /128 my_image_write [4] [10] write_image [10] /32768 fwrite [12] /130 fopen [24] /130 fclose [33] /395 fprintf [30] Instructor: Joseph Vybihal Page 6 of 9

7 Question 6: Debugging and Logic Error Answer the following questions about GDB: Assume a programmer wants to write a C program that strips all the blank spaces from the end of an array. It does this by putting a NULL character after the last nonblank character in the array. It is assumed that the array already stores a string, but there may be a number of blanks before the NULL. Below is this programmer s code. Note it is basically correct but there is more than one error in the code: 1. void strip (char *string) { 2. char *cp = string; 3. while (cp!= \0 ) 4. cp++; 5. do { 6. cp--; 7. } while ((cp > string) && (*cp == )); 8. if (*cp!= \0 ) 9. (cp+1) = \0 ; 10. else 11. string = \0 ; 12. } Now answer the following: 1. Identify the errors by studying the code and suggest corrections. 2. How would the program run with those errors present? Give a short description of the execution of the program given each error. 3. Assume you were this programmer and you were unable to identify the errors on your own so you decided to use gdb to help you. Give a line-by-line commandline example of how you would have used gdb to isolate where the errors where and what information lead to you determining the problem and solution. To do this start from the Unix prompt showing what command you would have used to compile the program, the command used to invoke gdb and then each gdb command with comments on the right-hand side describing why you entered each command. Instructor: Joseph Vybihal Page 7 of 9

8 Question 7: The Make Program A student has the following programming practises: There is always one.h file and one.c file (of the same name) in a module. For example, a module called mod would have mod.h and mod.c ; The corresponding.h file is always included in the.c file. The file will also include any other module it needs plus the standard C includes (e.g. <stdio.h>, etc.). The.h files may include other.h files. This student is currently writing, for her assignment, a program of 4 modules: m1, m2, m3 and main. She would like to name her executable ass3. Here are the include directives found in the four.h files: #include "m2.h" #include "m3.h" #include "m3.h" #include "m1.h" #include m2.h #include "m1.h" #include "m3.h" m1.h m2.h m3.h main.h In the meantime, this student does not use any tool to manage program compilation. The output of ls l is as follows: total 8 -rw student 734 Nov 17 10:02 m1.c -rw student 203 Nov 2 11:39 m1.h -rw student 2823 Nov 17 10:03 m1.o -rw student 425 Nov 16 20:21 m2.c -rw student 220 Nov 2 11:39 m2.h -rw student 2213 Nov 13 12:24 m2.o -rw student 323 Nov 16 22:38 m3.c -rw student 212 Nov 8 11:39 m3.h -rw student 2533 Nov 16 22:40 m3.o -rw student 3213 Nov 17 12:27 main.c -rw student 243 Nov 8 11:39 main.h -rw student Nov 10 11:39 main.o -rwx student Nov 10 11:39 ass3* From the above information, answer the following questions: 1. Write C pre-processor directives to insure that a header file is not included twice. 2. The student decides to use the make tool to facilitate project management. Write a makefile for her that gives an executable of her program when make is issued. The makefile must always work in the optimal way (i.e. compiles only the files that require compilation) 3. Now the student does a make with her makefile. What will be the output of ls l after this make? Giving only the filenames and times would suffice. (Current time is assumed to be Nov 17 14:00) 4. Adjust the make file so that it backs up all the source files to a backup directory before compilation. Instructor: Joseph Vybihal Page 8 of 9

9 Question 8: Source Management with RCS You are the team leader for a new project being proposed by the organization you are working for. You will be in charge of yourself and four other programmers. The organization is preparing a Specification Document and they have assigned to you the portion of the document referring to how the software will be optimally developed using RCS. Besides the four programmers who will need to access the software as writers of code, you will access the software as someone who will build the initial framework for each module of the program as well as test the code developed by your programmers and demo the program to management. In addition, management will want to have access to only the executable versions of the program for independent distribution of working versions of the program. The software under development is for an outside client. This client will have a contact person who will work closely with you providing you will internal company procedures and officiating at demos of the program. This individual will want to be able to see but not edit the developing code. This individual would also like to be able to get executable versions of the program for their own testing purposes. They would like to be able to do this independently. The following additional criteria are important to you: the code is managed in a secure environment, maintaining all versions of the program in recoverable form, want to insure that a single program is not undergoing multiple simultaneous revisions, and you want to see a log of all the changes in the file. Outline what you would suggest, with examples, should be put into this specification document. I am not asking you to write a paragraph but outline in bullet form what you would do. Instructor: Joseph Vybihal Page 9 of 9