Solved-Lab 1- Solution

Description

• Introduction to the Labs

If you have already used Unix before then most of the introduction below should be familiar and you can skip all the bits which you already know. For example, if you have used sunfire (by connecting to sunfire.comp.nus.edu.sg), then the main di erence is that perhaps you may not have used a graphical user interface (GUI) with Unix.

1.1 Logging In

Your tutor will give you a user name and password for the machines in the lab which are running Ubuntu 16.04.1 (one of the Linux distributions).

You may also want to install Ubuntu on your own machine { the simplest is to run it in a Virtual Machine such as Virtual Box. The closest distribution to the labs is 16.04.5. See ubuntu-setup.pdf for a rough guide to install using Virtual Box on Windows.

1.2 User Interfaces

When you use Linux, there is the command line interface and also the graphical desktop interface. We will mostly talk about the command line since that is closer to the OS.

1.2.1 The Command Line Interface

Try starting a terminal window. The terminal you started is actually composed of two programs. One is a graphical application which gives a window which emulates a terminal. The other is a shell which interprets ASCII command lines from the terminal. The shell uses a command line interface, because it is text based. You type in commands to a shell which performs the command(s) in that line of text. In Windows, there is also a command line shell, cmd which has a similar purpose.

There are a number of possible shells available, bash (Bourne Again SHell) is the default and is a GNU enhancement descended from the original Unix shell, the Bourne shell (sh). A list of shells of typical shells in Unix are: sh, bash, csh, ksh, tcsh, zsh. (Not all these shells may be available). You can stick to the default choice of bash unless you feel like using another shell ;-)

1.3 Using man

You should try out the Unix manual command: man. For example, it is self documenting: (the ‘$’ is meant to be the command-line shell prompt, you may see something else)

$ man man

Other commands which you may have tried are also in the manual page, eg. man date.

Take note that Unix man pages are written in a terse fashion. It is meant as a reference rather than a tutorial. An example of a long man page is the one for gcc.

The manual is in sections, and an entry might be in more than one section, e.g. man 1 login documents the command line login program, and man 3 login documents the les which record the users of the system. You can search for keywords with the -k option, eg. man -k login.

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As the manual is divided into sections, the various sections have their own introduction. Try man intro or man 1 intro. System calls are described in man 2 intro. The introduction for library functions is in man 3 intro.

1.4 Listing Files and Directories

The ls command will list all the les in the speci ed directories. You can use the -l option for a long listing format which displays information like size, permissions, owner, group, creation date, etc. If you do not specify a directory, by default it uses the current directory. The -R option causes ls to list recursively. Some examples to try:

$ ls

• ls -l

• ls /

• ls -l /

1.5 Editing Files

You can use various text editors either directly from the desktop or the terminal. Some editors are strictly text based and some have a graphical interface. Many editors are available such as: emacs, vi, vim (gvim, graphical), gedit, etc. Not all such editors may be installed. If you are using your own Ubuntu, apt can be used to install various software (see also the ubuntu-setup document). It is hard to recommend a speci c editor but in general it is better to learn/use a more powerful editor.

1.6 Logging Out

After you have nished, you MUST log out from Linux. Make sure you have saved any les rst. Choose logout from the top right-hand icon.

1.7 Backup and Transferring Files

Please see some of the options in ubuntu-setup.pdf.

1.8 Some Things to Remember!

The following points are to be noted for Unix in general and also Linux:

Unix is case sensitive. Most commands are lowercase.

Unlike Windows, Unix has no drive letters. Everything is in some directory.

Unix uses forward slash (/) to separate directory names, while Windows uses backslash (\).

The * wildcard is treated uniformly by Unix shells. In Windows, * works di erently for di erent programs.

It is worthwhile to look rst at the man page of a command

Try using various GUI and command line programs. Some programs need administrator privileges (in Unix, this is the root user who has superuser privileges) to run.

Happy Linux-ing!

• Sample Usage Session

Skip if you already know how to use Unix.

1. mkdir junk | makes a new directory junk

2. cd junk | change directory to it, you are now in junk

3. pwd | prints the path to your current directory (Print Working Directory)

4. echo abcd > test1.txt | makes a new le test1.txt with contents \abcd”

5. less test1.txt | display test1.txt using the pager. ‘q’ will quit from less and ‘h’ will give the help screen.

Select an editor to use (see previous editing section).

1. edit the le test1.txt, eg. gedit test1.txt

2. change \abcd” to something else

3. save the le in the editor (this is editor speci c)

4. cat test1.txt | concatenate 1 or more les to the output, so this displays test1.txt because output is the terminal

5. cat test1.txt test1.txt >test2.txt | test1.txt is concatenated twice and the output saved to test2.txt

6. cp test1.txt test3.txt | copies test1.txt to test3.txt

7. ls | listing shows the 3 les

8. rm test1.txt | deletes test1.txt

9. ls | only test2.txt and test3.txt

10. rmdir ../junk | try to remove directory, complains about non-empty directory

11. rm test*.txt; cd .. ; rmdir junk | no more les in directory, go up to parent directory, remove junk directory

12. logout | you can use the System menu in the GUI

13. Login. Read the man pages of all of the above

• Lab 1: C Exercises

3.1 Purpose

These exercises are to gain familiarity with writing some simple C programs, compiling, execution and debugging. It is meant to be straightforward. Only Exercise 4 and 5 are graded and to be submitted. If you are familiar with C programming, you can skip to Exercise 4.

3.2 Exercise 1: Hello (not graded)

The purpose of this simple exercise is simply to be able to edit, modify and compile a simple program.

Modify the program lab1-hello.c so that It then prints \Hello” followed by n exclamation marks and a newline. Eg. if the number was 3, then your program would print out

Hello!!!

It already reads in n and prints \Hello”. You should rst compile and run lab1-hello.c before modifying it so that you know how to use the C compiler.

Notes: Notice how the input is read into n, i.e. how n is passed to scanf().

To compile and then run: (assumes no errors from gcc)

gcc -o lab1-hello lab1-hello.c

./lab1-hello

The \-o” option tells gcc to create the executable le lab1-ex1. You may not need the \./”, it will depend on your shell settings. (If you use bash the setting is in the PATH environment variable, but do not worry if you do not understand PATH for now).

3.3 Exercise 2: String Length (not graded)

The purpose of this exercise is to show some code using strings and pointers in C. Look at, lab1-len.c written in a common C style (C coding style and source code formatting is quite typical). You should understand what the code is doing. It uses arguments to the process, as covered in Unix Processes in lectures. Compile and test it.

3.4 Exercise 3: Mystery Function (not graded)

C has bit manipulation operators. Bit manipulation is useful for low level code which might have to look at the actual bit representation and change it. The purpose of this exercise is to look at bit manipulation and also play with recursion:

expr₁ << expr₂ : shifts expr₁ left by the number of bits given in expr₂. Shift one bit left means that the least signi cant (the rightmost) bit becomes the second least signi cant bit and with a 0 replacing that rightmost bit. Eg. (2 << 1) 10₂ << 1 becomes 4 (100₂). Excess bits which are shifted o to the left are discarded.

expr₁ >> expr₂ : shifts expr₁ right by the number of bits given in expr₂. For an unsigned integer, 0 bits are shifted in from the left to replace the high bits. For signed integers, the behavior of the sign bit (leftmost bit) is implementation dependent in C | 0 bits are shifted in but the leftmost sign bit may or may not be preserved.

(Notes: Java is similar but has 2 versions, sign bit preservation (>>) or all 0-bits (>>>)).

The C code below is for a mystery function:

unsigned int mystery(unsigned int n)

{

unsigned int x;

if (n == 0)

return 0;

if (n & 1) x = 2;

else x = 0;

return x + (mystery(n >> 1) << 1);

}

3.4.1 Part 3A: Separate Compilation

We rst investigate how to do separate compilation. Create a le, lab1-mystery.c, containing only the mystery function. (This tests your use of an editor). The main program is in lab1-ex3.c which reads an integer n and prints out mystery(n).

Note: Note the use of a function prototype in lab1-ex3.c:

extern unsigned int mystery(unsigned int);

This is because in the main program, mystery() is used but the code is not declared. The function prototype is used to declare its return value and arguments.

To compile and then run:

• gcc -c lab1-mystery.c

• gcc -c lab1-ex3.c

• gcc -o lab1-ex3 lab1-mystery.o lab1-ex3.o

The rst two commands are to use gcc to compile and create the object code (.o les) from your C les. The command uses gcc as the linker to combine the object les into an executable program, lab1-ex3. You can run the executable created.

3.4.2 Part 3B: Investigating Local Variables

You know that mystery is recursive therefore there are many instances of the local variable x for each invocation of the function. How can you modify the code to show that there are various instances of x when mystery is run?

Remark:

This exercise links up with our coverage of stack frames.

3.4.3 Part 3C: Exploring/Debugging Mystery

Here we want to rst understand how to use the debugger to examine the execution of a program. To use the debugger you should compile your program with the -g option, eg. gcc -g -c lab1-mystery.c . The -g option tells the compiler to produce extra debugging information into the object/executable le which is then used by the debugger. (Use steps similar to 3A with \-g” to create the lab1ex3 executable).

There are several debuggers available. We will use gdb the GNU debugger. To use the debugger, instead of running your program directly, you run it under the control of the debugger,

$ gdb lab1ex3

A partial list of debugger commands are: (abbreviations for the commands is also give)

help (h): help on various commands

run (r): run the program, you can pass arguments the usual way on the command line, eg. r arg1 arg2

print (p): print an expression, eg. p x prints the variable x in mystery() provided you are inside the function. you can print either be a variable or any expression.

break (b): set a breakpoint – a running program will execute until it comes to speci ed breakpoint when it stops and control goes back to the debugger. Breakpoints could be a line number or a function, eg. b mystery will stop execution whenever mystery() is entered into.

step (s): steps execution to the next source line. If the statement contains a function, then steps into the function.

next (n): like step but treats a function call like one instruction so doesn’t stop inside the function.

continue (c): continue running after breakpoint. contains a function, then steps into the function.

list (l): list source code, can specify line or function, eg. list main . Without arguments, lists source code around current line.

backtrace (bt): prints the stack frames (see the lecture in comporg), i.e. it shows you what functions have been called to reach this point including the arguments.

More documentation on gdb can be found with the info command, e.g. info gdb. (Note that relies on the particular info page being present).

Try the following: (commands typed are indicated by the boxed text)

1. gdb lab1ex3 : start gdb

2. b mystery : set a breakpoint in mystery(), note that the current source line in C is printed

3. r : runs lab1ex3

4. execution of lab1ex3 stops at start of mystery()

5. l : list program source

6. p n : prints argument to mystery()

7. bt : you can see that main() calls mystery()

8. step a few times

9. c : continue running, will stop at next call to mystery (assumes initial n is big enough)

10. continue for a few recursive calls and then do a bt . You can see clearly the recursion which is going on. This is another way of answering part 3B.

11. q : (q)uit the debugger and executing program

You should now have a feel of using the debugger. In general, a combination of output printing as well as using the debugger is useful for nding and xing bugs.

The debugger is also useful for examining memory including looking at the stack. In this exercise, using the debugger will be one way of understanding how the mystery() function works.

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3.5 Exercise 4: Basic Palindrome (Graded: 70% of mark)

A palindrome is a string which when reversed is the same. For example, \ABBA” is a palindrome as reversing it gives the same result. However, \ABBB” is not a palindrome as it’s is \BBBA”. A classic palindrome is: \able was i ere i saw elba”.¹ Your task is to write the function:

int check_palindrome(char *s);

It returns 1 if the string s is a palindrome and returns 0 otherwise. Write your function in the le lab1-checkpal.c. Note that the le should only contain the check_palindrome function. It is called from the le lab1-ex4.c which illustrates how to read from standard input (called stdin) in Unix. (A Unix program has a notion of 3 kinds of input-output streams: standard input (for reading input), standard output, standard error).

The lab1-ex4.c illustrates the use of malloc(int n) which requests n bytes of memory from the OS. This comes from the process heap which we will cover in the Memory Unit. For now, just consider it as a form of object creation.

Note that lab1-ex4.c is not very robust and assumes that the input is always less than 128 char-acters. Furthermore, it does not do any form of error checking, but the code is only meant to be test check_palindrome(). Intentionally, Exercise 4 is meant to be straightforward as a basic graded exercise to write code with C pointers and C strings.

3.6 Exercise 5: Extended Palindrome (Graded: 30% of mark)

Your task is to write the function:

int read_palindrome(); // input comes from stdin

which will read one line from standard input and returns 1 if the line is a palindrome and 0 otherwise. A line is terminated by the newline character (’\n’) and the does not include the newline. The le lab1-readpal.c contains your read_palindrome function which will be called by the main program in lab1-ex5.c.

An important di erence from Exercise 4 is that there is no assumption about the length of the input. You are also not allowed to read the input twice, e.g. read the input, forget you read the input but remember the length, read the input again. which results in the input being read twice. You are also not allowed to create a very large bu er to store the input reasoning that the input line might be expected to be smaller than a very large bu er. The reason for this restriction is that we will consider the memory usage of the program. So a constant sized large bu er means that memory is wasted if the line is short. Your program, lab1-readpal.c, will be graded on correctness, CPU time and memory usage. You can obtain runtime statistics on a process as follows:

$ /usr/bin/time –verbose ./a.out

This will print various (accounting) statistics about the process which is executing the a.out binary when it terminates. The memory statistic to use is \Maximum resident set size” in KB. Note that Exercise 5 is harder than Exercise 4, there are many implementation strategies. Given the criteria, you should do your own testing for CPU time and memory usage on long strings.

You should note that as your code is contained in read_palindrome and any other associated functions, it can be called from any main program. Our test code will use a di erent main from lab1-ex5.c.

¹The story is that when Napoleon Bonaparte was exiled to the island of Elba, he uttered \Able was I ere I saw Elba”. We have made the string to be lower-case to be a strict palindrome. Some de nitions of palindrome to cater to English ignore case, punctuation, etc. but our de nition is for strict palindromes.

• Submission Instructions

Please follow the submission intructions to make grading easier. Suppose your name is \Dennis Ritchie”² and your student ID is U123Y. Given a lename such as ex8.c, then you should rename it to ritchie-U123Y-ex8.c to make the lename associated with your own student information. The start of every C le should have a comment block giving the information below, e.g.:

/*

• Name: Dennis Ritchie

• ID: U123Y

• file: ex8.c

*/

Submit a single zip le (do not submit other le types, e.g. arj) using the above format, e.g. ritchie-U123Y-lab1.zip containing the two les for Exercise 4 and 5 with lenames in the same style. In addition, there should be a comment block in Exercise 5 (lab1-readpal.c) to explain how your code deals with the unknown line length. Be careful that Exercise 4 and 5 les should not have main.

²https://en.wikipedia.org/wiki/Dennis_Ritchie

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