Exercise 2 Intermediate C Program Development

NWEN 241 Exercise 2
(Intermediate C Program Development)

1 Objective
The objective of this exercise is to familiarise you with an intermediate C development
tool, and write and debug programs using structures and pointers.
At the end of this exercise, you should submit the required files to the Assessment System
(https://apps.ecs.vuw.ac.nz/submit/NWEN241/Exercise_2) on or before the
submission deadline. You may submit as many times as you like in order to improve your
mark before the final deadline. Submissions beyond the deadline will not be marked and
will receive 0 marks.
2 Exercise Requirements
For NWEN 241, it is highly recommended that you undertake all development using the
computers in CO246. The computers in this lab use the Linux operating system. This
guide is written with the assumption that you are in CO246 lab.
If you are not able to go the lab, you can remotely access similar computers via secure
shell (ssh). Consult one of the remote study guides (see https://ecs.wgtn.ac.nz/
Courses/NWEN241_2023T1/RemoteStudyGuide) and follow one that suits you the
most.
3 GDB
In the previous exercise, you have learned basic debugging by looking at compiler
messages. There are many cases where this form of debugging is not sufficient. For
instance, a program may compile without errors and warnings, but during runtime, it
crashes or behaves unexpectedly on certain inputs. This situation requires the use of a
more advanced tool.
The tool that you will learn in this exercise is the GNU Debugger or gdb. Below are the
key features of gdb:
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
• It can print stack traces, stop program at predefined breakpoints, or step through
line by line
• It allows us to see values of each variable at each line
Start by writing this simple C program:
1 #include <stdio.h>
2
3 int main(void)
4 {
5 int i = 4;
6 while (i >= 0) {
7 printf("20/%d=%d\n", i, 20/i);
8 i--;
9 }
10 }
Save the file as buggy.c and compile with the -g option as follows
gcc buggy.c -g -o buggy
The -g option allows the executable file to be run by gdb. Before using gdb, execute
buggy by typing
./buggy
on the terminal. The program will terminate abnormally with an exception:
As you can see, the exception message is not helpful at all. We will now use gdb to
investigate this issue. To invoke gdb to load the executable file buggy, just type
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
gdb ./buggy
on the terminal. You should see something like this on the terminal:
When you see the (gdb) prompt, that means gdb is ready to accept commands. You may
now run the program within gdb by typing run on the (gdb) prompt. You should see
something like this:
You can see the program output prior to the exception, and more importantly, the details
of the exception and the exact line that caused the exception.
From the above output, you should see that the problem (arithmetic exception) happens
at line 7. Since there is only one arithmetic operation in that line (the operation 20/i), you
are certain that this is the issue. In fact, the problem occurs when i becomes 0 causing
the operation 20/i to throw the arithmetic exception because division by 0 is illegal. One
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
possible fix is to change the condition in the while-loop on line 6 from i >= 0 to i > 0.
If you want to know more about the context of the exception, you can use the command
list. This command shows the code around the exception.
Let’s do more advanced stuff with gdb. Let’s begin with breakpoints. If you want to
insert a breakpoint at line 7, just type the command break 7.
To run the program again, just type run. Answer yes when gdb asks whether you want
to start from the beginning. You should see this:
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
At this point, program execution paused at line 7 because of the breakpoint. You probably
want to view the value of some variables in the program at this point. You can use the
print command to do this. For instance, to view the value of i in our program, just type
print i.
The output tells you that the current value of i is 4. To let the program run execute the
current line, you can use either the step or next command. There is a subtle different
between these commands:
• step will step over functions
• next will actually step into functions
In our case, if we use next, gdb will enter into the printf() function and execute the
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
function line by line. We don’t want this. What we want is for printf() to be executed
as a single line, i.e., we want to step over printf(). The appropriate command for this
case is therefore step.
As you can see, line 7 is executed, generating the output 20/4=5. The program is now
halted at line 8. Try using the print command to display the value of i. Since line 8 is
not yet executed, i should still be 4.
Note that using print to track a variable is cumbersome. A better way is to use the
watch command. To watch variable i, just type watch i. This command will cause
program execution to be paused everytime i is modified.
If instead of stepping you want the program to continue execution until the next
breakpoint, just use the command continue.
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
Note that program is paused after the change of variable i from 4 to 3 because of our
watchpoint on i. Type continue to proceed.
If you want to delete all breakpoints, use the command delete. To delete a specific
breakpoint, specify the number, for instance, to delete the first breakpoint, type the
command delete 1. Try doing this followed by continue.
Finish the debugging until the exception is encountered.
To quit gdb, just type q.
Below are other useful gdb commands:
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
Command Purpose
bt Show stack trace
until Run until end of current loop
finish Run until end of current function
info break List all breakpoints
clear <fun> Delete breakpoint set at <fun>
x <addr> Print content at <addr>
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
4 Exercises
Download a copy of the base source files used in the activities from
https://ecs.wgtn.ac.nz/foswiki/pub/Courses/NWEN241_2023T1/
Exercises/nwen241_exercise2_files.zip.
Activity 1: Using GDB [50 Marks]
Using your favorite text editor, open the base source file activity1.c which should
contain the following code:
1 #include <stdio.h>
2 #include <ctype.h>
3
4 void capitalize(char *str)
5 {
6 int i=0;
7 while(str[i] != '\0') {
8 if (islower(str[i])) str[i] = toupper(str[i]);
9 i++;
10 }
11 }
12
13 int main(void)
14 {
15 char *s = "ABC123 is the most common password";
16 capitalize(s);
17 printf("%s\n", s);
18 }
Compile and run the program. The program should crash, with the rather terse message
Segmentation fault.
Now, you will need to find out why the program is crashing. Recompile the program with
the -g option. Load the program in gdb and run. Which line does the program crash?
What is the value of i when the program crashes? Why to do you think the program
crashes?
Fix the program so that it will not crash. You are only allowed to change one line: line
15 to be exact. Submit the fixed program to the Assessment System for marking. Click
Run checks to initiate auto-marking.
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
Activity 2: Structure [15 Marks]
Using your favorite text editor, open the base source file activity2.c which should
contain the following code::
1 #include <stdio.h>
2
3 // Define structure record
4
5 // Implement print_record() function
6
7 int main(void)
8 {
9 struct record rec;
10
11 scanf("%s %d %f", rec.name, &rec.age, &rec.height);
12 print_record(rec);
13 return 0;
14 }
Study the source file and do the following within the file:
1. Define a structure named record with the following fields:
• name: a string variable that can hold 40 characters (including null terminator)
• age: a short integer
• height: a single precision floating point number
2. Implement a function named print_record() which accepts a single argument
of type struct record and does not return anything. The function should use
printf() to display the name, age and height fields. The output must follow
this format:
Name : name
Age : age
Height: height
Note that denotes a single space character. The height should be displayed with
precision of 2 decimal places. As your submission is going to be auto-marked, do
not add any extra character in the output, including ’\n’ after the last line.
Compile and run the program. If you are happy with the program, submit it to the
Assessment System for marking. Click Run checks to initiate auto-marking.
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
Activity 3: Passing Pointer to a Structure [10 Marks]
One disadvantage of the code in Activity 2 is that the entire structure is copied to the
function. This can be highly inefficient, especially when the structure being passed is
large. An approach to address this issue is to pass pointer to a structure.
In this activity, you will repeat Activity 2, except that you will be passing a pointer to a
structure in the function invocation. open the base source file activity3.c using your
favorite text editor. The file should contain the following code:
1 #include <stdio.h>
2
3 // Define structure record
4
5 // Implement print_record_ptr() function
6
7 int main(void)
8 {
9 struct record rec;
10
11 scanf("%s %d %f", rec.name, &rec.age, &rec.height);
12 print_record_ptr(&rec);
13 return 0;
14 }
Study the source file and do the following within the file:
1. Define a structure named record with the following fields:
• name: a string variable that can hold 40 characters (including null terminator)
• age: a short integer
• height: a single precision floating point number
2. Implement a function named print_record_ptr() which accepts a single
argument of type pointer to struct record and does not return anything. The
function should use printf() to display the name, age and height fields. The
output must follow this format:
Name : name
Age : age
Height: height
Note that denotes a single space character. The height should be displayed with
precision of 2 decimal places. As your submission is going to be auto-marked, do
not add any extra character in the output, including ’\n’ after the last line.
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
Compile and run the program. If you are happy with the program, submit it to the
Assessment System for marking. Click Run checks to initiate auto-marking.
Activity 4: Pointers [25 Marks]
In this activity, you will use pointers to traverse an array and access array elements. Begin
by opening the base source file activity4.c using your favorite text editor. The file
should contain the following code:
1 #include <stdio.h>
2
3 #define MAX 100
4
5 // Implement find_max() function using pointer
6 int *find_max(int *a, int alen)
7 {
8 // These variables should be enough. Do not declare any other
variable.
9 int *max = a, *p = a;
10
11 // Use pointer p to iterate over arrays and access arrays elements
12 // Use max to point to the latest maximum value found
13 // Use either while-loop or for-loop to iterate
14
15
16 // max should point to the maximum
17 return max;
18 }
19
20 int main(void)
21 {
22 int n, array[MAX];
23 scanf("%d", &n);
24 for(int i=0; i<n; i++)
25 scanf("%d", array+i);
26 int *max = find_max(array, n);
27 printf("%d", *max);
28 return 0;
29 }
Study the source file and implement the find_max() function using pointers. The
function accepts two input arguments: the first argument is a pointer to the first element
of the array, and the second argument specifies the number of elements in the array. The
function must return a pointer to the element with maximum value.
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NWEN 241 2023-T1 (Intermediate C Program Development) Exercise 2
Compile and run the program. If you are happy with the program, submit it to the
Assessment System for marking. Click Run checks to initiate auto-marking.
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