The document discusses debugging techniques for finding and removing logical errors from programs. It describes common debugging techniques like print statements, core dump analysis, and execution monitoring. It then demonstrates how to use a debugger called DDD with a sample C program, including how to set breakpoints and conditional breakpoints to pause execution under certain conditions. The document stresses that prevention through writing clear, simple code is better than debugging, and provides some tips to aim for simplicity.

1. C
a series of presentations to help a bunch of
brilliant space scientists understand a
brilliant programming language

2. Debugging

3. Debugging
It is the process of finding and removing “bugs”
(logical errors) from a program

4. bluntly put …
… debugging is an art

5. depends on …
•Skill of programmer
•Complexity of the program
•Simplicity of the programming language
•Availability of debugging tools

6. Techniques
Print / Trace
It involves adding a ‘printf’ statement at various locations to
inspect the value stored in the variables
Core dump Analysis
Analyzing the memory snapshot of the program after the crash.
The operating system dumps the entire contents of the memory
related to the program into a core dump file. Used to detect
memory related bugs in your program.
Execution monitoring
Step by step execution of the program to analyze the execution
path and change in control or data.

7. Debugger
Debuggers are tools that enable
the programmer to monitor the
memory and/or execution of the
program.

8. Demonstration
Data Display Debugger (DDD)
•DDD is a well known C debugger in the Linux platform.
•It’s a GUI based front end for GNU Debugger (gdb) program.

9. Demonstration
//simple_loop.c
1. Compile the file using
#include <stdio.h> the –g flag in gcc
int main(void)
{
int i = 0; $> gcc –g simple_loop.c
printf("Entering Loopn");
for(i=0;i<10;i++)
{
printf("%d ",i);
} 2. Load ‘a.out’ in DDD
printf("Exiting Loopn");
return 0; $> ddd a.out &
}
-g flag stores extra information regarding the program which is required for the debugger

10. Demonstration
Data Window
Control
Panel
Source Code

11. Control Panel
Start the program
Step program until it reaches
a different source line
Step program, proceeding
through subroutine calls
Execute until source line
reaches greater than current
… rest for homework :P

12. Breakpoint
A breakpoint informs the debugger
to freeze execution at a chosen line.
A programmer can then evaluate all
the variables and stack of the
program.
Demonstration: setting a breakpoint in DDD

13. Breakpoint
1. Right click on the line you
wish to set the breakpoint
2. Select ‘Set Breakpoint’

14. Conditional Breakpoints
A breakpoint that informs the debugger
to freeze execution at a chosen line
when a desired condition is met .
Demonstration: setting a conditional breakpoint in DDD

15. Conditional Breakpoint
1. Right click on the new
breakpoint
2. Select ‘Properties’

16. Conditional Breakpoint
1. Set your required
condition in the
‘Condition’ text box
2. Click ‘Apply’

17. Conditional Breakpoints
//complex_loop.c
#include <stdio.h> How do I stop the
int main(void) execution when
{
int i = 0;
int j = 0; i = 0
int k = 0;
for(i = 0; i < 10; i++) j = 10
{
for(j = 0; j < 10000; j++)
{ k = 3000
for(k = 0; k < 10000; k++)
{
printf("%d ",i+j+k);
}
}
}
return 0;
}

18. Conditional Breakpoints

19. Conditional Breakpoints
//file_read.c
input.dat
#include <stdio.h>
#include <string.h> id animal
int main(void) 1 CAT
{ 2 SHEEP
FILE *input_fp = NULL;
char buff[255]; 3 WOLF
int id = 0; 4 DOG
char animal[16];
input_fp = fopen("input.dat","r"); 5 MONKEY
6 EAGLE
while(fgets(buff,sizeof(buff)-1,input_fp) != NULL)
{ 7 MAN
sscanf(buff,"%d %s %*[^n]",&id,animal);
printf("Animal at pos %d is %sn",id,animal);
}
fclose(input_fp);
return 0;
}
How do I stop the
id = 4
execution when

20. Conditional Breakpoints

21. Conditional Breakpoints
//file_read.c
input.dat
#include <stdio.h>
#include <string.h> id animal
int main(void) 1 CAT
{ 2 SHEEP
FILE *input_fp = NULL;
char buff[255]; 3 WOLF
int id = 0; 4 DOG
char animal[16];
input_fp = fopen("input.dat","r"); 5 MONKEY
6 EAGLE
while(fgets(buff,sizeof(buff)-1,input_fp) != NULL)
{ 7 MAN
sscanf(buff,"%d %s %*[^n]",&id,animal);
printf("Animal at pos %d is %sn",id,animal);
}
fclose(input_fp);
return 0;
}
How do I stop the
animal = MAN
execution when

22. Conditional Breakpoints

23. Final Words
Prevention is better than cure
Always strive for simplicity in your code. A good measure of
simplicity lies in the number of instructions used to get the
job done. Lesser the better.
Less conditional statements (if, else if , else)
More repeatable statements (loops, function recursion )
Less nested statements (loop in a loop, nested ifs)
Learn and be aware of standard library functions
Don’t re-invent the wheel, many open source library are
available to solve various problems in the scientific domain

24. what next?
File I/O Data types
Standard & structures
C Library
Pointers & Memory
Allocation Debugging
Macro and Version
Pre Processor Management
Multi file
projects

25. A man who wants to lead an orchestra
must turn his back on the crowd
- Max Lucado
thank you