Debugging Runtime Errors

1
Debugging Runtime Errors

• COMP103 Lab 4

2
Errors

• Compilation
• Syntax errors
• Compilers usually catch them at compilation time
• Runtime
• Array index out of bound
• Memory referencing errors
• File accessing errors
• Compilers usually generate NO errors but error
  messages appear at runtime

3
Runtime Errors
include ltiostreamgt using namespace std void
main() int k k 4
Wheres the problem?
k is not pointing to a valid address
4
Compiler Warnings

• Sometimes the compiler will generate warnings to
  you!
• Dont ignore them!

include ltiostreamgt using namespace std void
main() int k k 4
--------------------Configuration debug - Win32
Debug-------------------- Compiling... main.cpp d
\temp\103\tutorial\debug\main.cpp(6) warning
C4700 local variable 'k' used without having
been initialized Linking... debug.exe - 0
error(s), 1 warning(s)
Warning Message
5
Debugger

• However, compiler is not always helpful
• MS Visual C has a built-in debugger
• Support to run the program until reaching a
  breakpoint
• Support to run the statements line by line

6
Debugger

• Steps to use debugger
• Identify the possible locations for the bug
• Add a breakpoint to those locations (more
  breakpoints could also be added while you are
  debugging)
• Run the debugger

7
Debugger

• Right click on the suspicious lines
• Choose Insert/Remove Breakpoint

8
Debugger

• Choose Build-gtStart Debug-gtGo

stop at breakpoint
Note This statement is not executed yet!
9
Debugger

• Run the codes
• To the next breakpoint (pressing F5)
• Line by line
• Jump into the function
• Leave the function

10
Debugger

• Point to the variable to see its current value
• Look at some variables in the variables window

11
Final Note

• Compiler and debugger are helpful
• BUT dont rely on them to catch all your program
  bugs
• The most effective way is to design and write
  your code with care