Debugging

Debugging

methodical process of finding and reducing the number of bugs, or defects, in a computer program

Types of Bugs

• Bugs that occur during compilation time

• Bugs that occur during run time

What do Bugs occur during Compilation Time?

• Your program code is syntactically incorrect

• Your program violates common programming conventions, for example a variable is used before / without initialization

• Issues may appear as warnings

• Static analysis of your code detects that the program is invalid

What do Bugs occur during Run Time?

• Your program has a logical error

• It will still work but not as expected

Debugging Strategies

• Divide and conquer to debug a program:

  • Gradually remove/add code to create the smallest source file that contains the bug

• Approach 1: Remove code

  • Start with your code

  • Slowly remove code until program works well

  • Examine last removed lines

• Approach 2: Add code

  • Start with the smallest working program

  • Add functionality, until program breaks

  • Examine the last added lines

Debugging Using Logging (printf)

Insert printf in different places of the code to follow its flow, this approach is often helpful, but: 

• It generally takes a lot of printf statements

• Inclusion of printf could change the behaviour of the code (timing, stack, ...)

  • programs with printf may work, and then

  • the code fails when printf is removed (this is known as a ‘heisenbug’)

• This approach cannot examine program flow details,

  •  i.e., instruction-by-instruction inspection

Micro:bit and Printing on the Screen

You can use printf and scanf operations to print and read data using a serial/UART interface

• A communication interface between two computers

•  Transmits information sequentially one bit at a time

• You need a serial client to read and write data (i.e., screen)

Micro:bit - Sending Data through the Serial for Linux Systems

• Install the program “screen” if it is not already installed.

• Upload your code to your micro:bit device and open a terminal window

• Type ls /dev/ttyACM* to find out the device node that micro:bit has been assigned to. We are going to assume that the device node is /dev/ttyACM0

• Type screen /dev/ttyACM0 115200 to display the serial output of micro:bit on your screen. The value 115200 is the default baud rate (symbols/sec) of micro:bit

• To exit “screen”, press Ctrl-A and Ctrl-D. To return to “screen” type screen –r 

Micro:bit - Sending Data through the Serial for OSX Systems

Same as Linux but use /dev/cu.usbmodem* instead of /dev/ttyACM*

Micro:bit - Sending Data through the Serial for Windows Systems that use the Windows Subsystem for Linux (WSL)

• Open a terminal window as an administrator

• Install usbipd:

  • winget install --interactive --exact dorssel.usbipd-win 

• Type usbipd list to find out the bus IDs of currently connected USB devices. Let us assume that micro:bit uses bus 2-7

• Bind and then attach to the device through WSL:

  • usbipd bind –b 2-7 

  • usbipd attach –b 2-7 –w -a

• Open a WSL window as an administrator and follow the guidelines for Linux

Micro:bit - Debugging using Logging (printf)

What does a Debugger allow you to do?

• Step through a program (execute one instruction at a time)

• Set breakpoints (stop at checkpoints)

• Investigate machine state (memory, registers)

• Investigate crashes

What does a Debugger not do?

• Find problems for you (but it makes this job easier)

• Fix a problem (you have to do that…)

What Hardware support can Debuggers Require?

• In-System programming of logic devices, e.g., FPGAs (Field Programmable Gate Arrays) using Verilog or other Hardware Description Languages (HDLs)

• e.g., JTAG (Joint Test Action Group) to access debug interfaces

• Hardware support for code/data breakpoints (page fault)

What different Interfaces can Debuggers provide?

• Command line

• Graphical user interface (GUI)

The GNU DeBugger (GDB)

• Open-source debugger developed by the GNU project that also created the GNU Compiler Collection (GCC or gcc)

• Designed for the C language

• Command line interface but can be used with Integrated Development Environments (IDEs)

What are the Aims of GDB?

• Debugging C programs

• Understanding of system architecture

• Connection between hardware, assembler, C, applications

Debug Symbols

• Records associating code and variables with source code

• The flag –g, tells gcc to generate debug symbols

• To compile and then debug the program on this slide, type:

  • $ gcc -g SCC131_W10_Debug.cpp -o SCC131_W10_Debug

GDB - Example - Byte Order

Big Endian

the MSB is stored first

Little Endian

the LSB is stored first

Debugging Micro:bit

• Use DAPLink

• An On-Chip Debugger (OCD) allows remote debugging

• Integration with VS Code provides a powerful debugging environment