1. ecec 201 advanced programming for engineers
problem set 2
andrew r. cohen
due at end of lab during your assigned lab sessions
submission accepted until beginning of class next tuesday

2. ground rules
for this problem set, you can use vs code with your virtual box, or vs code with wsl (windows) or vs code native on linux. mac users can use vs code with gcc/gdb from the Xcode tools
in other words, you need vs code + gcc (and gdb). use whatever os / environment that works for you
use only the websites specifically referenced in this document, and your textbook. no google. no stack exchange.
no collaboration or discussion with *anyone* except Prof. Cohen or the TA’s. no exception.

3. know vs code
read this webpage
open your program helloWorld from last week (or create a new one)
using the vscode tools, find the definition of the printf function
try hovering over the printf command with your mouse, and verify that the definition pops up
put the cursor on the word ‘printf’ and press F12 (or right click and select ‘go to definition’)
Problem 0: what is the prototype (definition) for the printf function

4. return values
Question 1: what can you tell about the return value of the printf function from its prototype (definition)
from the vscode terminal, enter ‘man printf’
screenshot here
in the textbook find the appendices
what is the C operator symbol for the bitwise complement?
find the appendix listing the standard library functions
THIS IS SUPER USEFUL. BOOKMARK THIS!
look up the printf function in the textbook appendix defining stdlib functions
read appendix D (standard library functions) to familiarize yourself with the list – you will be expected to know these
Question 2: what is the return value of the printf function?

5. modify hello world
run the program and make sure it works as expected

6. breakpoints
THE MOST VALUABLE AND IMPORTANT WAY TO VERIFY THE CORRECTNESS OF YOUR CODE IS TO STEP THROUGH IN THE DEBUGGER LINE BY LINE!
code is not done if you haven’t touched every line

7. set a breakpoint in vs code
click on the line with the printf statement
from the debug menu, click ‘Toggle Breakpoint’
you must see a red dot appear on the line, to the left of the printf statement before you proceed!!!
click on the red dot with your mouse
verify it goes away
click again where the red dot was
to the left of the line number
verify the red dot shows up again
clicking like this is the usual way to set breakpoints

8. conditional breakpoints
right click on the red dot (breakpoint marker) and choose ‘edit breakpoint’
this is a very powerful way to control when your program breaks
for now, enter ‘1’ and run your program (F5)
next, right click again, edit breakpoint and enter ‘0’ for the condition
run the program again
Question 3: how does entering 0/1 for the breakpoint condition impact the program flow?

9. vs code debug tools set a breakpoint on the printf line
run the program (f5) till the breakpoint
on the debug tab, find the ‘locals’ window
Question 4: what is the value of the pszHello variable?
note 1 – this is a big deal. char *pszHello means that pszHello is a ‘pointer’ to a character. In other words, pszHello is a memory address. The contents of that memory address are a character.
note 2 – we’re allowing the precompiler here to allocate space for our pszHello string.
note 3 – more on this in chapters 11 and 17
Question 5: what is the value of the *pszHello character?

10. modifying variables set a breakpoint on the printf line and run to it
in the vscode locals window, expand the pszHello variable so the *pszHello value is shown
double click the ‘h’ (or right click and choose ‘modify’)
type ‘x’ instead
press F10 to step over the printf command
Question 6: screenshot

11. aha! vs code is lacking… here we have our first valid complaint
vs code for c/c++ is missing tools to effectively view and manipulate memory
the visual studio ide is far more sophisticated at this
visual studio allows you to view and modify variables in memory with far more ease and power then is possible here…
…but visual studio is heavy and closed source, so we’ll just keep on keepin’ on…
for more powerful debug tools, we can use gdb

12. debugging with gdb
from the terminal window in vs code (or from a stand-alone terminal)
enter ‘gdb [program name]’
e.g. gdb HelloWorld
then enter ‘run’
Question 6A: screenshot to show output

13. gdb see gdb.pdf on the website for details
enter ‘break 4’ ([4] use the linenumber of your program corresponding to the declaration of the pszHello variable)
enter ‘run’
you should see the program hit the breakpoint
enter ‘print pszHello’
enter ‘step’
Question 7: what is the value of pszHello before and after the line?

14. manipulating variables with gdb
run your program again, breaking at line the printf line
view the memory at address pszHello
run ‘x /20cb pszHello’
see gdb.pdf pp for details
screenshot
Question 8 : how does the printf command know when to stop printing characters for pszHello?
from the breakpoint, enter : set *((char *) pszHello)=‘y’
enter ‘continue’
redo this, but this time, modify the memory so the program prints ‘hello andy’
Question 9: screen shot

15. debugging and bash printf is one of the most useful debugging tools…
redirecting output to a file can be helpful as well
to do this, use the ‘>’ operator in the shell
from a vs code terminal, execute your program
note that the current folder is not on your path (by default), so you may need to execute ‘./helloWorld’
now redirect your output to a file
‘./helloWorld > hello.txt’
‘cat hello.txt’
from the console, enter ‘man tee’
the tee utility allows you to redirect to stdout and a file:
‘./helloWorld | tee helloTee.txt’
cat helloTee.txt
note the ‘|’ pipe operator – this redirects the output of a command to the stdin of another command
read pp in linuxfun.pdf
Question 10: write a shell command to make a sorted list of all the files in /etc that contain the string conf in their file name