15-213 Intro to Computer Systems: Frequently Asked Questions

Proxy Lab

How's proxylab graded?
- There will be no student demos.
- We will rely on a combination of autograder script results and code review.
- In some cases we will build and test your proxy by hand.
Why am I getting all these sig* warnings in csapp.c?
- You've probably added -std=c99 to your CFLAGS. Leave it out or set it to gnu99 instead.
What's the easiest way to get started testing my proxy?
- Have two terminal windows open, ssh to the same shark, start your proxy in one:
```
[myshark]$ ./proxy <portnum>
```
  and do the following in the other:
```
[myshark]$ export http_proxy=myshark:<portnum>
[myshark]$ wget -d www.cnn.com
```
  Note that the "-d" option provides debugging info with exact request/response headers.
- For fine-grain debugging, you can send headers from wget output line by line with nc:
```
[myshark]$ nc myshark <portnum>
GET http://www.cnn.com/ HTTP/1.0
...
```
I'm trying to test with nc. How do I generate \r\n?
- At the end of the line press Ctrl^V then enter to generate \r. Another enter will generate \n.

Cache Lab

Why are warnings like "unused parameter" being reported as errors?
- On some labs we have set flags (-Wall -Wextra) for the compiler to generate more warnings than usual. We also set a flag (-Werror) to treat warnings as errors.
- Any class of errors that can be detected by the compiler should be addressed sooner rather than later.
Why am I geting an error: implicit declaration of function 'getopt'?
- Reason: you are missing some header files in your include list.
- Fix: Add this to your other includes:
```
 #include  <getopt.h>
 #include  <stdlib.h>
 #include  <unistd.h>
 
```

General Programming Issues

When I try to run an executable file included in a lab handout, I get a "Permission denied" error. What should I do?
- The executable bit is not set. Run "chmod +x <filename>" to set the executable bit.
- This problem may occur if you extract the handout tarball on a Windows machine. Always extract the handout on a Linux machine.
How do I extract a tarball on a Linux machine?
- Run "tar -xvf <filename>" to extract it.
How do I avoid mixing tabs and spaces in my code?
- If using vim, add the following to your ~/.vimrc:
```
set expandtab
set tabstop=4
set shiftwidth=4
```
How do I make sure I don't have any non-ASCII characters in my code?
- Look at your code on autolab -- it will complain if you have non-ASCII characters.
- Run
```
 file mycode.c 
```
  You should expect to see: "ASCII C program text".
My code has non-ASCII characters, how do I find where they are?
- Try grepping for UTF-8 (common) characters in your code as follows:
```
grep --color='auto' -P -n "[\x80-\xFF]" mycode.c
```

General Course Issues

Why can't I access Autolab?
- Did you recently join the course? It is possible you do not yet have an account.
- Email the staff list 15-213-staff@cs.cmu.edu if you need an account created for you.
Why can't I access the shark machines?
- You should be able to access the shark machines with your Andrew credentials. Email the staff list 15-213-staff@cs.cmu.edu if you have trouble.
Must I work on the shark machines?
- Your work will be graded on the shark machines, so it is in your best interest to work there.
- For most labs, you may alternatively work on the Andrew Unix machines, which are nearly identical.
- You must complete bomblab and buflab on a shark machine.
Should I read the lab writeup before seeking help?
- Yes.
- Did we say, "Yes"?
- Yes, we did.
- Please read the lab writeups.
- :-)
How can I get help?
- Email us at 15-213-staff@cs.cmu.edu.
- Stop by during office hours.
- Schedule a 1:1 meeting with your instructors or TAs.
Must I attend a particular recitation?
- You may attend any recitation you wish as long as the instructor leading the recitation allows it.
- Please make sure the recitation you normally attend is the one specified on your Autolab account; if this is not the case, email the staff list.
I have some command line output that I'd like to send to the staff. Should I send it as a screenshot?
- No. Text (copy and paste) is fine.
My editor shows me that my C code looks fine, but when my TA prints it out it looks different!
- Your editor is configured to use tabs as a certain width. Please use spaces instead of tabs.
- To see what we'll see when we grade your submissions: run
```
a2ps -s2  --pretty-print --landscape --columns=2 --rows=1 --tabsize=4 --chars-per-line=80 somefile.c -o someotherfile.ps
```
- To replace all tabs with spaces, run
```
expand -t4 file > otherfile 
```