15-213 Intro to Computer Systems: Frequently Asked Questions

15-513 Specific Questions: 15-513 FAQ

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.

• I have trouble working on the shark machines because I'm uncomfortable using the shell or available editors.
  • Consider attending the Linux Boot Camp at the beginning of the semester.
  • Consult the quick-reference sheets posted on the Resources page.

• 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 am working from a Windows machine. How do I connect to the Shark machines?
  • Use the Cisco VPN client if you are off-campus: Cisco VPN client. There is a known issue with connection stability if you do not use the VPN client: your connection will randomly freeze or drop.
  • Use the CMU recommended Windows SSH/SFTP client: Tectia SSH/SFTP . If you need a lightweight SSH client you can use PuTTY instead: PuTTY
  • Avoid X-Windows (X-Term, XEmacs etc.) and the WIndows AFS client if you do not have experience with them. Instead open multiple Tectia SSH/SCP windows and use text-based editors on the Shark machine (emacs, vim or nano).
  • There are other nice tools like a Windows AFS client (OpenAFS), X-Windows support (X-Win 32) and the Linux-on-Windows port (cygwin. They are powerful tools in the hands of experts but often very painful to set up and can be the source of a lot of frustration and painful data loss when not used well. If you have no experience with them, leave the finger from them.

• 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