15-213/18-213/15-513 Intro to Computer Systems: Frequently Asked Questions

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. You have two options:
    • Reextract the files from the tarball (preferred).
    • Run “chmod +x <filename>” to set the executable bit (risky—the file itself may be corrupted).
  • 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.
  • Use this form to notify us, and then check again in 24 hours. Ask for help via a (private) post to Piazza 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. Ask for help via a (private) post to Piazza 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?
  • Post questions on Piazza (for assignments, exams, and logistics issues).
  • 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, let staff know via a Piazza post.

• 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.

• How should I submit code when I need help?
  • DON'T post it on Piazza, send it by email, or send us a screenshot! Submit it via Autolab and notify us that you have done so.

• 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