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