Part 1: SSH Keys to ieng6

Note: if you have a personal laptop you can set up ssh keys on it to be able to ssh to ieng6. If you're on windows, you’ll need a linux-like terminal, so use the terminal in VSCode, a program like git bash, or WSL for this.

If you don’t have a laptop or it isn’t set up with a linux terminal, work from your lab computer for today. You can set up ssh keys on your laptop at home on your own time.

  1. Generate a new pair of ssh keys (on your laptop, or on your lab computer)
  • cd to your .ssh directory, (if it doesn’t exist, first create it with mkdir ~/.ssh)
  • IF YOU ALREADY HAVE AN 'id_rsa', SKIP THIS STEP (otherwise you'll overwrite your github keys)
  • If you don't have an id_rsa, run: ssh-keygen -t rsa -b 4096 -C YOUR_EMAIL_ADDRESS
    • You'll be using the default filename and no password, so just press enter twice
  • Copy the contents of your new public key (you can print out the contents with cat)
  1. Copy your public key onto ieng6, into the file ~/.ssh/authorized_keys
  • On ieng6, in your .ssh directory, create a file called authorized_keys (or open it if it exists already)
  • Use vim to paste in the contents of the new public key into the authorized_keys file
    • (if your authorized_keys already existed, paste the new key on a new line at the end)
  1. Log out from ieng6 (use exit), and make sure you can ssh into it without typing your password

Part 2: Debugging with -Wall

Clone the repository we’ll be using for this lab (you’ll need to get the ssh URL). You’ll be working with one of the three programs, depending on your group: https://classroom.github.com/a/X-DdsPq-

Many common errors can be caught by the compiler, but a lot of these checks aren't enabled by default. We can ask the compiler to warn us by adding the -Wall flag ("- W(arn) all") when you compile:

$ gcc -Wall YOUR_CODE.c -o YOUR_CODE

Part 3: Debugging with gdb

Intro to gdb commands:

  • To use gdb, make sure you’ve compiled your program with “debug symbols”
$ gcc -g YOUR_CODE.c
  • Run your compiled code in gdb 
$ gdb YOUR_BINARY
  • This puts you into a gdb prompt: normal terminal commands don’t work here, and you can instead run gdb-specific commands
(gdb) run     # starts running your program
  • If your program stops running or segfaults in gdb, print out the backtrace (also called a “stack trace”)
(gdb) backtrace

or

(gdb) bt

Use the following commands to print out values at the point the program stopped:

  • (gdb) info locals
  • (gdb) info args
  • (gdb) print VALUE (or p VALUE)
    • You can print any variable or expression, e.g.
      • print x, p arr[5], p ((x & 0b1111) << 3)
    • You can also specify a format to print in
      • print/t (binary), print/x (hex), print/d (decimal)
  • (gdb) x ADDRESS
    • This prints out memory at an address, e.g. strings / arrays / pointers
  • (gdb) x/16cb str1
    • This prints the first 16 bytes of str1 as characters
  • (gdb) x/20xb str2
    • This prints 20 bytes of str2 in hex
  • (gdb) x/4dw arr
    • This prints 4 “words” (i.e. int32s) of arr, as decimal numbers
  • You can use the following reference card for reference on gdb commands, and format commands for x and print.

If you’re done early, use the reference card to try exploring other gdb commands!

Lab 3 Work Check-Off (due Monday, Oct 20):

In your copy of the lab3-buggy repo: fix the bugs in your assigned program, then commit and push your changes so they show up on GitHub.