CS61C Fall 2014 Lab 3 - C Pointers and GDB


This lab will introduce you to the C debugger gdb (and its cousin, cgdb). You will gain some practical experience using the gdb to debug a buggy C program, and then you'll write a few C programs that gdb will be helpful in debugging if you don't get it right on the first try.


Copy the contents of ~cs61c/labs/03 to a suitable location in your home directory.

$ cp -R ~cs61c/labs/03/ ~/lab03


Exercise 1: Debugger

For this exercise, you will find the GDB reference card useful. Compile hello.c with the "-g" flag:

$ gcc -g -o hello hello.c

This causes gcc to store information in the executable program for gdb to make sense of it. Now start the debugger, (c)gdb:

$ cgdb hello

If cgdb does not work, you can also use gdb to complete the following exercises (start gdb with gdb hello). The cgdb debugger is only installed on your cs61c-XXX accounts. Please use the hive machines or one of the computers in 271, 273, 275, or 277 soda to run cgdb, since our version of cgdb was built for Ubuntu.

Step through the whole program by:

  1. setting a breakpoint at main
  2. giving gdb's run command
  3. using gdb's single-step command

Type help from within gdb to find out the commands to do these things, or use the reference card.

cgdb vs gdb

In this exercise, we use cgdb to debug our programs. cgdb is identical to gdb, except it provides some extra nice features that make it more pleasant to use in practice. All of the commands on the reference sheet work in gdb. In cgdb, you can press ESC to go to the code window (top) and i to return to the command window (bottom) — similar to vim. The bottom command window is where you'll enter your gdb commands.

More gdb commands
Learning these commands will prove useful for the rest of this lab, and your C programming career in general. Create a text file containing answers to the following questions:

  1. How do you pass command line arguments to a program when using gdb?
  2. How do you set a breakpoint which only occurs when a set of conditions is true (e.g. when certain variables are a certain value)?
  3. How do you execute the next line of C code in the program after stopping at a breakpoint?
  4. If the next line of code is a function call, you'll execute the whole function call at once if you use your answer to #3. How do you tell GDB that you want to debug the code inside the function instead?
  5. How do you resume the program after stopping at a breakpoint?
  6. How can you see the value of a variable (or even an expression like 1+2) in gdb?
  7. How do you configure gdb so it prints the value of a variable after every step?
  8. How do you print a list of all variables and their values in the current function?
  9. How do you exit out of gdb?


Exercise 2: Debugging a buggy C program

You will now use your newly acquired gdb knowledge to debug a short C program! Consider the program ll_equal.c. Compile and run the program, and experiment with it. It will give you the following result:

$ gcc -g -o ll_equal ll_equal.c
$ ./ll_equal
equal test 1 result = 1
Segmentation fault

Now, start gdb on the program, following the instructions in exercise 1. Set a breakpoint in the ll_equal() function, and run the program. When the debugger returns at the breakpoint, step through the instructions in the function line by line, and examine the values of the variables. Pay attention to the pointers a and b in the function. Are they always pointed to the right address? Find the bug and fix it.


Exercise 3: Pointers and Structures in C

Here's one to help you in your interviews. In ll_cycle.c, complete the function ll_has_cycle() to implement the following algorithm for checking if a singly-linked list has a cycle.

  1. Start with two pointers at the head of the list. We'll call the first one tortoise and the second one hare.
  2. Advance hare by two nodes. If this is not possible because of a null pointer, we have found the end of the list, and therefore the list is acyclic.
  3. Advance tortoise by one node. (A null pointer check is unnecessary. Why?)
  4. If tortoise and hare point to the same node, the list is cyclic. Otherwise, go back to step 2.

After you have correctly implemented ll_has_cycle(), the program you get when you compile ll_cycle.c will tell you that ll_has_cycle() agrees with what the program expected it to output.


Exercise 4: Linear feedback shift register

In this exercise, you will implement a lfsr_calculate() function to compute the next iteration of a linear feedback shift register (LFSR). Applications that use LFSRs are: Digital TV, CDMA cellphones, Ethernet, USB 3.0, and more! This function will generate pseudo-random numbers using bitwise operators. For some more background, read the Wikipedia article on Linear feedback shift registers. In lfsr.c, fill in the function lfsr_calculate() so that it does the following:

Hardware diagram (see explanation below)

Explanation of the above diagram

After you have correctly implemented lfsr_calculate(), compile lfsr and run it. Your output should be similar to the following:

$ make
$ ./lfsr
My number is: 1
My number is: 5185
My number is: 38801
My number is: 52819
My number is: 21116
My number is: 54726
My number is: 26552
My number is: 46916
My number is: 41728
My number is: 26004
My number is: 62850
My number is: 40625
My number is: 647
My number is: 12837
My number is: 7043
My number is: 26003
My number is: 35845
My number is: 61398
My number is: 42863
My number is: 57133
My number is: 59156
My number is: 13312
My number is: 16285
 ... etc etc ... 
Got 65535 numbers before cycling!
Congratulations! It works!