U.C. Berkeley Spring 2008, CS61c

Lab1: Simple C and Number Representation







These lab exercises are intended to show you how to run a C program on the EECS instructional computers, to introduce you to the gdb debugger, and to get you thinking about the internal representations of numbers.


  • K&R: 1.1-1.5
  • P&H: 3.1-3.2


You need to have an EECS instructional account (cs61c-*) in order to do your lab work. Most importantly, you will need to log in to this account in order to enroll in the class. Don't forget to change your password (passwd) and finger information (chfn) once you have logged in.



Copy the contents of ~cs61c/labs/01 to a suitable location in your home directory.

$ mkdir ~/lab
$ gcp -R ~cs61c/labs/01/ ~/lab

Exercise 1: Simple C program

Fill in the blank in the following C program, also in output0.c, so that its output is a line containing 0. Don't change anything else in the program.

#include <stdio.h>
int main ( ) {
  int n;
  n = _____;
  printf ("%c\n", n);
  return 0;

To verify your solution, compile it and run the resulting binary:

$ gcc -c output0.c
$ gcc output0.o -o output0
$ ./output0


Show your TA how you run output0. 

Exercise 2: Debugger

Compile your solution to exercise 1 with the "-g" option. This causes gcc to store information in the executable program for gdb to make sense of it. Then single-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.


Set the breakpoint at main, and show your TA how you run upto that breakpoint. 
Show your TA how you single step a program. 

Exercise 3: Octal Dump

The program mysteryout apparently produces a blank line as output when it is executed. Find out what it really prints using the od (octal dump) command. Running "man od" will give you information on how it works. Print the output as hexadecimal numbers (hex). You will learn more about hex numbers in next lecture. Hint: The output is a sequence of 5 bytes. Therefore, it is best to look at the output byte by byte using -t switch of od.


Show the output of mysteryout in hexadecimal numbers. 
Show the output of mysteryout in decimal numbers. 

Exercise 4: The Biggest Integer

In Friday's class, we will discuss number representation. In particular, we are going to discuss unsigned integers and two's complement, the almost ubiquitous format for signed integers. Look at biggestInt.c. You may wish to read through the comments but at this point it is not critical that you understand exactly how the program works. Basically, it is a C program that will tell you some useful information about certain C data types. It does this by exploiting the fact that C does not check for overflow and wrap around conditions. Compile and run the program and answer the following questions:

  1. The first piece of information output by the program tells you the value of the most significant bit (MSB) of an unsigned int on your terminal. Based on this information, How many bits does the C data type unsigned int have?
  2. The second piece of information tells you the value of the largest positive signed long on your terminal. Based on this information, how many bits does the C data type long have?
  3. The third piece of information tells you the value of the most negative signed int on your machine. Based on this information, do the unsigned int and signed int have the same number of bits?
  4. 2's Complement number representations have one more negative number than they do positive numbers. That is, the most negative number does not have a positive counter-part. The final piece of information printed is the signed value of what you get when you try to negate the most negative value. Can you explain why this happens?


Answer to question 1  
Answer to question 2  
Answer to question 3  
Answer to question 4