CS 31: Weekly Lab: Week 1

Create a weeklylab subdirectory in your cs31 directory. In this directory you will create subdirectories for each week, into which you will copy files that we will work on in lab. This week you will create a week01 subdirectory. Your directory structure for cs31 will look something like this: 
              /
              |   
         --------------
        /     |        \
     bin     home      usr ...
              |
         ---------------
        /     |         \
    ...      you      ...
              |
       ----------------------
      /         |            \ 
   ...         cs31           ...
                |
        -------------------------
       /          |              \
     inclass     labs          weeklylab           
    /            /             /       \  
   01 ...       01  ...      week01    week02   ...
                             /
                        files you copy over
If you have not already done so, first run update31 to create the cs31/inclass/01 subdirectory and copy over files from lecture this week and files refered to in the "Intro to C for 31 Students" document off week 1 of the class schedule: 
    update31
Next, create the cs31/weeklylab/week01 directory structure and copy over some files into your week01 directory: 
 
    cd
    cd cs31
    mkdir weeklylab
    cd weeklylab
    mkdir week01
    cd week01
    pwd
    cp ~newhall/public/cs31/week01/* .
    ls
    Makefile  arrays.c  structs.c  testprog.c  types_scanf.c

Goals for this week:

  1. Practice compiling and running C programs: gcc and using make
  2. Practice with structs and arrays
  3. Introduction to Lab 1, running the starting point code, using the readfile library.
  4. If time, learn some basic gdb commands. We won't have time to learn gdb this week. However, there is some information about it on this page, and you should feel free to try out the C debugger, gdb, if you'd like on lab 1. Otherwise, use printf debug statments to debug your code. Next week in lab we will learn gdb to use for the lab 2 assignment.

C compiling and running

First, let's remind ourselves of the basics of compiling and running C programs.

Now, let's try it out on one of the files you copied over (we will use the -g and -o outfile options to gcc): 

gcc -g -o testprog testprog.c
./testprog
With the code you copied over is a Makefile. In this file are rules for compiling executables from the .c source files that are executed by typing in the make command. make is very convinent way to compile without having to type in a long gcc command every time, you just need to type make: 
make         # this will compile all files as specified by the all: rule

make clean   # this removes all files generated by make (the can be rebuilt)

Stucts, Arrays and scanf

In the code you copied over are some examples of scanf, and of arrays, structs and arrays of structs.
  1. We are going start by just looking at the scanf example, and try running to see what is happening.
  2. Next we are going to look at the structs example, and write and test out a lot of code here.
  3. The arrays.c file contains the solution to the swap function we implemented in lecture today. You can try it out on your own.

GDB intro

gdb is the gnu debugger for C and C++ programs. Over the course of the semester will will use gdb and learn more and more features. Today, we will learn just a few basics.

To use the debugger, you usually want to compile your C program with the -g flag to add debugging information to the a.out file (this allows gdb to map machine code to C program code that the programmer understands). 

$ gcc -g -o testprog testprog.c
Actually, the makefile already has this rule for us, so let's just run make Next, we will run the executable file inside the gdb debugger: 
$ gdb ./testprog
The first thing we get is the gdb prompt (our program has not yet started). Typically we will set a break point at main. A breakpoint tells gdb to grab control at a certain point in the execution, in this case right before the first instruction in main is executed: 
(gdb) break main
Next, we will enter the run command at the gdb prompt to tell gdb to start running our program: 
(gdb) run
The run command will start your program running, and gdb will only gain control again when a breakpoint is hit.

There are a few other main gdb commands we will learn today, the first is next (or just n for next), which tells gdb to execute the next instruction and then grab control again: 

(gdb) next    # this will execute the instruction x = 10
The list command lists the C source code around the point where we are in the execution: 
(gdb) list
list with a line number lists the source code around that line: 
(gdb) list 30
cont tells gdb to let the program continue running. Since we have no more breakpoints it will run until termination.

Now lets add a breakpoint in the function mystery, and rerun the program: 

(gdb) break mystery
Lets set a breakpoint at line 20, right before the call to mystery, then type cont to continue execution from breakpoint in main : 
(gdb) break 20
(gdb) cont
(gdb) list
We can use the print command to print out expressions in the program, so let's print out the values of the arguments passed to mystery, and type cont to run until the next break point is hit: 
(gdb) print a    # print out the value of the variable a
(gdb) print (a - 4) # print out the value of the expression (a - 4)
(gdb) list

The where or bt command list the call stack: 

(gdb) where
Lets step through some of the mystery function's execution, and print out some of its parameters and locals.

When you are all done using gdb, type the command quit.

Now let's just try running commandargs in gdb, to see how to run this program: 

gdb ./commandargs
For programs that take command line arguments, list the arguments after the run command: 
(gdb) break main
(gdb) run 6 4 hello
Examine some run-time state of this program, and then quit gdb

We will talk more about C and gdb over the course of the semester.
Off my "Unix and CS Info Pages and Links" page are some C programming and gdb references that will be useful this semester:

Lab 1 Intro

Go back to the course schedule and select the lab 1 link from this week, and we will discuss lab 1: CS21 class schedule