Lab Exercises: C && gdb

Getting started with Systems programming
  [~]$ cd ~/cs31/weeklylab
  [weeklylab]$ mkdir week02 && cd week02
  [week02]$ cp ~adanner/public/cs31/week02/* ./
  [week02]$ ls
  arrays.c  Makefile  testprog.c  types_scanf.c

Goals for this week:

  1. Practice compiling and running C programs: gcc and using make
  2. Practice with scanf, arrays, and the absolute basics of memory addresses in C
  3. Introduction to Lab 2, running the starting point code, using the readfile library.
  4. Learn some basic gdb commands. We won't have time to learn all of 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 2. Otherwise, use printf debug statements to debug your code. We will make further use of gdb, and learn more about it, throughout the semester.

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
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 convenient 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 (they can be rebuilt)

Arrays and scanf

In the code you copied over are some examples of scanf, and of arrays.
  1. We are going to start by looking at arrays.c, which contains examples of how to use (and abuse) arrays in C.
  2. Then, we are going to look at the types_scanf.c example, and try running to see what is happening.
  3. Talking about scanf() will lead naturally to an introduction to pointers in C. Later in the semester, we'll talk about the basics of how to define them and use them. We'll also talk about the relationship between pointers and arrays.

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.

If you use gdb to help you debug the lab this week, you may need to know how to use it for programs that take command line arguments. For such programs, simply list the arguments after the run command:

(gdb) break main
(gdb) run 6 4 hello

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

Lab 2 Intro

In Lab 2, you will be writing some C code to perform a sorting task.