CS 31: Weekly Lab: Week 1

Goals for this week:

  1. Set up our Swarthmore GitHub accounts.
  2. Practice compiling and running C programs: gcc and using make.
  3. Learn the basics of printing variables in the GDB debugger.
  4. Introduction to Lab 1 and C syntax.

One-time git / GitHub configuration

Before using Swarthmore's GitHub Enterprise, you'll need to complete some one time setup steps.

We'll come back to git again later, after we go over a few examples.

Lab examples

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           ...
       /                     \
     labs                 weeklylab
    /                     /       \
   ...                 week01    week02   ...
                  Files you'll copy over

Let's start by creating this directory structure and copying over some files:

    cd                              # cd with no argument switches to your home directory
    mkdir cs31                      # create a 'cs31' directory
    cd cs31
    mkdir labs weeklylab            # create a 'weeklylab' directory
    cd weeklylab
    mkdir week01                    # create a 'week01' directory
    cd week01
    pwd                             # print the current directory name

    # Copy some example files
    cp ~kwebb/public/cs31/week01/* .
    Makefile  README  printtypes.c  testprog.c  testprog_nocoments.c

C code: compiling and running

(For more info, refer to the basics of compiling and running C programs.)

C is a compiled language. Compiling is a process that translates C language code text to a machine code program that the system knows how to execute (the computer doesn't understand high-level languages like C or Python or Java, instead it understands low-level machine code). If compilation succeeds (there are no syntax errors in your code), the compiler creates an executable file named a.out that you can run.

We'll use the gnu compiler, gcc, to translate C to an executable form:

  gcc testprog.c

Then enter the path name of executable file on the command line to run it:


gcc supports command line options to include debug information in the executable file (-g) and to specify the name of the executable file (-o filename) rather than use the default "a.out". Let's try it out on one of the files you copied over:

  gcc -g -o testprog testprog.c

Along with the code you copied over, there's 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

To clean up all the files that were compiled by make, you can run make clean:

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

C code, C types, and printf

The files testprog.c and printtypes.c have some examples of printf format strings for printing out values of different types. Use it as a reference for printing out values in the lab assignment. Below is just a little info about printf and C types.

Let's open testprog.c and look for examples the following things:

Now let's compile and run the program:


C has many different types for storing integer values. These types differ by the number of bytes they use to store values, and by whether or not they store both positive and negative values.

1 byte: 2 bytes: 4 bytes: 4 or 8 bytes:
(depends on architecture)
8 bytes:
char short int long long long
unsigned char unsigned short unsigned int unsigned long unsigned long long

When you allocate a variable of a specific type, you get a storage location of the appropriate number of bytes associated with that variable name:

  int x;             // 4 bytes of storage space to store a signed integer value
  unsigned char ch;  // 1 byte of storage space to store an unsigned integer value

C functions

The syntax for functions in C is similar to that in Python, except that C function definitions must define the return type of the function and type and name of each parameter.

  // sum: a function that computes the sum of two values
  //  x, y: two int parameters (the values to add)
  //  returns: an int value (the sum of its 2 parameter values)
int sum(int x, int y) {
   int z;      // a local variable declaration
   z = x + y;  // an assignment statment
   return z;   // return the value of the expression z

// a function that does not return a value has return type void
void blah( ) {
   printf("this function is called just for its side effects\n");

int main() {
   int p;      // local variable declaration

   p = sum(7, 12);   // call to function that returns an int value
   printf("%d\n", p);
   blah();           // call to void function
   return 0;

Try out:

Try adding a simple function to testprog.c, something like sum above, then make a call to in from the main function. Compile testprog.c and try running it to see what happens.

printf formatted output

printf uses placeholders for specifying how a value should be printed (how its series of bytes be interpreted). See printtypes.c for examples. Here is a brief summary:

### Specifying the numeric representation:
   %d: print out value in decimal  (base 10)
   %u: print out value in unsigned decimal  (base 10)
   %x: print out value in hexidecimal  (base 16)
   %o: print out value in octal (base 8)
   (there is no formatting option to display the value in binary)

### Specifying the type:
   %d:   int     (to print numeric values of int, short, and char args)
   %ld:  long int
   %lld: long long int
   %u:   unsigned
   %lu:  long unsigned
   %llu: long long unsigned
   %p:   an address value
   %f:   float or double
   %lf:  double
   %e:   float or double in scientific notation
   %g:   float in either %e or %f format
   %c:   char (ex. 'x')
   %s:   string  (ex.  "hello there")

### Specifying field width:
   %5d: print out the value in decimal in a field of width 5
   %-5d: print out the value in decimal in a field of width 5, left justified
   %6.4f: print out a float in a field with of 6 with a precision of 4

C debugger: gdb

The GNU debugger, gdb, is the C debugger we will use in this class. Usually, we will use gdb to debug a program, but this week we are going to use gdb as calculator.

gdb's print command can be used to print the value of a expression in different representations (binary, decimal, hex); you can use it as a simple calculator to verify answers to hex, binary, and decimal arithmetic. For this use of gdb, we don't have to have an executable to run gdb on. We can just run gdb, and then call its print command:

$ gdb
# print an expression in different representations:
# (/t in binary, /x  in hexidecimal, default is decimal):
(gdb) print/t 1234       # print/t: print decimal value 1234 in binary format
                         # p is shorthand for print
(gdb) p/x 1234           # p/x: print value in hexidecimal format
(gdb) p/d 1234           # p/d: print value in decimal format (the default)

# 0x is the prefix for a hexidecimal literals
# 0b is the prefix for a binary literals
# no prefix:  for decimal literals
(gdb) p 0xabf1           # print the hex value abf1 as decimal
(gdb) p 0b0101           # print the binary value 0101 as decimal
(gdb) p/t 0x1234         # print the hex value 0x1234 as binary
                         # (note: leading 0's are not printed out)
(gdb) p/d 0b1010         # print the binary value 01010 as decimal
(gdb) p/x  0b10100000    # print a binary value in hex
(gdb) p/t 0b101001001 + 0xa2  # add a binary and a hex value, print result in binary

# you can re-cast a value as a specific C type:
(gdb) p/t (char)(12)    # tell gdb that the value 12 is a char (1 byte) and print as binary
(gdb) p/t (char)(-12)   # print -12 char value (1 byte) in binary

Lab 1 Intro

Next, we'll take a look at the first lab assignment.