Commodore Free 30/Commodore_Free_Issue_30_2009_Commodore_Computer_Club.d64/c part 1.1

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Alternative Programming Languages: C
Part 1 By Paul Davis


In the first article of this series we
explored some of the capabilities of
the Forth language. Forth has many
unusual characteristics that make it
well suited to programming on the
Commodore, but it's a language that
polarises opinion. Some people love
it, some hate it, & some just can't
seem to wrap their head around it. So
this time we're going to look at a
more mainstream language, C.

C is a compiled language. It's not
like BASIC or Forth where there's an
interpreter you can type commands into
& experiment with. Program code is
entered into text files which are then
run through a compiler to translate
the source code into an executable
program. This makes programming in C
a bit more involved than you may be
used to. Nevertheless, C is a very
powerful & flexible language & is
well worth adding to your programming
repertoire.

Although there are several
implementations of C that run on the
Commodore, the lack of memory & poor
disk drive performance makes writing
programs a laborious process. With
that in mind, it provides a good
opportunity to introduce the concept
of cross-development. That is, using a
modern computer to edit & compile
our programs into a form that can be
run either in an emulator or on a real
Commodore machine.

As was the case with the Forth
article, this is not a complete
tutorial. It's merely an intro to
some of the features & programming
style of the language to give you an
idea of how it can be used. I'm
going to follow a similar format as
before & present some example programs
with a brief commentary on how they
work.

Getting started
We're going to be using a freeware C
compiler called cc65 which is
available in executable form for
Windows, & as source code that can be
compiled for OS X & Linux. The cc65
website can be found at:

 http://www.cc65.org/

We will also be using the VICE
emulator to test our programs. If you
don't already have this, it can be
downloaded from:

http://www.viceteam.org/

The first step is to download &
install cc65. Point your browser or
favourite FTP client at the download
area:

 ftp://ftp.musoftware.de/pub/uz/cc65/

then follow the relevant instructions
for your OS below.

Windows XP & Vista
Download the files called
cc65-win32-2.12.0-1.zip &
cc65-c64-2.12.0-1.zip. Next, create a
directory where you want to install
the compiler. This can be anywhere you
like, although I would recommend
avoiding the 'Program Files' directory
(especially on Vista) or directories
with spaces in their name. This
article will assume the directory is
C:\cc65. To install the program,
extract the zip files you downloaded
into the cc65 directory you have just
created.

Before we can use the compiler we need
to set up our programming environment
at the command prompt. Open a command
prompt window (hold down the Windows
key, press R then type 'cmd' & press
Enter) & type the following
commands:

  path %path%;c:\cc65\bin
  set CC65_INC=c:\cc65\include
  set CC65_LIB=c:\cc65\lib

These instructions tell the command
shell where to find the cc65 programs
and tell cc65 where to find various
files that are needed to compile a
program.

You will need to enter these commands
each time you open a new command
prompt window. If you want the
settings to be permanent, here's how
to do it. Press Windows + Break (or
right-click on 'My Computer' & choose
'Properties') to bring up the System
Properties dialog. On Vista select
'Advanced system settings' from the
task panel on the left & confirm the
UAC dialog. In the System Properties
dialog click on the 'Advanced' tab,
then on the 'Environment Variables'
button. In the bottom panel for system
variables click on 'New' then enter

 'CC65_INC' for the name &

 'c:\cc65\include' for the value

 & click 'OK'. Do the same for the

 'CC65_LIB'

 variable with the value 'c:\cc65\lib'

Next, scroll down the list to find the
entry for 'Path' & double click it.
Click on the value field & move the
cursor to the end of the value then
add ';c:\cc65\bin' to the end & click
'OK'. Click on the'OK' button of the
Environment Variable & System
Properties dialogs to complete the
settings.

Finally, we need to create a directory
within the cc65 directory to store the
files we will create during this
tutorial.

  cd c:\cc65
  mkdir tut
  cd tut

OS X & Linux
To keep these instructions as short as
possible I'm going to describe the
procedure for OS X & assume Linux
users will know how to deal with any
differences.

First, to compile cc65 we need to
install the Xcode developer tools.
Insert the OS X installation DVD that
was shipped with your Mac & follow the
links to install Xcode.

Next, download the file called
cc65-sources-2.12.0.tar.bz2 from the
cc65 web site. You will not need to
download any of the other files.
Assuming the file has been saved to
your 'Downloads' directory, open a new
Finder window & navigate to that
directory. Double-click on the
cc65-sources file & the Archive
Utility will extract it, creating a
directory called 'cc65-2.12.0' in the
Downloads directory.

Now we need to work at a command line,
so navigate to Applications then
Utilities & double-click the Terminal
program.

Enter this command to change the
current directory to the cc65
directory in Downloads:

  cd /Downloads/cc65-2.12.0

Now type this command to build the
cc65 program suite:

  make -f make/gcc.mak

This will take a minute or two to run.
When the command prompt returns enter
these lines:

  sudo mkdir /usr/local
  sudo make -f make/gcc.mak install

You will probably need to enter the
administrator password to run those
commands. They will install cc65 into
/usr/local/lib/cc65 & the
executables into /usr/local/bin.

At the end of the installation, there
will be a message listing two
variables that need to be set up. We
will do this in a minute, but first we
need to create a work directory to
store the files that will be created
during this tutorial. Enter these
lines:

  cd /Documents
  mkdir cc65
  cd cc65

To make setting up the environment
variables a bit easier we will create
a script to do it for us. Enter this
command:

  xed -xc init.sh

The Xcode editor will start up &
show a blank document. Enter the
following lines
into the file:

  export CC65_INC=/usr/local/lib/
   cc65/include
  export CC65_LIB=/usr/local/lib/
   cc65/lib
  alias edit='xed -xc'

Press Cmd+W & confirm the dialog to
save the file. Finally, enter this
command in the terminal window to run
our script:

  . init.sh

That's a dot & a space before the
init.sh. This command will need to be
run in any new terminal window when
you want to use cc65.

Creating your first program
Because C is a compiled language, the
program code is entered into plain
text files. These files should have a
'.c' extension. The cc65 compiler is
then used to translate the source code
file into an executable program.

To keep things as simple as possible
we will use the standard tools
provided by the operating system. On
Windows we will use Notepad for
entering the source code. On OS X we
will be using the Xcode editor which
you have already seen. If you are
using Linux replace 'notepad' or
'edit' in the instructions below with
your favourite text file editor.
Windows users enter this command:

  notepad first.c

Notepad will ask if you want to create
a new file, answer yes. OS X users
enter this command:

  edit first.c

Now type in (or copy & paste) the
program below. To get the # on a Mac
with a UK keyboard press alt/option &
3.

  // My first C program

  #include <stdio.h>

  void main(void)

 +
      puts("It works!");
 !

Press Ctrl+S on Windows or Cmd+S on OS
X to save the file. Now flip back to
the command prompt/terminal window.
Windows users enter the command:

  dir

OS X or Linux users enter the command:

  ls -l

The directory should show our
'first.c' file. Now we need to compile
this into a C64 program. To do this,
enter the command:

  cl65 first.c

If all goes well, the command prompt
should return with no other output
(cc65 follows the Unix philosophy of
'no news is good news'). If you get an
error message, swap back to the editor
window, make any necessary corrections
save the file, then enter the cl65
command again at the command prompt.

Once your program has compiled without
errors, enter the 'dir' (or 'ls')
command again and you should see a
couple of new files, 'first.o' and
'first'. The 'first.o' file is an
intermediary file created during the
compilation. The file called 'first'
is our C64 program.

Next, load up x64, the VICE C64
emulator. Select the option called
'Autostart' or 'Smart attach' in the
'File' menu. On Windows navigate to
the c:\cc65\tut directory & select
'All files (*.*)' as the file type
filter. OS X users navigate to the
Documents/cc65 directory. Now double
click on the file called 'first' &
our program should load & run,
displaying 'It works!' on the screen.

That is, in a nutshell, the process
for creating programs in C. This
'edit, compile, run' sequence of
actions will soon become 2nd nature.

Okay, let's take a look at the program
code in more detail. The first thing
to notice is that the code is written
predominantly in lower case. C is case
sensitive & all of its built-in
keywords must be entered in lower
case. The first line in the program is
a comment line. The double slash
characters mark the start of a comment
& all text after the // up to the end
of the line is ignored. C also
supports comments that span multiple
lines. All text between /* and */
markers will be ignored.

Skip past the #include line for now,
we will come back to this in a moment.

C programs are broken down into
separate named blocks of code called
'functions'. Each function can take
any number of parameters & can
optionally return a single value. The
next line in the program creates a
function called 'main'. The word
'void' before the function name
declares the type of the return value,
in this case, void because we are not
going to return a value from the
function. The parameters of a function
are listed after its name inside
brackets. Again, 'void' here means the
function takes no parameters.

Typically, many programs follow the
convention of using lower case for
function names, often with multiple
words separated by underscores. For
example, 'move', 'fire' or
'show_high_scores'. Sometimes you
may see programs or libraries that use
mixed case function names such as
'NewList' & 'AddListitem'. These are
not enforced by the compiler, they are
just conventions used to make names
consistent & readable.

The function called 'main' serves a
special purpose in C. It is auto-
matically called when the program is
run & therefore marks the starting
point of the program. When all the
instructions in the main function have
completed, the program will exit &
return back to BASIC. Every C program
must have a 'main' function.

The body of a function is contained
within curly braces. This is the way C
groups together lines of code into
discrete blocks. Our 'main' function
only contains one instruction that
calls another function named 'puts'.
The brackets contain the arguments
passed to that function, in this
example the string "It works!"
delimited by double quotes. The 'puts'
function is one of many standard
functions defined by the C language &
is used to print out a string. Notice
that the instruction is followed by a
semi-colon. This must be used to mark
the end of every separate instruction
in C.

Before a function can be called, C
needs to have been previously told
about its name, what parameters it
takes & what type of value it returns.
The C compiler uses this info to check
that the arguments you pass to the
function match up with the parameters
it expects. C is quite strict in this
regard. For example, it would not
allow you to pass a number to the
'puts' function which expects a string
So, how does the compiler already know
about 'puts' in our example program?

The answer lies with the #include line
at the start of the program. This
instruction reads in another file
containing declarations of functions
that are in the standard library, one
of which is 'puts'. These files are
called 'header' files & have the
extension '.h'. The one used here is
'stdio.h', short for 'standard input
and output'. It contains functions
used to read & write data to files &
other devices including the screen.

The core C language itself is very
simple, consisting only of a few
keywords for declaring functions &
variables, testing conditions, looping
& arithmetic operations. The rest of
the features are provided by libraries
of functions. The standard C language
provides a good number of library
functions for many purposes including
string, file & memory handling. The
cc65 compiler provides some additional
libraries & you can also create your
own libraries of often-used functions.

Variables
Now let's try using variables in C.
Close the 'first.c' file & create a
new file called 'var.c' at the command
prompt:

  notepad var.c

Enter the following program into this
file:

 #include <stdio.h>

 void main(void)

 +
  int n = 1000;
  char c = 'X';
  char s[] = "Commodore";
  int a[5] = 1, 10, 100, 1000, 10000;!
  int i;

  printf("n = %d\n", n);
  printf("c = %c\n", c);
  printf("s = %s\n", s);
  printf("a =\n");

  for (i = 0; i < 5; ++i)
 +
      printf("%5d\n", a[i]);
  !
 !

Save the file & compile it at the
command prompt using the cl65 command
as
before:

  cl65 var.c

When the program has compiled
correctly, switch back to VICE. You
could run the program using the same
auto-load method as before but,
because VICE has now set up device 8
to point to our tutorial directory
during the last auto-load, you can
also enter the familiar load command
directly into the C64 emulation:

  LOAD "VAR",8
  RUN

The program should display the values
of the variables. 'n' is a number
variable, 'c' is a character, 's' is a
string & 'a' is an array of numbers.
These short variable names have been
chosen so the code fits in the
narrow columns of the magazine.
Normally, variables would have longer,
more descriptive names!

Variables declared inside a function
must be positioned at the start of the
code block, that is, after the opening
curly brace before any other inst-
ructions. These variables will be
'local' to the function. In other
words, they are only accessible by
that function & only exist temporarily
while the function is running. You may
also create global variables in C by
declaring them outside of any
function. We will see an example of
global variables later.

Variable declarations follow this
general format:

  type name = value;

All variables in C must be declared to
be a specific type. The type dictates
what kind of data can be stored, the
limit on its values & how much memory
the variable takes up. The most
commonly used types in cc65 are:

  char - characters or whole numbers
   from -128 to 127
  int - whole numbers from -32768 to
   32767
  long - whole numbers up to +/-
   2,147,483,647

These are 'signed' types because they
allow both positive & negative
numbers. C also supports 'unsigned'
types that allow larger positive-only
values:

  unsigned char - numbers from 0 to
   255
  unsigned int - numbers from 0 to
   65535
  unsigned long - numbers up to
   4,294,967,295

The variable name can be any comb-
ination of letters, numbers & under-
scores (although it can't start with a
number). Conventionally, variables
have lower case names, often with
words separated by underscores. For
example, 'level', 'score',
'high_score' etc.

A variable doesn't have to be assigned
an initial value. Any variable without
an explicit value will contain random
garbage. To avoid errors in your
programs it's usually a good idea to
give variables an explicit initial
value.

Arrays
C has no built-in type for strings,
although it does allow a literal
string to be created by putting it in
double quotes. In C, a string is
simply considered to be an array of
characters.

Arrays are declared by putting the
size of the array in square brackets
after the variable name. In the
example program, the variable 's' is
declared with empty brackets so the
array will be as long as it needs to
be to fit the string.

The variable 'a' is declared as length
5 & has 5 values assigned to it.
Notice the use of curly braces again
for grouping these values together.

To access an element in an array, the
index number of that element is put in
square brackets after the variable
name. Index numbers always start at
zero & go up to the array length -1.
In the example program, we could
access the first element of the
array with a[0] & the last element
with a[4]. Often, we would use some
kind of loop to iterate through all
the elements in an array & use a
variable as the index.

        CONTINUED IN PART 1.2