Structure of C Programs

---

Objectives

Having completed this section you should know about:

1. C's character set



2. C's keywords



3. the general structure of a C program



4. that all C statement must end in a ;



5. that C is a free format language



6. all C programs us header files that contain standard library
   functions.

---

C's Character Set

C does not use, nor requires the use of, every character found on
a modern computer keyboard. The only characters required by the C Programming Language are as follows:

• A - Z



• a -z



• 0 - 9



• space . , : ; ' $ "



• # % & ! _ {} [] () < > |



• + - / * =

The use of most of this set of characters will be discussed
throughout the course.

---

The form of a C Program

All C programs will consist of at least one function, but it is
usual (when your experience grows) to write a C program that
comprises several functions. The only function that has to be
present is the function called main. For more
advanced programs the main function will act as a controlling function calling other functions in their turn to do
the dirty work! The main function is the first
function that is called when your program executes.

C makes use of only 32 keywords

which combine with the formal syntax to the form the C
programming language. Note that all keywords are written in lower
case - C, like UNIX, uses upper and lowercase text to mean
different things. If you are not sure what to use then always use
lowercase text in writing your C programs. A keyword may not be
used for any other purposes. For example, you cannot have a
variable called auto.

---

The layout of C Programs

The general form of a C program is as follows (don't worry about
what everything means at the moment - things will be explained
later):

pre-processor directives
global declarations
main()
{
   local variables to function main ;
   statements associated with function main ;
}
f1()
{
   local variables to function 1 ;
   statements associated with function 1 ;
}
f2()
{
   local variables to function f2 ;
   statements associated with function 2 ;
}
.
.
.
etc

Note the use of the bracket set () and {}. () are used in
conjunction with function names whereas {} are used as to delimit
the C statements that are associated with that function. Also
note the semicolon - yes it is there, but you might have missed
it! a semicolon (;) is used to terminate C statements. C is a
free format language and long statements can be continued,
without truncation, onto the next line. The semicolon informs the
C compiler that the end of the statement has been reached. Free
format also means that you can add as many spaces as you like to
improve the look of your programs.

A very common mistake made by everyone, who is new to the C
programming language, is to miss off the semicolon. The C
compiler will concatenate the various lines of the program
together and then tries to understand them - which it will not be
able to do. The error message produced by the compiler will
relate to a line of you program which could be some distance from
the initial mistake.

---

Pre-processor Directives

C is a small language but provides the programmer with all the
tools to be able to write powerful programs. Some people don't
like C because it is too primitive! Look again at the set of href="cccckey.html">keywords that comprises the C language
and see if you can find a command that allows you to print to the
computer's screen the result of, say, a simple calculation. Don't
look too hard because it doesn't exist.

It would be very tedious, for all of us, if every time we
wanted to communicate with the computer we all had to write our
own output functions. Fortunately, we do not have to. C uses
libraries of standard functions which are included when we build
our programs. For the novice C programmer one of the many
questions always asked is does a function already exist for
what I want to do? Only experience will help here but we do
include a function listing as part of this course.

All programs you will write will need to communicate to the
outside world - I don't think I can think of a program that
doesn't need to tell someone an answer. So all our C programs
will need at least one of C's standard libraries which deals with
standard inputting and outputting of data. This library is called
stdin.h and it is declared in our programs before
the main function. The .h extension
indicates that this is a header file.

I have already mentioned that C is a free format language and
that you can layout your programs how you want to using as much
white space as you like. The only exception are statements
associated with the pre-processor.

All pre-processor directives begin with a # and the must start
in the first column. The commonest directive to all C programs
is:

#include <stdio.h>

Note the use of the angle brackets (< and >) around the
header's name. These indicate that the header file is to be
looked for on the system disk which stores the rest of the C
program application. Some text books will show the above
statement as follows:

#include "stdio.h"

The double quotes indicate that the current working directory
should be searched for the required header file. This will be
true when you write your own header files but the standard header
files should always have the angle brackets around them.

NOTE: just to keep you on your toes - pre-processor
statements, such as include, DO NOT use
semi-colons as delimiters! But don't forget the # must be in the 1st coloumn