Introduction to C

Origin of C

The C Programming Language was initially developed by Denis Ritchie using a Unix system in 1972. This was varied and modified until a standard was defined by Brian Kernighan and Dennis Ritchie in 1978 in "The C Programming Language".

By the early 80's many versions of C were available which were inconsistent with each other in many aspects. This led to a standard being defined by ANSI in 1983. It is this standard this set of notes primarily addresses.

Why use C ?

Industry Presence : Over the last decade C has become one of the most widely used development languages in the software industry. Its importance is not entirely derived from its use as a primary development language but also because of its use as an interface language to some of the newer “visual” languages and of course because of its relationship with C++.

Middle Level : Being a Middle level language it combines elements of high level languages with the functionality of assembly language. C supports data types and operations on data types in much the same way as higher level languages as well as allowing direct manipulation of bits, bytes, words and addresses as is possible with low level languages.

Portability : With the availability of compilers for almost all operating systems and hardware platforms it is easy to write code on one system which can be easily ported to another as long as a few simple guidelines are followed.

Flexibility : Supporting its position as the mainstream development language C can be interfaced readily to other programming languages.

Malleable : C, unlike some other languages, offers little restriction to the programmer with regard to data types -- one type may be coerced to another type as the situation dictates. However this feature can lead to sloppy coding unless the programmer is fully aware of what rules are being bent and why.

Speed : The availability of various optimising compilers allow extremely efficient code to be generated automatically.

1.2 The “Hello World” Program

A C program consists of one or more functions or code modules. These are essentially groups of instructions that are to be executed as a unit in a given order and that can be referenced by a unique name. Each C program must contain a  main() function. This is the first function called when the program starts to run. Note that while "main" is not a C keyword and hence not reserved it should be used only in this context.

A C program is traditionally arranged in the following order but not strictly as a rule.  

 

Function prototypes and global data declarations

The main() function

Function definitions

Consider first a simple C program which simply prints a line of text to the computer screen. This is traditionally the first C program you will see and is commonly called the “Hello World” program for obvious reasons.

     #include <stdio.h>

     void main()

     {

     /* This is how comments are implemented in C 

                to comment out a block of text */

     // or like this for a single line comment

     printf( "Hello World\n" ) ;

     }

As you can see this program consists of just one function the mandatory main function. The parentheses, ( ), after the word main indicate a function while the curly braces, { }, are used to denote a block of code -- in this case the sequence of instructions that make up the function.

Comments are contained within a /* ... */ pair in the case of a block comment or a double forward slash, //, may be used to comment out the remains of a single line of test.

The line

     printf("Hello World\n " ) ;

is the only C statement in the program and must be terminated by a semi-colon.

The statement calls a function called printf which causes its argument, the string of text within the quotation marks, to be printed to the screen. The characters \n are not printed as these characters are interpreted as special characters by the printf function in this case printing out a newline on the screen. These characters are called escape sequences in C and cause special actions to occur and are preceded always by the backslash character, \ .

 All C compiler include a library of standard C functions such as printf which allow the programmer to carry out routine tasks such as I/O, maths operations, etc. but which are not part of the C language, the compiled C code merely being provided with the compiler in a standard form.

Header files must be included which contain prototypes for the standard library functions and declarations for the various variables or constants needed. These are normally denoted by a .h extension and are processed automatically by a program called the Preprocessor prior to the actual compilation of the C program.

The line

     #include <stdio.h>

instructs the preprocessor to include the file stdio.h into the program before compilation so that the definitions for the standard input/output functions including printf will be present for the compiler. The angle braces denote that the compiler should look in the default “INCLUDE” directory for this file. A pair of double quotes indicate that the compiler should search in the specified path e.g.

     #include “d:\myfile.h”

 NB :  C is case sensitive i.e. printf() and Printf() would be regarded as two different functions.

 The C Programming Environment

Program development is nowadays carried out in specifically designed software systems or workbenches with editing, compilation, linking, debugging and execution facilities built in. In this course we will be making use of a Microsoft system but the features found in this are to be found in one form or another in almost all modern systems.

The first phase of development involves the creation and editing of a file containing the appropriate C instructions which will be stored using a file extension of .c normally to invoke the C compiler, e.g. fname.c.

The next step is to take the C program and to compile it into object code or machine language code. The C compiler includes the aforementioned preprocessor which is called automatically before the code translation takes place. This preprocessor acts on special commands or directives from the programmer to manipulate the text of the C code before compilation commences. These directives might involve including other source files in the file to be compiled, replacing special symbols with specific replacement text, etc. Once this is done the C code is translated into object code and stored in a file with the extension .obj, e.g. fname.obj.

The final phase in building the executable program is called linking. After the compilation stage the C code has been translated into machine recognisable code but is in a somewhat unconnected state. The program invariably contains references to standard library functions or functions contained in other libraries or modules which must be connected to the C program at link time. This simply involves linking the machine code for these functions with the program’s object code to complete the build process and produce an executable file with an extension .exe e.g. fname.exe.

The executable program can be loaded and run from within the programming environment itself or may be run from the host environment directly. If it executes as expected that is the end of the task. However if this does not happen it may require the use of the debugger to isolate any logical problems. The debugger allows us to step through the code instruction by instruction or up to predefined break-points and to look at the values of variables in the code in order to establish where errors are introduced.