Introduction to C Programming

Programmers using MSDOS should note that versions of printf() supplied with MSDOS compilers normally convert \n to a carriage return and line feed (new-line) character pair, there is no need to try and include a carriage return character in a string. On Unix systems this is the responsibility of the display driving software.

There are several other characters that cannot conveniently be included in a string but which can be included using the \ notation. The complete list is

\a	"BELL" - i.e. a beep
\b	Backspace 
\f	Form Feed (new page) 
\n	New Line (line feed)
\r	Real carriage return
\t	Tab
\v	Vertical Tab
\\ 	Backslash
\'	Single Quote
\"	Double Quote
\?	Question Mark

It is also possible to include arbitrary characters in strings by including the three digit octal representation of the character preceded by a \ symbol. For example the "hello world" program could be written in this form.

main()
{
	printf("\150ello, world\n");
}

The effect is actually to send the string of binary 1s and 0s equivalent to octal 150 to the output device which should respond in the manner its designers intended it to. You can use this convention to send all sorts of unusual special characters to all sorts of unusual output devices.

Program Layout

The hello world program listed at the start of this chapter is laid out in accordance with normal C conventions and practice. The C programming language is "free format" which means you can lay out the text of programs in whatever form takes your fancy provided, of course, you don't run words together. The "hello world" program could have been written in this form

main(){printf("hello, world\n");}

main
(
)
{
printf
(
"hello, world\n"
)
;
}

The indentation of the word printf in the original version of the hello world program is achieved by the use of a single TAB character not a sequence of spaces. TAB characters may be generated by hitting the TAB key on normal keyboards, occasionally the TAB key carries a mysterious symbol consisting of 2 arrows. Multiple TAB characters should be used to indent C programs in preference to sequences of spaces. The use of TABs makes for smaller source files, less typing and tidier layout. All C compilers understand the use of TABs. Unfortunately some word processors and editors have strange ideas about standard TAB settings, for C source TABs should be set every 8 characters.

All but the briefest programs will benefit from descriptive comments included in the source. In the C programming language anything enclosed between the character pair /* and the character pair */ is treated as a comment. The compiler treats any comment, however long, as if it were a single space. Comments may appear in any reasonable place in the source. A commented version of the "hello world" program appears below.

/*	This is the first C program

	Author	: J.P.H.Burden
	Date	: 22 September 1992
	System	: IBM 6150 + AIX Version 2.1
*/
main()
{
	printf("hello, world\n");
}

main()
{
	printf("hello, world\n");
/*	
	some very clever code I haven't
	quite sorted out yet.
*/
        printf("the solution to the problem of \
life, the universe and everything is");
/*	
	more very clever code still to be
	developed
*/
}

main()
{
	printf("hello, world\n");
/*
	OK let's stop pretending we can write
	really clever programs
	/*	
		some very clever code I haven't
		quite sorted out yet.
	*/
		printf("the solution to the problem of \
	life, the universe and everything is");
	/*	
		more very clever code still to be
		developed
	*/
*/
}

"comm2.c", line 17: syntax error before or at: /
Compilation failed

Programming Errors

It is quite likely that your attempt to write a hello world program will be completely successful and will work first time. It will probably be the only C program you ever write that works first time. There are many different types of mistake you might make when writing C programs. These can result in messages from the compiler, or sometimes the linker, programs that bring the computer grinding to a halt or programs that simply produce the wrong answer to whatever problem they were trying to solve or programs that get stuck in a never-ending loop. As a beginner you are more likely to encounter compiler or linker error messages, particularly if you have already done some programming in another programming language. The rest of this section illustrates some possible errors and their consequences.

In the first example the programmer has forgotten that the C programming language is case sensitive.

MAIN()
{
	printf("hello, world\n");
}

ld: Undefined -
	.main
	_main
	_end

ld: Undefined symbol 
   _main 
Compilation failed

Linker Error: Undefined symbol _main in function main

hw.c

Microsoft (R) Overlay Linker  Version 3.65   
Copyright (C) Microsoft Corp 1983-1988.  All rights reserved.

Object Modules [.OBJ]: HW.OBJ 
Run File [HW.EXE]: HW.EXE /NOI
List File [NUL.MAP]: NUL
Libraries [.LIB]: SLIBCEC.LIB 

LINK : error L2029: Unresolved externals:



_main in file(s):
 C:\MSC\LIB\SLIBCEC.LIB(dos\crt0.asm)

There was 1 error detected

All these errors reflect the fact that the linker cannot put the program together properly. On Unix based systems the linker is usually called "ld". Along with the user supplied main() function all C programs include something often called the run-time support package which is actually the code that the operating system kicks into life when starting up your program. The run-time support package then expects to call the user supplied function main(), if there is no user supplied main() then the linker cannot finish the installation of the run-time support package. In this case the user had supplied MAIN() rather than main(). "MAIN" is a perfectly valid C function name but it isn't "main".

The rather extensive set of messages from Microsoft C 5.1 were partly generated by the compiler, which translated the program without difficulty, and partly generated by the linker. The reference to "crt0" is, in fact, a reference to the C Run Time package which tries to call main() to start the program running.

In the second example the programmer, probably confusing C with another programming language, had used single quotes rather than double quotes to enclose the string passed to printf().

main()
{
	printf('hello, world\n');
}

"hw.c", line 3: too many characters in character constant
"hw.c", line 3: warning: Character constant contains more than one byte

"hw.c", line 3: too many characters in character constant
Compilation failed

Error C:\ISPTESTS\HW.C 3:
Character constant too long in function main

hw.c
hw.c(3) : error C2015: too many chars in constant

In the third example the programmer, again possibly confused by another programming language, had missed out the semi-colon on line 3.

main()
{
	printf("hello, world\n")
}

"hw.c", line 4: syntax error

"hw.c", line 4: syntax error at or near symbol }
Compilation failed

Error C:\ISPTESTS\HW.C 4:
Statement missing ; in function main

hw.c
hw.c(4) : error C2143: syntax error : missing ';' before '}'

In the fourth example the programmer wrote print() rather than printf().

main()
{
	print("hello, world\n");
}

ld: Undefined -
	.print
	_print

ld: Undefined symbol 
   _print 
Compilation failed

Linking C:\ISPTESTS\HW.C 4:
Linker Error: Undefined symbol _print in module HW.C

hw.c

Microsoft (R) Overlay Linker  Version 3.65   
Copyright (C) Microsoft Corp 1983-1988.  All rights reserved.

Object Modules [.OBJ]: HW.OBJ 
Run File [HW.EXE]: HW.EXE /NOI
List File [NUL.MAP]: NUL
Libraries [.LIB]: SLIBCEC.LIB 

LINK : error L2029: Unresolved externals:



_print in file(s):
 HW.OBJ(hw.c)

There was 1 error detected

In the final example the programmer left out the parentheses immediately after main.

main
{
	printf("hello, world\n");
}

"hw.c", line 2: syntax error
"hw.c", line 3: illegal character: 134 (octal)
"hw.c", line 3: cannot recover from earlier errors: goodbye!

"hw.c", line 2: syntax error at or near symbol {
"hw.c", line 2: unknown size
Compilation failed

Error C:\ISPTESTS\HW.C 2: Declaration syntax error

hw.c
hw.c(2) : error C2054: expected '(' to follow 'main'

You may find it interesting to try the various erroneous versions of the "hello world" program on your particular system. Do you think your compiler generates more helpful error messages?

If you are using Turbo C you will see the following message, even when compiling a correct version of the hello world program

Warning C:\ISPTESTS\HW.C 4:
   Function should return a value in function main

The message means that the user supplied function main() should return a value to the run-time support package. There is no requirement to do so and most compilers recognise main() as a special case in this respect. Returning a value to the run-time support package should not be confused with returning a value, sometimes known as an exit code, to the host operating system.

C Standards and History

The C programming language was developed in the years 1969 to 1973, although the first published description did not appear until the book The C Programming Language" written by Brian Kernighan and Dennis Ritchie was published in 1978. The early versions of the C language were strongly influenced by a language called BCPL which itself was a derviative of Algol.

The early development of C was closely linked to the development of the Unix operating system. Large portions of the code of the Unix operating system were eventually written in C and problems encountered in transferring Unix to various computers were reflected into the design of the C language. The modest hardware available to the Unix developers was also reflected in the language design, most notably the use of separate library functions for operations such as input and output. Until the early 1980s the language was almost exclusively associated with Unix.

The widespread introduction of microprocessor based computer systems in the early 1980s also saw a rapid growth in the use of C on such systems. C compilers were known to be small and many of the start-up operations that produced the early microprocessor based computer systems were staffed by ex-students who had encountered the language whilst at university.

As early as 1982 it became clear that the informal description of the language in Kernighan & Ritchie's book was not good enough. ANSI established a committee known as X3J11 in 1983. This committee produced a report defining the language at the end of 1989. The report was known as X3.159 but the standard was soon taken over by ISO with the designation ISO/IEC 9899-1990. This version of the language is known as ANSI-C to distinguish it from the earlier version of the language described in Kernighan and Ritchie's book. The earlier version of the language is known as K&R C. C++ and Objective-C are different languages developed from C. The GNU C compiler, often known by the command that invokes it, gcc , is public domain software available for both Unix and MSDOS based systems. It supports a version of the language close to the ANSI standard.

All the code presented in these notes, unless specifically indicated otherwise, confirms to the ANSI standard. Many compiler writers and vendors produce C compilers that will compile code conforming to the ANSI standard but they also provide a variety of extensions to suit the particular target environment. Such extensions are ususally extra library routines for functions such as PC screen handling and interfacing direct to MSDOS system functions. Sometimes the extensions include extra features introduced into the language, usually to cope with some of the problems of memory management on MSDOS based systems. In the Unix environment such extensions are less common, Unix systems are normally supplied with a compiler and the extensions appear as extra libraries for functions such as driving X-Windows displays or network communications.

All C programmers ought to be aware of what is and what isn't standard in their particular environment. The better compiler writers usually make this fairly clear in their manuals.

It is also often possible to obtain further special purpose application specific libraries for use with C compilers to provide facilities such as database handling, graphics, numerical analysis etc.

C and C++

Although it is the intention of these notes to stick rigidly to the ANSI standard for C, it is quite possible you will be using a compiler, such as Turbo C, that is really a C++ compiler. This is possible because C++ is a strict superset of C which means that any C program should be acceptable to a C++ compiler. In fact some C++ compilers operate by first converting the C++ code into C. As well as the ideas associated with object-oriented programming, C++ introduces a few extra features that the C programmer should be aware of, these include an alternative syntax for comments, several extra keywords that cannot be used in normal programming and a very simple form of input and output known as stream IO.

The alternative comment syntax uses the symbol // to mark the start of a comment and the end of a line to indicate the end of a comment. This means that you cannot have multi-line comments in C++ and it also means that you cannot have a comment in the middle of a line of code. If you have ever programmed in assembler language you'll be familiar with this style of commenting. The problem of nested comments does not arise.

Of course, C++ compilers will still accept C style comments marked by symbols "/*" and "*/". You shouldn't mix the two styles in the same program.

Here is an example of the hello world program written using C++ commenting style.

//          A simple program to demonstrate
//          C++ style comments
//
//          The following line is essential
//          in the C++ version of the hello
//          world program
//
#include	<stdio.h>
main()
{
	printf("hello, world\n");  // just like C
}

The C++ stream output facility is illustrated by the following program. The basic syntax of C++ stream output is

Note that the program, like the previous example, requires the use of "#include" and a header file name. The meaning of "#include" will be discussed in a later chapter.

#include	<iostream.h>
main()
{
	cout << "hello, world\n";
}

Character Codes

Here is a list of ASCII character codes in octal and hexadecimal notation along with the equivalent C escape sequence where relevant. This table is not comprehensive.

21 041 !  22 042 "  23 043 #  24 044 $  25 045 %  26 046 &  
27 047 '  28 050 (  29 051 )  2a 052 *  2b 053 +  2c 054 ,  
2d 055 -  2e 056 .  2f 057 /  30 060 0  31 061 1  32 062 2  
33 063 3  34 064 4  35 065 5  36 066 6  37 067 7  38 070 8  
39 071 9  3a 072 :  3b 073 ;  3c 074 <  3d 075 =  3e 076 >  
3f 077 ?  40 100 @  41 101 A  42 102 B  43 103 C  44 104 D  
45 105 E  46 106 F  47 107 G  48 110 H  49 111 I  4a 112 J  
4b 113 K  4c 114 L  4d 115 M  4e 116 N  4f 117 O  50 120 P  
51 121 Q  52 122 R  53 123 S  54 124 T  55 125 U  56 126 V  
57 127 W  58 130 X  59 131 Y  5a 132 Z  5b 133 [  5c 134 \  
5d 135 ]  5e 136 ^  5f 137 _  60 140 `  61 141 a  62 142 b  
63 143 c  64 144 d  65 145 e  66 146 f  67 147 g  68 150 h  
69 151 i  6a 152 j  6b 153 k  6c 154 l  6d 155 m  6e 156 n  
6f 157 o  70 160 p  71 161 q  72 162 r  73 163 s  74 164 t  
75 165 u  76 166 v  77 167 w  78 170 x  79 171 y  7a 172 z  
7b 173 {  7c 174 |  7d 175 }  7e 176 ~   

Exercises

1. Your first task, of course, is to enter, compile and run the hello world program on your computer system. Note the commands you used to compile the program, any messages produced and the names of any files generated.

2. Now try some of the programs with errors. Note the error messages produced by your compiler. Do you think they are more infomrative than those described in the notes ?

3. What is the effect of typing the hello world program using round brackets (parentheses) rather than curly brackets (braces) ?

4. Write a C program that produces the following output

   	****************
           *  hello world *
   	****************
   

   using multiple printf() calls.

5. Write a C program that prints hello world vertically on the screen.

6. Write a C program that prints hello world on the screen and then "beeps".

7. On many displays the character sequence ESC [ 2 J causes the display to go blank. Use this information to write a C program that clears the screen. (If you are working on a PC running MSDOS you'll need to ensure that ANSI.SYS is operational)

8. Write a program that displays hello world in the centre of an otherwise blank screen.