C language Introduction

 C is a powerful and compact computer language that allows you to write programs that specify exactly what you want your computer to do. You’re in charge: you create a program, which is just a set of instructions, and your computer will follow them.

Programming in C isn’t difficult, as you’re about to find out. I’m going to teach you all the fundamentals of  C programming in an enjoyable and easy-to-understand way, and by the end of this chapter you’ll have written your first few C programs. It’s as easy as that!

In this chapter you’ll learn:

•       What the C language standard is

•       What the standard library is

•       How to create C programs

•       How C programs are organized

•       How to write your own program to display text on the screen

The C Language

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C is remarkably flexible. It has been used for developing just about everything you can imagine by way of a computer program, from accounting applications to word processing and from games to operating systems. It is not only the basis for more advanced languages, such as C++, it is also used currently for developing mobile phone apps in the form of Objective C. Objective C is standard C with a thin veneer of object-oriented programming capability added. C is easy to learn because of its compactness. Thus, C is an ideal first language if you have ambitions to be a programmer. You’ll acquire sufficient knowledge for practical application development quickly and easily.

The C language is defined by an international standard, and the latest is currently defined by the document  ISO/IEC 9899:2011. The current standard is commonly referred to as C11, and the language that I describe in this book conforms to C11. You need to be aware that some elements of the language as defined by C11 are optional.  This implies that a C compiler that conforms to the C11 standard may not implement everything in the standard.  (A compiler is just a program that converts your program written in terms you understand into a form your computer understands.) I will identify any language feature in the book that is optional so far as C11 is concerned, just so you are aware that it is possible that your compiler may not support it.

It is also possible that a C11 compiler may not implement all of the language features mandated by the C11 standard. It takes time to implement new language capabilities, so compiler developers will often take an incremental approach to implementing them. This provides another reason why a program may not work. Having said that, I can confirm from my own experience that the most common reason for things not working in a C program, at least 99.9% of the time, is that a mistake has been made.

The Standard Library

The standard library for C is also specified within the C11 standard. The standard library defines constants, symbols, and functions that you frequently need when writing a C program. It also provides some optional extensions to the basic C language. Machine-dependent facilities such as input and output for your computer are implemented by the standard library in a machine-independent form. This means that you write data to a disk file in C in the same way on your PC as you would on any other kind of computer, even though the underlying hardware processes are quite different. The standard functionality that the library contains includes capabilities that most programmers are likely to need, such as processing text strings or math calculations. This saves you an enormous amount of effort that would be required to implement such things yourself.

The standard library is specified in a set of standard files called header files. Header files always have names with the extension .h. To make a particular set of standard features available in your C program file, you just include the appropriate standard header file in a way that I’ll explain later in this chapter. Every program you write will make use of the standard library. A summary of the header files that make up the standard library is in Appendix E.

Learning C

If you are completely new to programming, there are some aspects of C that you do not need to learn, at least not the first time around. These are capabilities that are quite specialized or used relatively infrequently. I have put all these together in Chapter 14 so you will learn about them  when you are comfortable with the rest.

Although the code for all the examples is available for download from the Apress web site (http://www.apress.com), I recommend that you type in all the examples in the book, even when they are very simple. Keying stuff in makes it less likely that you will forget things later. Don’t be afraid to experiment with the code. Making mistakes is very educational in programming. The more mistakes you make early on, the more you are likely to learn.

Creating C Programs

There are four fundamental stages, or processes, in the creation of any C program:

•       Editing

•       Compiling

•       Linking

•       Executing

You’ll soon know all these processes like the back of your hand because you’ll be carrying them out so often. First, I’ll explain what each process is and how it contributes to the development of your C program.

Editing

Editing is the process of creating and modifying C source code—the name given to the program instructions you write. Some C compilers come with a specific editor program that provides a lot of assistance in managing your programs. In fact, an editor often provides a complete environment for writing, managing, developing, and testing your programs.

This is sometimes called an integrated development environment (IDE).

You can also use a general-purpose text editor to create your source files, but the editor must store the code as plain text without any extra formatting data embedded in it. Don’t use a word processor such as Microsoft Word; word processors aren’t suitable for producing program code because of the extra formatting information they store along with the text. In general, if you have a compiler system with an editor included, it will provide a lot of features that make it easier to write and organize your source programs. There will usually be automatic facilities for laying out the

programming in C

program text appropriately and color highlighting for important language elements, which not only makes your code more readable but also provides a clear indicator when you make errors when keying in such words.

If you’re working with Linux, the most common text editor is the Vim editor. Alternately you might prefer to use the GNU Emacs editor. With Microsoft Windows, you could use one of the many freeware and shareware programming editors. These will often provide help in ensuring your code is correct, with syntax highlighting and autoindenting. There is also a version of Emacs for Microsoft Windows. The Vi and VIM editors from the UNIX environment are available for Windows too, and you could even use Notepad++ (http://notepad-plus-plus.org/).

Of course, you can also purchase one of the professionally created programming development environments that support C, such as those from Borland or Microsoft, in which case you will have very extensive editing capabilities. Before parting with your cash though, it’s a good idea to check that the level of C that is supported conforms to the current C standard, C11. With some of the products out there that are primarily aimed at C++ developers, C has been left behind somewhat.

Compiling

The compiler converts your source code into machine language and detects and reports errors in the compilation process. The input to this stage is the file you produce during your editing, which is usually referred to as a source file.

The compiler can detect a wide range of errors that are due to invalid or unrecognized program code, as well as structural errors where, for example, part of a program can never be executed. The output from the compiler is known as object code and it is stored in files called object files, which usually have names with the extension .obj in the Microsoft Windows environment, or .o in the Linux/UNIX environment. The compiler can detect several different kinds of errors during the translation process, and most of these will prevent the object file from being created.

The result of a successful compilation is a file with the same name as that used for the source file, but with the .o or .obj extension.

If you’re working in UNIX, at the command line, the standard command to compile your C programs will be cc (or the GNU’s Not UNIX [GNU] compiler, which is .gcc). You can use it like this:

 

cc -c myprog.c

 

where myprog.c is the name of the source file that contains the program you want to compile. Note that if you omit

the -c flag, your program will automatically be linked as well. The result of a successful compilation will be an object file.

Most C compilers will have a standard compile option, whether it’s from the command line (such as cc myprog.c) or a menu option from within an IDE (where you’ll find a Compile menu option). Compiling from within an IDE is generally much easier than using the command line.

Compilation is a two-stage process. The first stage is called the preprocessing phase, during which your code may be modified or added to, and the second stage is the actual compilation that generates the object code. Your source file can include preprocessing macros, which you use to add to or modify the C program statements. Don’t worry if this doesn’t make complete sense now. It will come together for you as the book progresses.

Linking

The linker combines the object modules generated by the compiler from source code files, adds required code modules from the standard library supplied as part of C, and welds everything into an executable whole. The linker also detects and reports errors; for example, if part of your program is missing or a nonexistent library component  is referenced.

In practice, a program of any significant size will consist of several source code files, from which the compiler generates object files that need to be linked. A large program may be difficult to write in one working session, and it may be impossible to work with as a single file. By breaking it up into a number of smaller source files that each provide a coherent part of what the complete program does, you can make the development of the program a lot easier. The source files can be compiled separately, which makes eliminating simple typographical errors a bit easier.