# C Programming/Print version

C Programming

The current, editable version of this book is available in Wikibooks, the open-content textbooks collection, at
https://en.wikibooks.org/wiki/C_Programming

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# Why learn C?

C is the most commonly used programming language for writing operating systems. The first operating system written in C is Unix. Later operating systems like GNU/Linux were all written in C. Not only is C the language of operating systems, it is the precursor and inspiration for almost all of the most popular high-level languages available today. In fact, Perl, PHP, Python and Ruby are all written in C.

By way of analogy, let's say that you were going to be learning Spanish, Italian, French, or Romanian. Do you think knowing Latin would be helpful? Just as Latin was the basis of all of those languages, knowing C will enable you to understand and appreciate an entire family of programming languages built upon the traditions of C. Knowledge of C enables freedom.

### Why C and not assembly language?

While assembly language can provide speed and maximum control of the program, C provides portability.

Different processors are programmed using different Assembly languages and having to choose and learn only one of them is too arbitrary. In fact, one of the main strengths of C is that it combines universality and portability across various computer architectures while retaining most of the control of the hardware provided by assembly language.

For example, C programs can be compiled and run on the HP 50g calculator (ARM processor), the TI-89 calculator (68000 processor), Palm OS Cobalt smartphones (ARM processor), the original iMac (PowerPC), the Arduino (Atmel AVR), and the Intel iMac (Intel Core 2 Duo). Each of these devices has its own assembly language that is completely incompatible with the assembly language of any other.

Assembly, while extremely powerful, is simply too difficult to program large applications and hard to read or interpret in a logical way. C is a compiled language, which creates fast and efficient executable files. It is also a small “what you see is all you get” language: a C statement corresponds to at most a handful of assembly statements, everything else is provided by library functions.

So is it any wonder that C is such a popular language?

Like toppling dominoes, the next generation of programs follows the trend of its ancestors. Operating systems designed in C always have system libraries designed in C. Those system libraries are in turn used to create higher-level libraries (like OpenGL, or GTK), and the designers of those libraries often decide to use the language the system libraries used. Application developers use the higher-level libraries to design word processors, games, media players and the like. Many of them will choose to program in the language that the higher-level library uses. And the pattern continues on and on and on...

### Why C, and not another language?

The primary design of C is to produce portable code while maintaining performance and minimizing footprint (CPU time, memory usage, disk I/O, etc.). This is useful for operating systems, embedded systems or other programs where performance matters a lot (“high-level” interface would affect performance). With C it’s relatively easy to keep a mental picture of what a given line really does, because most of the things are written explicitly in the code. C has a big codebase for low level applications. It is the “native” language of UNIX, which makes it flexible and portable. It is a stable and mature language which is unlikely to disappear for a long time and has been ported to most, if not all, platforms.

One powerful reason is memory allocation. Unlike most programming languages, C allows the programmer to write directly to memory. Key constructs in C such as structs, pointers and arrays are designed to structure and manipulate memory in an efficient, machine-independent fashion. In particular, C gives control over the memory layout of data structures. Moreover dynamic memory allocation is under the control of the programmer (which also means that memory deallocation has to be done by the programmer). Languages like Java and Perl shield the programmer from having to manage most details of memory allocation and pointers (except for memory leaks and some other forms of excess memory usage). This can be useful since dealing with memory allocation when building a high-level program is a highly error-prone process. However, when dealing with low-level code such as the part of the OS that controls a device, C provides a uniform, clean interface. These capabilities just do not exist in most other languages.

While Perl, PHP, Python and Ruby may be powerful and support many features not provided by default in C, they are not normally implemented in their own language. Rather, most such languages initially relied on being written in C (or another high-performance programming language), and would require their implementation be ported to a new platform before they can be used.

As with all programming languages, whether you want to choose C over another high-level language is a matter of opinion and both technical and business requirements could dictate which language is required.

# History

The field of computing as we know it today started in 1947 with three scientists at Bell Telephone Laboratories—William Shockley, Walter Brattain, and John Bardeen—and their groundbreaking invention: the transistor. In 1956, the first fully transistor-based computer, the TX-0, was completed at MIT. The first integrated circuit was created in 1958 by Jack Kilby at Texas Instruments, but the first high-level programming language existed even before then.

"The Fortran project" was originally developed in 1954 by IBM. A shortening of "The IBM Mathematical Formula Translating System", the project had the purpose of creating and fostering development of a procedural, imperative programming language that was especially suited to numeric computation and scientific computing. It was a breakthrough in terms of productivity and programming ease (compared to assembly language) and speed (Fortran programs ran nearly as fast as, and in some cases, just as fast as, programs written in assembly). Furthermore, Fortran was written at a high-enough level (and thus was machine independent enough) to become the first widely adopted programming language. The Algorithmic Language (Algol 58) was derived from Fortran in 1958 and evolved into Algol 60 in 1960. The Combined Programming Language (CPL) was then created out of Algol 60 in 1963. In 1967, it evolved into Basic CPL, which was itself, the base for B in 1969. Finally, B, the root of C, was created in 1971.

C was the direct successor of B, a stripped down version of BCPL, created by Ken Thompson at Bell Labs, that was also a compiled language - User's Reference to B, used in early internal versions of the UNIX operating system. As noted in Ritchie's C History : "The B compiler on the PDP-7 did not generate machine instructions, but instead 'threaded code', an interpretive scheme in which the compiler's output consists of a sequence of addresses of code fragments that perform the elementary operations. The operations typically — in particular for B — act on a simple stack machine". Thompson and Dennis Ritchie, also working at Bell Labs, improved B and called the result NB. Further extensions to NB created its logical successor, C. Most of UNIX was rewritten in NB, and then C, which resulted in a more portable operating system.

The portability of UNIX was the main reason for the initial popularity of both UNIX and C. Rather than creating a new operating system for each new machine, system programmers could simply write the few system-dependent parts required for the machine, and then write a C compiler for the new system. Since most of the system utilities were thus written in C, it simply made sense to also write new utilities in C.

The American National Standards Institute began work on standardizing the C language in 1983, and completed the standard in 1989. The standard, ANSI X3.159-1989 "Programming Language C", served as the basis for all implementations of C compilers. The standards were later updated in 1990 and 1999, allowing for features that were either in common use, or were appearing in C++.

# What You Need Before You Can Learn

## Getting Started

The goal of this book is to introduce you to and teach you the C programming language. Basic computer literacy is assumed, but no special knowledge is needed.

Before you can start programming in C, you will need a C compiler. A compiler is a program that converts C code into executable machine code.[1]

Popular C compilers/IDEs include:

Microsoft Visual Studio Community Visual Studio Windows Proprietary, free of charge Powerful and student-friendly version of an industry standard compiler.
Xcode Xcode macOS, OSX Proprietary, free of charge Default IDE on macOS
Tiny C Compiler (TCC) tinycc GNU/Linux, Windows LGPL Small, fast and simple compiler.
Clang clang GNU/Linux, Windows, Unix, OS X University of Illinois/NCSA License A free, permissively licensed front-end using a LLVM backend.
GNU C Compiler gcc GNU/Linux, MinGW or mingw-w64 (Windows), Unix, OS X. GPL The De facto standard. Ships with most Unix systems.

The minimum software requirements to program in C is a text editor, as opposed to a word processor. A plain text Notepad editor can be used but it does not offer any advanced capabilities such as syntax highlighting and code completion. There are many text editors (see List of Text Editors), among the most popular are Notepad++ for Windows as well as Atom, Sublime Text, gedit, Vim and Emacs which are also available on other operating systems (“cross-platform”). These text editors come with syntax highlighting and line numbers, which makes code easier to read at a glance, and to spot syntax errors.

Though not absolutely needed, many programmers prefer and recommend using an Integrated development environment (IDE) instead of a text editor. An IDE is a suite of programs that developers need, combined into one convenient package, usually with a graphical user interface. These programs include a text editor and file browser and are sometimes bundled with an easily accessible compiler. They also typically include a debugger, a tool that will enable you to do such things as step through the program you develop manually one source code line at a time, or alter data as an aid to finding and correcting programming errors.

Many IDEs do not offer their users a console-based interface to the compiler and for executing the developed program but offer only graphical buttons. For beginners it is recommended not to use such an IDE, since it hides most of what is going on. Using the command line builds up familiarity with the toolchain. Such an IDE may still be useful to somebody with programming experience who knows how the IDE works. So as a general guideline: Do not use an IDE unless you know what the IDE does!

Other popular compilers/IDEs include:

Eclipse CDT Eclipse Windows, Mac OS X, GNU/Linux Free/Libre and Open Source Eclipse IDE for C/C++ developement, a popular open source IDE.
Netbeans Netbeans Cross-platform CDDL and GPL 2.0 A Good comparable matured IDE to Eclipse.
GNOME Builder Builder GNU/Linux GPL A feature-rich but simple IDE for the GNOME desktop environment.
Anjuta Anjuta GNU/Linux GPL An extensible GTK+3 IDE for the GNOME desktop environment.
Geany geany Cross-platform GPL A lightweight cross-platform GTK+ notepad based on Scintilla, with basic IDE features.
KDevelop KDevelop Cross-platform GPL A cross-platform IDE for the KDE project.
Little C Compiler (LCC) lcc Windows Open Source but not Libre Small open source compiler.
Xcode Xcode Mac OS X Proprietary, free of charge Available free of charge at Mac App Store.
Pelles C Pelles C Windows, Pocket PC Proprietary, free of charge A complete C development kit for Windows.
Dev-C++ Dev C++ Windows GPL Updated version of the formerly popular Bloodshed Dev-C++.
Microsoft Visual Studio Community Visual Studio Windows Proprietary, free of charge Microsoft’s compiler already mentioned above comes bundled with an IDE.
CodeLite CodeLite Cross-platform GPL 2 Free IDE for C/C++ development.
Code::Blocks Code::Blocks Cross-platform GPL 3.0 Built to meet users' most demanding needs. Very extensible and fully configurable.

On GNU/Linux, GCC is almost always included by default.

On Microsoft Windows, Dev-C++ is recommended for beginners because it is easy to use, free, and simple to install. Although the initial developer (Bloodshed) hasn’t updated it since 2005, a new version appeared in 2011, made by an independent programmer, and is being actively developed.[2] An alternate option for those working only in the Windows environment is the proprietary Microsoft Visual Studio Community which is free of charge and has an excellent debugger.

On Mac OS X, the Xcode IDE provides the compilers needed to compile various source files. The newer versions do not include the command line tools. They need to be downloaded via Xcode->Preferences->Downloads.

## Footnotes

1. Actually, GCC’s (GNU C Compiler) cc (C Compiler) translates the input .c file to the target CPU’s assembly, output is written to an .s file. Then as (assembler) generates a machine code file from the .s file. Pre-processing is done by another sub-program cpp (C PreProcessor), which is not to be confused with c++ (a compiler for another programming language).
2. http://orwelldevcpp.blogspot.com/

# Obtaining a Compiler

### Dev-C++

Dev C++ is an Integrated Development Environment(IDE) for the C++ programming language, available from Bloodshed Software. An updated version is available at Orwell Dev-C++.
C++ is a programming language which contains within itself most of the C language, plus extensions. Most C++ compilers will compile C programs, sometimes with a few adjustments (like invoking them with a different name or command line switch). Therefore, you can use Dev C++ for C development.

However, Dev C++ is not the compiler. It is designed to use the MinGW or Cygwin versions of GCC - both of which can be obtained as part of the Dev C++ package, although they are completely different projects.
Dev C++ simply provides an editor, syntax highlighting, some facilities for the visualisation of code (like class and package browsing) and a graphical interface to the chosen compiler. Because Dev C++ analyses the error messages produced by the compiler and attempts to distinguish the line numbers from the errors themselves, the use of other compiler software is discouraged since the format of their error messages is likely to be different.

The latest version of Dev-C++ is a beta for version 5. However, it still has a significant number of bugs. All the features are there, and it is quite usable. It is considered one of the best free software C IDEs available for Windows.

A version of Dev C++ for Linux is in the pipeline. It is not quite usable yet, however. Linux users already have a wealth of IDEs available. (e.g. KDevelop and Anjuta.) Most of the graphical text editors, and other common editors such as emacs and vim, support syntax highlighting.

Steps for Obtaining Dev-C++ if You're on Windows
2. The setup is pretty straight forward. Make sure the compiler option is ticked.
3. You can now use the environment provided by the software to write and run your code.
4. OPTIONALLY: "C:\Program Files (x86)\Dev-Cpp\MinGW64\bin" can be added to the global PATH variable of the operating system to compile with gcc from a command prompt.

### GCC

The GNU Compiler Collection (GCC) is a free/libre set of compilers developed by the Free Software Foundation.

Steps for Obtaining the GCC Compiler if You're on GNU/Linux

On GNU/Linux, Installing the GNU C Compiler can vary in method from distribution to distribution. (Type in cc -v to see if it is installed already.)

• For Ubuntu, install the GCC compiler (along with other necessary tools) by using sudo apt install build-essential, or by using Synaptic. You do not need Universe enabled.
• For Debian, install the GCC compiler (as root) by using apt install gcc.
• For Fedora Core, install the GCC compiler (as root) by using yum install gcc.
• For Redhat, get a GCC RPM, e.g. using Rpmfind and then install (as root) using rpm -ivh gcc-version-release.arch.rpm
• For Mandrake, install the GCC compiler (as root) by using urpmi gcc
• For Slackware, the package is available on their website - simply download, and type installpkg gcc-xxxxx.tgz
• For Gentoo, you should already have GCC installed as it will have been used when you first installed. To update it run (as root) emerge -uav gcc.
• For Arch Linux, install the GCC compiler (as root) by using pacman -S gcc.
• If you cannot become root, get the GCC tarball from ftp://ftp.gnu.org/ and follow the instructions in it to compile and install in your home directory. Be warned though, you need a C compiler to do that - yes, GCC itself is written in C.
• You can use a commercial C compiler/IDE.

Steps for Obtaining the GCC Compiler if You're on BSD Family Systems

Steps for Obtaining the GCC Compiler if You're on Windows

There are two ways to use GCC on Windows: Cygwin and MinGW. Applications compiled with Cygwin will not run on any computer without Cygwin, so MinGW is recommended. MinGW is simpler to install, and takes less disk space.

To get MinGW, do this:

2. Once the download is finished, open it and follow the instructions. You can also choose to install additional compilers, or the tool Make, but these aren't necessary.
3. Now you need to set your PATH. Right-click on "My computer" and click "Properties". Go to the "Advanced" tab and click on "Environment variables". Go to the "System variables" section and scroll down until you see "Path". Click on it, then click "edit". Add ";C:\mingw\bin\" (without the quotes) to the end.
4. To test if GCC works, open a command prompt and type "gcc". You should get the message "gcc: fatal error: no input files compilation terminated.". If you get this message, GCC is installed correctly.

To get Cygwin, do this:

1. Go to http://www.cygwin.com and click on the "Install Cygwin Now" button in the upper right corner of the page.
2. Click "run" in the window that pops up, and click "next" several times, accepting all the default settings.
4. When the "Select Packages" window appears, scroll down to the heading "Devel" and click on the "+" by it. In the list of packages that now displays, scroll down and find the "gcc-core" package; this is the compiler. Click once on the word "Skip", and it should change to some number like "3.4" etc. (the version number), and an "X" will appear next to "gcc-core" and several other related packages that will now be downloaded.
5. Click "next" and the compiler as well as the Cygwin tools should start downloading; this could take a while. While you're waiting for the installation to finish, download any text-editor designed for programming. While Cygwin does include some, you may prefer doing a web search to find other alternatives. While using a stock text editor is possible, it is not ideal.
6. Once the Cygwin downloads are finished and you have clicked "next", etc. to finish the installation, double-click the Cygwin icon on your desktop to begin the Cygwin "command prompt". Your home directory will automatically be set up in the Cygwin folder, which now should be at "C:\cygwin" (the Cygwin folder is in some ways like a small unix/linux computer on your Windows machine -- not technically of course, but it may be helpful to think of it that way).
7. Type "gcc" at the Cygwin prompt and press "enter"; if "gcc: no input files" or something like it appears you have succeeded and now have the gcc compiler on your computer (and congratulations -- you have also just received your first error message!).

Third option is to use WSL:

1. Go to http://aka.ms/wsldocs and follow the steps to install WSL
2. Go to https://aka.ms/vscode and follow the steps to install VSCode
3. Follow the guide and choose Get Started with C++ and WSL
4. As a result you will need to install possibly Ubuntu and set-up accordingly installing GCC like the Linux guide above.

The current stable (usable) version of GCC is 4.9.1 published on 2014-07-16, which supports several platforms. In fact, GCC is not only a C compiler, but a family of compilers for several languages, such as C++, Ada, Java, and Fortran.

### Embedded systems

• Most CPUs are microcontrollers in embedded systems, often programmed in C, but most of the compilers mentioned above (except GCC) do not support such CPUs. For specialized compilers that do support embedded systems, see Embedded Systems/C Programming.

### Other C compilers

We have a long list of C compilers in a much later section of this Wikibook. Which of those compilers would be suitable for beginning C programmers, that we should say a few words about getting started with that particular compiler in this section of this Wikibook?

# Intro Exercise

## The "Hello, World!" Program

Tradition dictates that we begin with a very simple program, which simply displays the characters "Hello, World!" on the screen and immediately exits. Type the following source code in your preferred text editor/IDE and save this in a file named hello.c.

#include <stdio.h>

int main(void)
{
printf("Hello, World!\n");
return 0;
}


### Source code analysis

Below are described the parts the program is composed of. The various details will be introduced and explained in later chapters.

#include <stdio.h>


This is a preprocessor directive. Preprocessor directives instruct a part of the compiler - the preprocessor - to modify the code we've written before it is compiled. In this case, the #include directive retrieves C code from the stdio.h file found in the standard library. Files used in this way are called header files and are saved with the .h extension. The stdio.h file contains many functions defined according to the C standard. For this program, the only function we need from stdio.h is the printf function.

int main(void)


The function named main is the starting point of all C programs. In computer science, the term function tends to be used a bit more loosely than in mathematics, since functions often express imperative ideas (as in the case of C) - that is, how-to process, instead of declarations. For now, suffice it to say, functions let us define a complex process that we want to reference frequently.

    printf("Hello World!\n");


This line is of particular interest because it produces the actual output on the console (also known as the terminal in the context of Unix-like operating systems), a traditional text-based interface to system utilities and programs.

    return 0;


When terminating our program, it is useful to be able to let the operating system know whether or not the program succeeded. We do this with an exit status, which is sent to the operating system with a return statement in the main function. In this case, we provide an exit status of 0 to indicate that execution succeeded without error. As our programs grow in complexity, we can use other integers as codes to indicate various types of errors. This style of providing exit status is a long standing convention[1].

### Compiling

#### Unix-like

If you are using a Unix(-like) system, such as GNU/Linux, Mac OS X, or Solaris, it will probably have GCC installed, otherwise on Linux you can install it using yum or apt-get commands depending on your distribution. Open the virtual console or a terminal emulator and enter the following (be certain your current working directory is the one containing your source code):

gcc hello.c


By default gcc will generate our executable binary with the name a.out. To run your new generated program type:

./a.out


You should see Hello, World! printed after the last prompt.

To see the exit status of the last program you ran, type on your shell command:

echo ?  This shows the value the main function has returned, which is 0 in the above example. There are a lot of options you can use with the gcc compiler. For example, if you want the output to have a name other than a.out, you can use the -o option. The following shows a few examples: -o indicates that the next parameter is the name of the resulting program (or library). If this option is not specified, the compiled program will, for historic reasons, end up in a file called "a.out" or "a.exe" (for cygwin users). -Wall indicates that gcc should warn about many types of suspicious code that are likely to be incorrect. You can use these options to create a program called "helloworld" instead of "a.out" by typing: gcc -o helloworld hello.c -Wall  Now you can run it by typing: ./helloworld  All the options are well documented in the manual[2] for GCC. #### On IDEs If you are using an IDE you may have to select console project, and to compile you just select build from the menu or the toolbar. The executable will appear inside the project folder, but you should have a menu button so you can just run the executable from the IDE. The process is roughly the same on all IDEs. ## References # Preliminaries Before learning C syntax and programming constructs, it is important to learn the meaning of a few key terms that are central in understanding C. ## Block Structure, Statements, Whitespace, and Scope Sentences delimited with '/*' and '*/' are comments, and the compiler ignores them. They are described in Programming Structure and Style Next we'll discuss the basic structure of a C program. If you're familiar with PASCAL, you may have heard it referred to as a block-structured language. C does not have complete block structure (and you'll find out why when you go over functions in detail) but it is still very important to understand what blocks are and how to use them. So what is in a block? Generally, a block consists of executable statements. But before we delve into blocks, let's examine statements. One way to describe statements is they are the text (and surrounding whitespace) the compiler will attempt to turn into executable instructions. A simpler definition is statements are bits of code that do things. For example: int i = 6;  This declares an integer variable, which can be accessed with the identifier 'i', and initializes it to the value 6. The various data types are introduced in the chapter Variables. You might have noticed the semicolon at the end of the statement. Statements in C always end with a semicolon (;). Leaving off the semicolon is a common mistake many people make, beginners and experts alike! So until it becomes second nature, be sure to double check your statements! Since C is a "free-format" language, several statements can share a single line in the source file, like this: /* this declares the variables 'i', 'test', 'foo', and 'bar' note that ONLY the variable named 'bar' is set to six! */ int i, test, foo, bar = 6;  There are several kinds of statements. You've already seen some of them, such as the assignment (i = 6;). A substantial portion of this book deals with statement construction. Back to our discussion of blocks. In C, blocks begin with an opening brace "{" and end with a closing brace "}". Blocks can contain other blocks which can contain their own blocks, and so on. Let's look at a block example. int main(void) { /* this is a 'block' */ int i = 5; { /* this is also a 'block', nested inside the outer block */ int i = 6; } return 0; }  You can use blocks with the preceding statements, such as the main function declaration (and other statements we've not yet covered), but you can also use blocks by themselves. Whitespace refers to the tab, space and newline characters that separate the text characters that make up the source code. Like many things in life, it's hard to appreciate whitespace until it's gone. To a C compiler, the source code  printf("Hello world"); return 0;  is the same as  printf("Hello world"); return 0;  which is also the same as  printf ( "Hello world") ; return 0;  The compiler simply ignores most whitespace (except, for example, when it separates return from 0). However, it is common practice to use spaces (or tabs) to organize source code for human readability. Most of the time we do not want other functions or other programmer's routines accessing data we are currently manipulating, which is why it is important to understand the concept of scope. Scope describes the level at which a piece of data or a function is visible. There are two types of scope in C, local and global. When we speak of global scope, we're referring to something that can be seen or manipulated from anywhere in the program. Local scope applies to a program element that can be seen or manipulated only within the block in which it was declared. Let's look at some examples to get a better idea of scope. int i = 5; /* this is a 'global' variable, it can be accessed from anywhere in the program */ /* this is a function, all variables inside of it are "local" to the function. */ int main(void) { int i = 6; /* 'i' now equals 6 */ printf("%d\n", i); /* prints a '6' to the screen, instead of the global variable of 'i', which is 5 */ return 0; }  That shows an example of local and global. But what about different scopes inside of functions? (you'll learn more about functions later, for now, just focus on the "main" part.) /* the main function */ int main(void) { /* this is the beginning of a 'block', you read about those above */ int i = 6; /* this is the first variable of this 'block', 'i' */ { /* this is a new 'block', and because it's a different block, it has its own scope */ /* this is also a variable called 'i', but in a different 'block', because it's in a different 'block' than the first variable named 'i', it doesn't affect the first one! */ int i = 5; printf("%d\n", i); /* prints a '5' onto the screen */ } /* now we're back into the first block */ printf("%d\n", i); /* prints a '6' onto the screen */ return 0; }  ## Basics of Using Functions Functions are a big part of programming. A function is a special kind of block that performs a well-defined task. If a function is well-designed, it can enable a programmer to perform a task without knowing anything about how the function works. The act of requesting a function to perform its task is called a function call. Many functions require a function call to hand it certain pieces of data needed to perform its task; these are called arguments. Many functions also return a value to the function call when they're finished; this is called a return value (the return value in the above program is 0). The things you need to know before calling a function are: • What the function does • The data type (discussed later) of the arguments and what they mean • The data type of the return value and what it means Many functions use the return value for the result of a computation. Some functions use the return value to indicate whether they successfully completed their work. As you have seen in the intro exercise, the main function uses the return value to provide an exit status to the operating system. All code other than global data definitions and declarations needs to be a part of a function. Usually, you're free to call a function whatever you wish to. The only restriction is that every executable program needs to have one, and only one, main function, which is where the program begins executing. We will discuss functions in more detail in a later chapter, C Programming/Procedures and functions. ## The Standard Library In 1983, when C was in the process of becoming standardized, the American National Standards Institute (ANSI) formed a committee to establish a standard specification of C known as "ANSI C". That standard specification created a basic set of functions common to each implementation of C, which is referred to as the Standard Library. The Standard Library provides functions for tasks such as input/output, string manipulation, mathematics, files, and memory allocation. The Standard Library does not provide functions that are dependent on specific hardware or operating systems, like graphics, sound, or networking. In the "Hello, World" program, a Standard Library function is used (printf) which outputs lines of text to the standard output stream. ## References # Basics of Compilation Having covered the basic concepts of C programming, we can now briefly discuss the process of compilation. Like any programming language, C by itself is completely incomprehensible to a microprocessor. Its purpose is to provide an intuitive way for humans to provide instructions that can be easily converted into machine code that is comprehensible to a microprocessor. The compiler is what translates our human-readable source code into machine code. To those new to programming, this seems fairly simple. A naive compiler might read in every source file, translate everything into machine code, and write out an executable. That could work, but has two serious problems. First, for a large project, the computer may not have enough memory to read all of the source code at once. Second, if you make a change to a single source file, you would have to recompile the entire application. To deal with these problems, compilers break the job into steps. For each source file (each .c file), the compiler reads the file, reads the files it references via the #include directive, and translates them to machine code. The result of this is an "object file" (.o). After all the object files are created, a "linker" program collects all of the object files and writes the actual executable program. That way, if you change one source file, only that file needs to be recompiled, after which, the application will need to be re-linked. Without going into details, it can be beneficial to have a superficial understanding of the compilation process. ## Preprocessor The preprocessor provides the ability for the inclusion of so called header files, macro expansions, conditional compilation and line control. Many times you will need to give special instructions to your compiler. This can be done by inserting preprocessor directives into your code. When you begin compiling your code, a special program called the preprocessor scans the source code and performs simple substitution of token strings for others according to predefined rules. The C preprocessor is not a part of the C language. All preprocessor directives begin with the hash character (#). You can see one preprocessor directive in the Hello world program. Example:  #include <stdio.h>  This directive causes the stdio header to be included into your program. Other directives such as #pragma control compiler settings and macros. The result of the preprocessing stage is a text string. You can think of the preprocessor as a non-interactive text editor that prepares your code for compilation. The language of preprocessor directives is agnostic to the grammar of C, so the C preprocessor can also be used independently to process other kinds of text files. ## Syntax Checking This step ensures that the code is valid and will sequence into an executable program. Under most compilers, you may get messages or warnings indicating potential issues with your program (such as a conditional statement always being true or false, etc.) When an error is detected in the program, the compiler will normally report the file name and line that is preventing compilation. ## Object Code The compiler produces a machine code equivalent of the source code that can be linked into the final program. At this point the code itself can't be executed, as it requires linking to do so. It's important to note after discussing the basics that compilation is a "one way street". That is, compiling a C source file into machine code is easy, but "decompiling" (turning machine code into the C source that creates it) is not. Decompilers for C do exist, but the code they create is hard to understand and only useful for reverse engineering. ## Linking Linking combines the separate object files into one complete program by integrating libraries and the code and producing either an executable program or a library. Linking is performed by a linker program, which is often part of a compiler suite. Common errors during this stage are either missing or duplicate functions. ## Automation For large C projects, many programmers choose to automate compilation, both in order to reduce user interaction requirements and to speed up the process by recompiling only modified files. Most Integrated Development Environments (IDE's) have some kind of project management which makes such automation very easy. However, the project management files are often usable only by users of the same integrated development environment, so anyone desiring to modify the project would need to use the same IDE. On UNIX-like systems, make and Makefiles are often used to accomplish the same. Make is traditional and flexible and is available as one of the standard developer tools on most Unix and GNU distributions. Makefiles have been extended by the GNU Autotools, composed of Automake and Autoconf for making software compilable, testable, translatable and configurable on many types of machines. Automake and Autoconf are described in detail in their respective manuals. The Autotools are often perceived to be complicated and various simpler build systems have been developed. Many components of the GNOME project now use the declarative Meson build system which is less flexible, but instead focuses on providing the features most commonly needed from a build system in a simple way. Other popular build systems for programs written in the C language include CMake and Waf. Once gcc is installed, it can be called with a list of c source files that have been written but not yet compiled. e.g. if the file main.c includes functions described in myfun.h and implemented in myfun_a.c and myfun_b.c, then it is enough to write  gcc main.c myfun_a.c myfun_b.c  myfun.h is included in main.c, but if it is in a separate header file directory, then that directory can be listed after a "-I " switch. In larger programs, Makefiles and gnu make program can compile c files into intermediate files ending with suffix .o which can be linked by gcc. How to compile each object file is usually described in the Makefile with the object file as a label ending with a colon followed by two spaces (tabs often cause problems) followed by a list of other files that are dependencies, e.g. .c files and .o files compiled in another section, and on the next line, the invocation of gcc that is required. Typing man make or info make often gives the information needed to on how to use make, as well as gcc. Although gcc has a lot of option switches, one often used is -g to generate debugging information for gdb to allow gdb to show source code during a step-through of the machine code program. gdb has an 'h' command that shows what it can do, and is usually started with 'gdb a.out' if a.out is the anonymous executable machine code file that was compiled by gcc. ## References # Programming Structure and Style ## C Structure and Style This is a basic introduction to good coding style in the C Programming Language. It is designed to provide information on how to effectively use indentation, comments, and other elements that will make your C code more readable. It is not a tutorial on actual C programming. As a beginning programmer, the point of creating structure in the program code might not be clear, as the compiler doesn't care about the difference. However, as programs become complex, chances are that writing the program has become a joint effort. (Or others might want to see how it was accomplished. Or you may have to read it again years later.) Well-written code also helps you get an overview of what the code does. In the following sections, we will attempt to explain good programming practices that will in turn make your programs clearer. ## Introduction In C, programs are composed of statements. Statements are terminated with a semi-colon, and are collected in sections known as functions. By convention, a statement should be kept on its own line, as shown in the example below:  #include <stdio.h> int main(void) { printf("Hello, World!\n"); return 0; }  The following block of code is essentially the same. While it contains exactly the same code, and will compile and execute with the same result, the removal of spacing causes an essential difference: it's harder to read.  #include <stdio.h> int main(void) {printf("Hello, World!\n");return 0;}  The simple use of indents and line breaks can greatly improve code readability without impacting code performance. Readable code makes it much easier to see where functions and procedures end and which lines are part of which loops and procedures. This lesson is going to focus on improving the coding style of an example piece of code which applies a formula and prints the result. Later, you'll see how to write code for such tasks in more detail. For now, focus on how the code looks, not what it does. ## Line Breaks and Indentation The addition of white space inside your code is arguably the most important part of good code structure. Effective use of white space can create a visual scale of how your code flows, which can be very important when returning to your code when you want to maintain it. ### Line Breaks With minimal line breaks, code is barely human-readable, and may be hard to debug or understand: 1 #include <stdio.h> 2 int main(void) { int revenue = 80; int cost = 50; int roi; roi = (100 * (revenue - cost)) / cost; if (roi >= 0) { printf ("%d\n", roi); } return 0; }  Rather than putting everything on one line, it is much more readable to break up long lines so that each statement and declaration goes on its own line. After inserting line breaks, the code will look like this:  1 #include <stdio.h> 2 int main(void) { 3 int revenue = 80; 4 int cost = 50; 5 int roi; 6 roi = (100 * (revenue - cost)) / cost; 7 if (roi >= 0) { 8 printf ("%d\n", roi); 9 } 10 return 0; 11 }  ### Blank Lines Blank lines should be used to offset the main components of your code. Always use them • After preprocessor directives. • After new variables are declared. • Use your own judgment for finding other places where components should be separated. Based on these two rules, there should now be at least two line breaks added. • After line 1, because line 1 has a preprocessor directive. • After line 5, because line 5 contains a variable declaration. This will make the code much more readable than it was before: The following lines of code have line breaks between functions, but without indentation.  1 #include <stdio.h> 2 3 int main(void) { 4 5 int revenue = 80; 6 int cost = 50; 7 8 int roi; 9 10 roi = (100 * (revenue - cost)) / cost; 11 12 if (roi >= 0) { 13 printf ("%d\n", roi); 14 } 15 16 return 0; 17 }  But it's still not as readable as it can be. ### Indentation Many text editors automatically indent appropriately when you hit the enter/return key. Although adding simple line breaks between key blocks of code can make code easier to read, it provides no information about the block structure of the program. Using the tab key can be very helpful. Indentation visually separates paths of execution by moving their starting points to a new column. This simple practice will make it much easier to read and understand code. Indentation follows a fairly simple rule: • All code inside a new block should be indented by one tab[1] more than the code in the previous path. Based on the code from the previous section, there are two blocks requiring indentation: • Lines 4 to 16 • Line 13  1 #include <stdio.h> 2 3 int main(void) { 4 5 int revenue = 80; 6 int cost = 50; 7 8 int roi; 9 10 roi = (100 * (revenue - cost)) / cost; 11 12 if (roi >= 0) { 13 printf ("%d\n", roi); 14 } 15 16 return 0; 17 }  It is now fairly obvious as to which parts of the program fit inside which blocks. You can tell which parts of the program the coder has intended to be conditional, and which ones he or she has not. Although it might not be immediately noticeable, once many nested paths get added to the structure of the program, the use of indentation can be very important. Thus, indentation makes the structure of your program clear. It is claimed that research has shown that an indentation size between 2 to 4 characters is easier to read than 8 character indents[2]. However, an indent of 8 characters may still be in use for some systems[3]. ## Comments Comments in code can be useful for a variety of purposes. They provide the easiest way to set off specific parts of code (and their purpose); as well as providing a visual "split" between various parts of your code. Having good comments throughout your code will make it much easier to remember what specific parts of your code do. Comments in modern flavors of C (and many other languages) can come in two forms: 1 //Single Line Comments (added by C99 standard, famously known as c++ style of comments)  and 1 /*Multi-Line 2 Comments 3 (only form of comments supported by C89 standard)*/  Note that Single line comments are a more recent addition to C, so some compilers may not support them. A recent version of GCC will have no problems supporting them. This section is going to focus on the various uses of each form of commentary. ### Single-line Comments Single-line comments are most useful for simple 'side' notes that explain what certain parts of the code do. The best places to put these comments are next to variable declarations, and next to pieces of code that may need explanation. Comments should make clear the intention and ideas behind the corresponding code. What is immediately obvious from reading the code does not belong in a comment. Based on our previous program, there are various good places to place comments • Line 5 and/or 6, to explain what 'int revenue' and 'int cost' represent, • Line 8, to explain what the variable 'roi' is going to be used for, • Line 10, to explain the idea of the calculation, • Line 12, to explain the purpose of the 'if'. This will make our program look something like #include <stdio.h> int main(void) { int revenue = 80; // as of 2016 int cost = 50; int roi; // return on investment in percent roi = (100 * (revenue - cost)) / cost; // formula from accounting book if (roi >= 0) { // we don't care about negative roi printf ("%d\n", roi); } return 0; }  ### Multi-line Comments Single-line comments are a new feature, so many C programmers only use multi-line comments. Multi-line comments are most useful for long explanations of code. They can be used as copyright/licensing notices, and they can also be used to explain the purpose of a block of code. This can be useful for two reasons: They make your functions easier to understand, and they make it easier to spot errors in code. If you know what a block is supposed to do, then it is much easier to find the piece of code that is responsible if an error occurs. As an example, suppose we had a program that was designed to print "Hello, World! " a certain number of lines, a specified number of times. There would be many for loops in this program. For this example, we shall call the number of lines i, and the number of strings per line as j. A good example of a multi-line comment that describes 'for' loop i's purpose would be:  /* For Loop (int i) Loops the following procedure i times (for number of lines). Performs 'for' loop j on each loop, and prints a new line at end of each loop. */  This provides a good explanation of what i's purpose is, whilst not going into detail of what j does. By going into detail over what the specific path does (and not ones inside it), it will be easier to troubleshoot the path. Similarly, you should always include a multi-line comment before each function, to explain the role, preconditions and postconditions of each function. Always leave the technical details to the individual blocks inside your program - this makes it easier to troubleshoot. A function descriptor should look something like:  /* Function : int hworld (int i,int j) Input : int i (Number of lines), int j (Number of instances per line) Output : 0 (on success) Procedure: Prints "Hello, World!" j times, and a new line to standard output over i lines. */  This system allows for an at-a-glance explanation of what the function should do. You can then go into detail over how each aspect of the program is achieved later on in the program. Finally, if you like to have aesthetically-pleasing source code, the multi-line comment system allows for the easy addition of comment boxes. These make the comments stand out much more than they would without otherwise. They look like this.  /*************************************** * This is a multi line comment * That is nearly surrounded by a * Cool, starry border! ***************************************/  Applied to our original program, we can now include a much more descriptive and readable source code: #include <stdio.h> int main(void){ /************************************************************************************ * Function: int main(void) * Input : none * Output : Returns 0 on success * Procedure: Prints 2016's return on investment in percent if it is not negative. ************************************************************************************/ int revenue = 80; // as of 2016 int cost = 50; int roi; // return on investment in percent roi = (100 * (revenue - cost)) / cost; // formula from accounting book if (roi >= 0) { // we don't care about negative roi printf ("%d\n", roi); } return 0; }  This will allow any outside users of the program an easy way to comprehend what the code functions are and how they operate. It also inhibits uncertainty with other like-named functions. A few programmers add a column of stars on the right side of a block comment:  /*************************************** * This is a multi line comment * * that is completely surrounded by a * * cool, starry border! * ***************************************/  But most programmers don't put any stars on the right side of a block comment. They feel that aligning the right side is a waste of time. Comments written in source files can be used for documenting source code automatically by using popular tools like Doxygen.[4][5] ## References 1. Several programmers recommend "use spaces for indentation. Do not use tabs in your code. You should set your editor to emit spaces when you hit the tab key." [1] [2] Other programmers disagree. [3] [4] Regardless of whether you prefer spaces or tabs, make sure you keep it consistent within the projects you are working on. Mixing tabs and spaces can cause code to become unreadable. 2. https://www.kernel.org/doc/html/latest/process/coding-style.html Linux Kernel Coding Style 3. "Coding Conventions for C++ and Java" "all the block comments illustrated in this document have no pretty stars on the right side of the block comment. This deliberate choice was made because aligning those pretty stars is a large waste of time and discourages the maintenance of in-line comments.", 4. c2:BigBlocksOfAsterisks,"Code craft" by Pete Goodliffe page 82,Falvotech "C Programming Style Guide", Fedora Directory Server Coding Style # Variables Like most programming languages, C is able to use and process named variables and their contents. Variables are simply names used to refer to some location in memory – a location that holds a value with which we are working. It may help to think of variables as a placeholder for a value. You can think of a variable as being equivalent to its assigned value. So, if you have a variable i that is initialized (set equal) to 4, then it follows that i + 1 will equal 5. Since C is a relatively low-level programming language, before a C program can utilize memory to store a variable it must claim the memory needed to store the values for a variable. This is done by declaring variables. Declaring variables is the way in which a C program shows the number of variables it needs, what they are going to be named, and how much memory they will need. Within the C programming language, when managing and working with variables, it is important to know the type of variables and the size of these types. A type’s size is the amount of computer memory required to store one value of this type. Since C is a fairly low-level programming language, the size of types can be specific to the hardware and compiler used – that is, how the language is made to work on one type of machine can be different from how it is made to work on another. All variables in C are typed. That is, every variable declared must be assigned as a certain type of variable. ## Declaring, Initializing, and Assigning Variables Here is an example of declaring an integer, which we've called some_number. (Note the semicolon at the end of the line; that is how your compiler separates one program statement from another.) int some_number;  This statement means we're declaring some space for a variable called some_number, which will be used to store integer data. Note that we must specify the type of data that a variable will store. There are specific keywords to do this – we'll look at them in the next section. Multiple variables can be declared with one statement, like this: int anumber, anothernumber, yetanothernumber;  We can also declare and assign some content to a variable at the same time. int some_number = 3;  This is called initialization. In early versions of C, variables had to be declared at the beginning of a block. In C99 it is allowed to mix declarations and statements arbitrarily – but doing so is not usual, because it is rarely necessary, some compilers still don’t support C99 (portability), and it may, because it is uncommon yet, irritate fellow programmers (maintainability). After declaring variables, you can assign a value to a variable later on using a statement like this: some_number = 3;  You can also assign a variable the value of another variable, like so: anumber = anothernumber;  Or assign multiple variables the same value with one statement: anumber = anothernumber = yetanothernumber = 3;  This is because the assignment x = y returns the value of the assignment, y. For example, some_number = 3 returns 3. That said, x = y = z is really a shorthand for x = (y = z). ### Naming Variables Variable names in C are made up of letters (upper and lower case) and digits. The underscore character ("_") is also permitted. Names must not begin with a digit. Unlike some languages (such as Perl and some BASIC dialects), C does not use any special prefix characters on variable names. Some examples of valid (but not very descriptive) C variable names: foo Bar BAZ foo_bar _foo42 _ QuUx  Some examples of invalid C variable names: 2foo (must not begin with a digit) my foo (spaces not allowed in names)foo    (\$ not allowed -- only letters, and _)
while   (language keywords cannot be used as names)


As the last example suggests, certain words are reserved as keywords in the language, and these cannot be used as variable names.

It is not allowed to use the same name for multiple variables in the same scope. When working with other developers, you should therefore take steps to avoid using the same name for global variables or function names. Some large projects adhere to naming guidelines[1] to avoid duplicate names and for consistency.

In addition there are certain sets of names that, while not language keywords, are reserved for one reason or another. For example, a C compiler might use certain names "behind the scenes", and this might cause problems for a program that attempts to use them. Also, some names are reserved for possible future use in the C standard library. The rules for determining exactly what names are reserved (and in what contexts they are reserved) are too complicated to describe here[citation needed], and as a beginner you don't need to worry about them much anyway. For now, just avoid using names that begin with an underscore character.

The naming rules for C variables also apply to naming other language constructs such as function names, struct tags, and macros, all of which will be covered later.

## Literals

Anytime within a program in which you specify a value explicitly instead of referring to a variable or some other form of data, that value is referred to as a literal. In the initialization example above, 3 is a literal. Literals can either take a form defined by their type (more on that soon), or one can use hexadecimal (hex) notation to directly insert data into a variable regardless of its type.[citation needed] Hex numbers are always preceded with 0x. For now, though, you probably shouldn't be too concerned with hex.

## The Four Basic Data Types

In Standard C there are four basic data types. They are int, char, float, and double.

### The int type

The int type stores integers in the form of "whole numbers". An integer is typically the size of one machine word, which on most modern home PCs is 32 bits (4 octets). Examples of literals are whole numbers (integers) such as 1, 2, 3, 10, 100... When int is 32 bits (4 octets), it can store any whole number (integer) between -2147483648 and 2147483647. A 32 bit word (number) has the possibility of representing any one number out of 4294967296 possibilities (2 to the power of 32).

If you want to declare a new int variable, use the int keyword. For example:

int numberOfStudents, i, j=5;


In this declaration we declare 3 variables, numberOfStudents, i and j, j here is assigned the literal 5.

### The char type

The char type is capable of holding any member of the execution character set. It stores the same kind of data as an int (i.e. integers), but typically has a size of one byte. The size of a byte is specified by the macro CHAR_BIT which specifies the number of bits in a char (byte). In standard C it never can be less than 8 bits. A variable of type char is most often used to store character data, hence its name. Most implementations use the ASCII character set as the execution character set, but it's best not to know or care about that unless the actual values are important.

Examples of character literals are 'a', 'b', '1', etc., as well as some special characters such as '\0' (the null character) and '\n' (newline, recall "Hello, World"). Note that the char value must be enclosed within single quotations.

When we initialize a character variable, we can do it two ways. One is preferred, the other way is bad programming practice.

The first way is to write

char letter1 = 'a';


This is good programming practice in that it allows a person reading your code to understand that letter1 is being initialized with the letter 'a' to start off with.

The second way, which should not be used when you are coding letter characters, is to write

char letter2 = 97; /* in ASCII, 97 = 'a' */


This is considered by some to be extremely bad practice, if we are using it to store a character, not a small number, in that if someone reads your code, most readers are forced to look up what character corresponds with the number 97 in the encoding scheme. In the end, letter1 and letter2 store both the same thing – the letter 'a', but the first method is clearer, easier to debug, and much more straightforward.

One important thing to mention is that characters for numerals are represented differently from their corresponding number, i.e. '1' is not equal to 1. In short, any single entry that is enclosed within 'single quotes'.

There is one more kind of literal that needs to be explained in connection with chars: the string literal. A string is a series of characters, usually intended to be displayed. They are surrounded by double quotations (" ", not ' '). An example of a string literal is the "Hello, World!\n" in the "Hello, World" example.

The string literal is assigned to a character array, arrays are described later. Example:

const char MY_CONSTANT_PEDANTIC_ITCH[] = "learn the usage context.\n";
printf("Square brackets after a variable name means it is a pointer to a string of memory blocks the size of the type of the array element.\n");


### The float type

float is short for floating point. It stores inexact representations of real numbers, both integer and non-integer values. It can be used with numbers that are much greater than the greatest possible int. float literals must be suffixed with F or f. Examples are: 3.1415926f, 4.0f, 6.022e+23f.

It is important to note that floating-point numbers are inexact. Some numbers like 0.1f cannot be represented exactly as floats but will have a small error. Very large and very small numbers will have less precision and arithmetic operations are sometimes not associative or distributive because of a lack of precision. Nonetheless, floating-point numbers are most commonly used for approximating real numbers and operations on them are efficient on modern microprocessors.[2] Floating-point arithmetic is explained in more detail on Wikipedia.

float variables can be declared using the float keyword. A float is only one machine word in size. Therefore, it is used when less precision than a double provides is required.

### The double type

The double and float types are very similar. The float type allows you to store single-precision floating point numbers, while the double keyword allows you to store double-precision floating point numbers – real numbers, in other words. Its size is typically two machine words, or 8 bytes on most machines. Examples of double literals are 3.1415926535897932, 4.0, 6.022e+23 (scientific notation). If you use 4 instead of 4.0, the 4 will be interpreted as an int.

The distinction between floats and doubles was made because of the differing sizes of the two types. When C was first used, space was at a minimum and so the judicious use of a float instead of a double saved some memory. Nowadays, with memory more freely available, you rarely need to conserve memory like this – it may be better to use doubles consistently. Indeed, some C implementations use doubles instead of floats when you declare a float variable.

If you want to use a double variable, use the double keyword.

## sizeof

If you have any doubts as to the amount of memory actually used by any variable (and this goes for types we'll discuss later, also), you can use the sizeof operator to find out for sure. (For completeness, it is important to mention that sizeof is a unary operator, not a function.) Its syntax is:

sizeof object
sizeof(type)


The two expressions above return the size of the object and type specified, in bytes. The return type is size_t (defined in the header <stddef.h>) which is an unsigned value. Here's an example usage:

size_t size;
int i;
size = sizeof(i);


size will be set to 4, assuming CHAR_BIT is defined as 8, and an integer is 32 bits wide. The value of sizeof's result is the number of bytes.

Note that when sizeof is applied to a char, the result is 1; that is:

sizeof(char)


always returns 1.

## Data type modifiers

One can alter the data storage of any data type by preceding it with certain modifiers.

long and short are modifiers that make it possible for a data type to use either more or less memory. The int keyword need not follow the short and long keywords. This is most commonly the case. A short can be used where the values fall within a lesser range than that of an int, typically -32768 to 32767. A long can be used to contain an extended range of values. It is not guaranteed that a short uses less memory than an int, nor is it guaranteed that a long takes up more memory than an int. It is only guaranteed that sizeof(short) <= sizeof(int) <= sizeof(long). Typically a short is 2 bytes, an int is 4 bytes, and a long either 4 or 8 bytes. Modern C compilers also provide long long which is typically an 8 byte integer.

In all of the types described above, one bit is used to indicate the sign (positive or negative) of a value. If you decide that a variable will never hold a negative value, you may use the unsigned modifier to use that one bit for storing other data, effectively doubling the range of values while mandating that those values be positive. The unsigned specifier also may be used without a trailing int, in which case the size defaults to that of an int. There is also a signed modifier which is the opposite, but it is not necessary, except for certain uses of char, and seldom used since all types (except char) are signed by default.

The long modifier can also be used with double to create a long double type. This floating-point type may (but is not required to) have greater precision than the double type.

To use a modifier, just declare a variable with the data type and relevant modifiers:

unsigned short int usi;  /* fully qualified -- unsigned short int */
short si;                /* short int */
unsigned long uli;       /* unsigned long int */


## const qualifier

When the const qualifier is used, the declared variable must be initialized at declaration. It is then not allowed to be changed.

While the idea of a variable that never changes may not seem useful, there are good reasons to use const. For one thing, many compilers can perform some small optimizations on data when it knows that data will never change. For example, if you need the value of π in your calculations, you can declare a const variable of pi, so a program or another function written by someone else cannot change the value of pi.

Note that a Standard conforming compiler must issue a warning if an attempt is made to change a const variable - but after doing so the compiler is free to ignore the const qualifier.

## Magic numbers

When you write C programs, you may be tempted to write code that will depend on certain numbers. For example, you may be writing a program for a grocery store. This complex program has thousands upon thousands of lines of code. The programmer decides to represent the cost of a can of corn, currently 99 cents, as a literal throughout the code. Now, assume the cost of a can of corn changes to 89 cents. The programmer must now go in and manually change each entry of 99 cents to 89. While this is not that big of a problem, considering the "global find-replace" function of many text editors, consider another problem: the cost of a can of green beans is also initially 99 cents. To reliably change the price, you have to look at every occurrence of the number 99.

C possesses certain functionality to avoid this. This functionality is approximately equivalent, though one method can be useful in one circumstance, over another.

### Using the const keyword

The const keyword helps eradicate magic numbers. By declaring a variable const corn at the beginning of a block, a programmer can simply change that const and not have to worry about setting the value elsewhere.

There is also another method for avoiding magic numbers. It is much more flexible than const, and also much more problematic in many ways. It also involves the preprocessor, as opposed to the compiler. Behold...

### #define

When you write programs, you can create what is known as a macro, so when the computer is reading your code, it will replace all instances of a word with the specified expression.

Here's an example. If you write

#define PRICE_OF_CORN 0.99


when you want to, for example, print the price of corn, you use the word PRICE_OF_CORN instead of the number 0.99 – the preprocessor will replace all instances of PRICE_OF_CORN with 0.99, which the compiler will interpret as the literal double 0.99. The preprocessor performs substitution, that is, PRICE_OF_CORN is replaced by 0.99 so this means there is no need for a semicolon.

It is important to note that #define has basically the same functionality as the "find-and-replace" function in a lot of text editors/word processors.

For some purposes, #define can be harmfully used, and it is usually preferable to use const if #define is unnecessary. It is possible, for instance, to #define, say, a macro DOG as the number 3, but if you try to print the macro, thinking that DOG represents a string that you can show on the screen, the program will have an error. #define also has no regard for type. It disregards the structure of your program, replacing the text everywhere (in effect, disregarding scope), which could be advantageous in some circumstances, but can be the source of problematic bugs.

You will see further instances of the #define directive later in the text. It is good convention to write #defined words in all capitals, so a programmer will know that this is not a variable that you have declared but a #defined macro. It is not necessary to end a preprocessor directive such as #define with a semicolon; in fact, some compilers may warn you about unnecessary tokens in your code if you do.

## Scope

In the Basic Concepts section, the concept of scope was introduced. It is important to revisit the distinction between local types and global types, and how to declare variables of each. To declare a local variable, you place the declaration at the beginning (i.e. before any non-declarative statements) of the block to which the variable is deemed to be local. To declare a global variable, declare the variable outside of any block. If a variable is global, it can be read, and written, from anywhere in your program.

Global variables are not considered good programming practice, and should be avoided whenever possible. They inhibit code readability, create naming conflicts, waste memory, and can create difficult-to-trace bugs. Excessive usage of globals is usually a sign of laziness or poor design. However, if there is a situation where local variables may create more obtuse and unreadable code, there's no shame in using globals.

## Other Modifiers

Included here, for completeness, are more of the modifiers that standard C provides. For the beginning programmer, static and extern may be useful. volatile is more of interest to advanced programmers. register and auto are largely deprecated and are generally not of interest to either beginning or advanced programmers.

### static

static is sometimes a useful keyword. It is a common misbelief that the only purpose is to make a variable stay in memory.

When you declare a function or global variable as static, you cannot access the function or variable through the extern (see below) keyword from other files in your project. This is called static linkage.

When you declare a local variable as static, it is created just like any other variable. However, when the variable goes out of scope (i.e. the block it was local to is finished) the variable stays in memory, retaining its value. The variable stays in memory until the program ends. While this behaviour resembles that of global variables, static variables still obey scope rules and therefore cannot be accessed outside of their scope. This is called static storage duration.

Variables declared static are initialized to zero (or for pointers, NULL[3][4]) by default. They can be initialized explicitly on declaration to any constant value. The initialization is made just once, at compile time.

You can use static in (at least) two different ways. Consider this code, and imagine it is in a file called jfile.c:

#include <stdio.h>

static int j = 0;

void up(void)
{
/* k is set to 0 when the program starts. The line is then "ignored"
* for the rest of the program (i.e. k is not set to 0 every time up()
* is called)
*/
static int k = 0;
j++;
k++;
printf("up() called.   k= %2d, j= %2d\n", k , j);
}

void down(void)
{
static int k = 0;
j--;
k--;
printf("down() called. k= %2d, j= %2d\n", k , j);
}

int main(void)
{
int i;

/* call the up function 3 times, then the down function 2 times */
for (i = 0; i < 3; i++)
up();
for (i = 0; i < 2; i++)
down();

return 0;
}


The j variable is accessible by both up and down and retains its value. The k variables also retain their value, but they are two different variables, one in each of their scopes. Static variables are a good way to implement encapsulation, a term from the object-oriented way of thinking that effectively means not allowing changes to be made to a variable except through function calls.

Running the program above will produce the following output:

up() called.   k=  1, j=  1
up() called.   k=  2, j=  2
up() called.   k=  3, j=  3
down() called. k= -1, j=  2
down() called. k= -2, j=  1


Features of static variables :

    1. Keyword used        - static
2. Storage             - Memory
3. Default value       - Zero
4. Scope               - Local to the block in which it is declared
5. Lifetime            - Value persists between different function calls
6. Keyword optionality - Mandatory to use the keyword


### extern

extern is used when a file needs to access a variable in another file that it may not have #included directly. Therefore, extern does not actually carve out space for a new variable, it just provides the compiler with sufficient information to access the remote variable.

Features of extern variable :

    1. Keyword used        - extern
2. Storage             - Memory
3. Default value       - Zero
4. Scope               - Global (all over the program)
5. Lifetime            - Value persists till the program's execution comes to an end
6. Keyword optionality - Optional if declared outside all the functions


### volatile

volatile is a special type of modifier which informs the compiler that the value of the variable may be changed by external entities other than the program itself. This is necessary for certain programs compiled with optimizations – if a variable were not defined volatile then the compiler may assume that certain operations involving the variable are safe to optimize away when in fact they aren't. volatile is particularly relevant when working with embedded systems (where a program may not have complete control of a variable) and multi-threaded applications.

### auto

auto is a modifier which specifies an "automatic" variable that is automatically created when in scope and destroyed when out of scope. If you think this sounds like pretty much what you've been doing all along when you declare a variable, you're right: all declared items within a block are implicitly "automatic". For this reason, the auto keyword is more like the answer to a trivia question than a useful modifier, and there are lots of very competent programmers that are unaware of its existence.

Features of automatic variables :

    1. Keyword used        - auto
2. Storage             - Memory
3. Default value       - Garbage value (random value)
4. Scope               - Local to the block in which it is defined
5. Lifetime            - Value persists while the control remains within the block
6. Keyword optionality - Optional


### register

register is a hint to the compiler to attempt to optimize the storage of the given variable by storing it in a register of the computer's CPU when the program is run. Most optimizing compilers do this anyway, so use of this keyword is often unnecessary. In fact, ANSI C states that a compiler can ignore this keyword if it so desires – and many do. Microsoft Visual C++ is an example of an implementation that completely ignores the register keyword.

Features of register variables :

    1. Keyword used        - register
2. Storage             - CPU registers (values can be retrieved faster than from memory)
3. Default value       - Garbage value
4. Scope               - Local to the block in which it is defined
5. Lifetime            - Value persists while the control remains within the block
6. Keyword optionality - Mandatory to use the keyword


## References

1. Examples of naming guidelines are those of the GNOME Project or the parts of the Python interpreter that are written in C.
2. Representations of real numbers other than floating-point numbers exist but are not fundamental data types in C. Some C compilers support fixed-point arithmetic data types, but these are not part of standard C. Libraries such as the GNU Multiple Precision Arithmetic Library offer more data types for real numbers and very large numbers.
3. [5] - What is NULL and how is it defined?
4. [6] - NULL or 0, which should you use?

# Simple Input and Output

When you take time to consider it, a computer would be pretty useless without some way to talk to the people who use it. Just like we need information in order to accomplish tasks, so do computers. And just as we supply information to others so that they can do tasks, so do computers.

These supplies and returns of information to a computer are called input and output. 'Input' is information supplied to a computer or program. 'Output' is information provided by a computer or program. Frequently, computer programmers will lump the discussion in the more general term input/output or simply, I/O.

In C, there are many different ways for a program to communicate with the user. Amazingly, the most simple methods usually taught to beginning programmers may also be the most powerful. In the Hello, World! example at the beginning of this text, we were introduced to a Standard Library file stdio.h, and one of its functions, printf(). Here we discuss more of the functions that stdio.h gives us.

## Output using printf()

Recall from the beginning of this text the demonstration program duplicated below:

#include <stdio.h>

int main(void)
{
printf("Hello, World!");
return 0;
}


If you compile and run this program, you will see the sentence below show up on your screen:

  Hello, World! 

This amazing accomplishment was achieved by using the function printf(). A function is like a "black box" that does something for you without exposing the internals inside. We can write functions ourselves in C, but we will cover that later.

You have seen that to use printf() one puts text, surrounded by quotes, in between the parentheses. We call the text surrounded by quotes a literal string (or just a string), and we call that string an argument to printf.

As a note of explanation, it is sometimes convenient to include the open and closing parentheses after a function name to remind us that it is, indeed, a function. However usually when the name of the function we are talking about is understood, it is not necessary.

As you can see in the example above, using printf() can be as simple as typing in some text, surrounded by double quotes (note that these are double quotes and not two single quotes). So, for example, you can print any string by placing it as an argument to the printf() function:

  printf("This sentence will print out exactly as you see it..."); 

And once it is contained in a proper main() function, it will show:

  This sentence will print out exactly as you see it... 

### Printing numbers and escape sequences

#### Placeholder codes

The printf() function is a powerful function, and is probably the most-used function in C programs.

For example, let us look at a problem. Say we want to calculate: 19 + 31. Let's use C to get the answer.

We start writing

#include "stdio.h" // this is important, since printf
// can't be used without this header

int main(void)
{
printf("19+31 is");


But here we are stuck! printf() only prints strings! Thankfully, printf has methods for printing numbers. What we do is put a placeholder format code in the string. We write:

    printf("19+31 is '''%d'''", 19+31);


The placeholder %d literally "holds the place" for the actual number that is the result of adding 19 to 31.

These placeholders are called format specifiers. Many other format specifiers work with printf(). If we have a floating-point number, we can use %f to print out a floating-point number, decimal point and all. Other format specifiers are:

• %d - int (same as %i)
• %ld - long int (same as %li)
• %f - float
• %lf - double[1]
• %c - char
• %s - string
• %x - hexadecimal

A complete listing of all the format specifiers for printf() is on Wikipedia.

#### Tabs and newlines

What if, we want to achieve some output that will look like:

   1905
312 +
-----


printf() will not put line breaks in at the end of each statement: we must do this ourselves. But how?

What we can do is use the newline escape character. An escape character is a special character that we can write but will do something special onscreen, such as make a beep, write a tab, and so on. To write a newline we write \n. All escape characters start with a backslash.

So to achieve the output above, we write

    printf(" 1905\n312 +\n-----\n");


or to be a bit more clear, we can break this long printf statement over several lines. So our program will be

#include <stdio.h>

int main(void)
{
printf(" 1905\n");
printf("312 +\n");
printf("-----\n");
printf("%d", 1905+312);
return 0;
}


There are other escape characters we can use. Another common one is to use \t to write a tab. You can use \a to ring the computer's bell, but you should not use this very much in your programs, as excessive use of sound is not very friendly to the user.

## Other output methods

### puts()

The puts() function is a very simple way to send a string to the screen when you have no placeholders or variables to be concerned about. It works very much like the printf() function we saw in the "Hello, World!" example:

    puts("Print this string.");


will print to the screen:

  Print this string.


followed by the newline character (as discussed above). (The puts function appends a newline character to its output.)

## Input using scanf()

The scanf() function is the input method equivalent to the printf() output function - simple yet powerful. In its simplest invocation, the scanf format string holds a single placeholder representing the type of value that will be entered by the user. These placeholders are mostly the same as the printf() function - %d for integers, %f for floats, and %lf for doubles.

There is, however, one variation to scanf() as compared to printf(). The scanf() function requires the memory address of the variable to which you want to save the input value. While pointers (variables storing memory addresses) can be used here, this is a concept that won't be approached until later in the text. Instead, the simple technique is to use the address-of operator, &. For now it may be best to consider this "magic" before we discuss pointers.

A typical application might be like this:

#include "stdio.h"

int main(void)
{
int a;

printf("Please input an integer value: ");
scanf("%d", &a);
printf("You entered: %d\n", a);

return 0;
}


If you were to describe the effect of the scanf() function call above, it might read as: "Read in an integer from the user and store it at the address of variable a ".

If you are trying to input a string using scanf, you should not include the & operator. The code below will produce a runtime error and the program will likely crash.

    scanf("%s", &a);


The correct usage would be:

    scanf("%s", a);


This is because, whenever you use a format specifier for a string (%s), the variable that you use to store the value will be an array and, the array names (in this case - a) themselves point out to their base address and hence, the address of operator is not required.

(Although, this is vulnerable to Buffer overflow. fgets() is preferred to scanf()).

Note on inputs: When data is typed at a keyboard, the information does not go straight to the program that is running. It is first stored in what is known as a buffer - a small amount of memory reserved for the input source. Sometimes there will be data left in the buffer when the program wants to read from the input source, and the scanf() function will read this data instead of waiting for the user to type something. Some may suggest you use the function fflush(stdin), which may work as desired on some computers, but isn't considered good practice, as you will see later. Doing this has the downfall that if you take your code to a different computer with a different compiler, your code may not work properly.

## References

1. Actually %f prints doubles as well, but the use of %f for input is different. For more details, see the Wikipedia article on C data types.

# Operators and type casting

## Operators and Assignments

C has a wide range of operators that make simple math easy to handle. The list of operators grouped into precedence levels is as follows:

### Primary expressions

Identifiers are names of things in C, and consist of either a letter or an underscore ( _ ) optionally followed by letters, digits, or underscores. An identifier (or variable name) is a primary expression, provided that it has been declared as designating an object (in which case it is an lvalue [a value that can be used as the left side of an assignment expression]) or a function (in which case it is a function designator).

A constant is a primary expression. Its type depends on its form and value. The types of constants are character constants (e.g. ' ' is a space), integer constants (e.g. 2), floating-point constants (e.g. 0.5), and enumerated constants that have been previously defined via enum.

A string literal is a primary expression. It consists of a string of characters within double quotes ( " ).

A parenthesized expression is a primary expression. It consists of an expression within parentheses ( ( ) ). Its type and value are those of the non-parenthesized expression within the parentheses.

In C11, an expression that starts with _Generic followed by (, an initial expression, a list of values of the form type: expression where type is either a named type or the keyword default, and ) constitute a primary expression. The value is the expression that follows the type of the initial expression or the default if not found.

### Postfix operators

First, a primary expression is also a postfix expression. The following expressions are also postfix expressions:

A postfix expression followed by a left square bracket ([), an expression, and a right square bracket (]) in sequence constitutes an invocation of the array subscript operator. One of the expressions shall have type "pointer to object type" and the other shall have an integer type; the result type is type. Successive array subscript operators designate an element of a multidimensional array.

A postfix expression followed by parentheses or an optional parenthesized argument list indicates an invocation of the function call operator. The value of the function call operator is the return value of the function called with the provided arguments. The parameters to the function are copied on the stack by value (or at least the compiler acts as if that is what happens; if the programmer wanted the parameter to be copied by reference, then it is easier to pass the address of the area to be modified by value, then the called function can access the area through the respective pointer). The trend for compilers is to pass the parameters from right to left onto the stack, but this is not universal.

A postfix expression followed by a dot (.) followed by an identifier selects a member from a structure or union; a postfix expression followed by an arrow (->) followed by an identifier selects a member from a structure or union who is pointed to by the pointer on the left-hand side of the expression.

A postfix expression followed by the increment or decrement operators (++ or -- respectively) indicates that the variable is to be incremented or decremented as a side effect. The value of the expression is the value of the postfix expression before the increment or decrement. These operators only work on integers and pointers.

### Unary expressions

First, a postfix expression is a unary expression. The following expressions are all unary expressions:

The increment or decrement operators followed by a unary expression is a unary expression. The value of the expression is the value of the unary expression after the increment or decrement. These operators only work on integers and pointers.

The following operators followed by a cast expression are unary expressions:

Operator     Meaning
========     =======
&         Address-of; value is the location of the operand
*         Contents-of; value is what is stored at the location
-         Negation
+         Value-of operator
!         Logical negation ( (!E) is equivalent to (0==E) )
~         Bit-wise complement


The keyword sizeof followed by a unary expression is a unary expression. The value is the size of the type of the expression in bytes. The expression is not evaluated.

The keyword sizeof followed by a parenthesized type name is a unary expression. The value is the size of the type in bytes.

### Cast operators

A cast expression is a unary expression.

A parenthesized type name followed by any expression, including literals, is a cast expression. The parenthesized type name has the effect of forcing the cast expression into the type specified by the type name in parentheses. For arithmetic types, this either does not change the value of the expression, or truncates the value of the expression if the expression is an integer and the new type is smaller than the previous type.

An example of casting a float as an int:

float pi = 3.141592;
int truncated_pi = (int) pi; // truncated_pi == 3


An example of casting a char as an int:

char my_char = 'A';
int my_int = (int) my_char; // On machines which use ASCII as the character set, my_int == 65


In C, simple math is very easy to handle. The following operators exist: + (addition), - (subtraction), * (multiplication), / (division), and % (modulus); You likely know all of them from your math classes - except, perhaps, modulus. It returns the remainder of a division (e.g. 5 % 2 = 1). (Modulus is not defined for floating-point numbers, but the math.h library has an fmod function.)

Care must be taken with the modulus, because it's not the equivalent of the mathematical modulus: (-5) % 2 is not 1, but -1. Division of integers will return an integer, and the division of a negative integer by a positive integer will round towards zero instead of rounding down (e.g. (-5) / 3 = -1 instead of -2). However, it is always true that for all integer a and nonzero integer b, ((a / b) * b) + (a % b) == a.

There is no inline operator to do exponentiation (e.g. 5 ^ 2 is not 25 [it is 7; ^ is the exclusive-or operator], and 5 ** 2 is an error), but there is a power function.

The mathematical order of operations does apply. For example (2 + 3) * 2 = 10 while 2 + 3 * 2 = 8. Multiplicative operators have precedence over additive operators.

#include <stdio.h>

int main(void)
{
int i = 0, j = 0;

/* while i is less than 5 AND j is less than 5, loop */
while( (i < 5) && (j < 5) )
{
/* postfix increment, i++
*     the value of i is read and then incremented
*/
printf("i: %d\t", i++);

/*
* prefix increment, ++j
*     the value of j is incremented and then read
*/
printf("j: %d\n", ++j);
}

printf("At the end they have both equal values:\ni: %d\tj: %d\n", i, j);

getchar(); /* pause */
return 0;
}


will display the following:

i: 0    j: 1
i: 1    j: 2
i: 2    j: 3
i: 3    j: 4
i: 4    j: 5
At the end they have both equal values:
i: 5    j: 5


### The shift operators (which may be used to rotate bits)

Shift functions are often used in low-level I/O hardware interfacing. Shift and rotate functions are heavily used in cryptography and software floating point emulation. Other than that, shifts can be used in place of division or multiplication by a power of two. Many processors have dedicated function blocks to make these operations fast -- see Microprocessor Design/Shift and Rotate Blocks. On processors which have such blocks, most C compilers compile shift and rotate operators to a single assembly-language instruction -- see X86 Assembly/Shift and Rotate.

#### shift left

The << operator shifts the binary representation to the left, dropping the most significant bits and appending it with zero bits. The result is equivalent to multiplying the integer by a power of two.

#### unsigned shift right

The unsigned shift right operator, also sometimes called the logical right shift operator. It shifts the binary representation to the right, dropping the least significant bits and prepending it with zeros. The >> operator is equivalent to division by a power of two for unsigned integers.

#### signed shift right

The signed shift right operator, also sometimes called the arithmetic right shift operator. It shifts the binary representation to the right, dropping the least significant bit, but prepending it with copies of the original sign bit. The >> operator is not equivalent to division for signed integers.

In C, the behavior of the >> operator depends on the data type it acts on. Therefore, a signed and an unsigned right shift looks exactly the same, but produces a different result in some cases.

#### rotate right

Contrary to popular belief, it is possible to write C code that compiles down to the "rotate" assembly language instruction (on CPUs that have such an instruction).

Most compilers recognize this idiom:

  unsigned int x;
unsigned int y;
/* ... */
y = (x >> shift) | (x << (32 - shift));


and compile it to a single 32 bit rotate instruction. [1] [2]

On some systems, this may be "#define"ed as a macro or defined as an inline function called something like "rightrotate32" or "rotr32" or "ror32" in a standard header file like "bitops.h". [3]

#### rotate left

Most compilers recognize this idiom:

  unsigned int x;
unsigned int y;
/* ... */
y = (x << shift) | (x >> (32 - shift));


and compile it to a single 32 bit rotate instruction.

On some systems, this may be "#define"ed as a macro or defined as an inline function called something like "leftrotate32" or "rotl32" in a header file like "bitops.h".

### Relational and equality operators

The relational binary operators < (less than), > (greater than), <= (less than or equal), and >= (greater than or equal) operators return a value of 1 if the result of the operation is true, 0 if false.

The equality binary operators == (equals) and != (not equals) operators are similar to the relational operators except that their precedence is lower.

### Bitwise operators

The bitwise operators are & (and), ^ (exclusive or) and | (inclusive or). The & operator has higher precedence than ^, which has higher precedence than |.

The values being operated upon must be integral; the result is integral.

One use for the bitwise operators is to emulate bit flags. These flags can be set with OR, tested with AND, flipped with XOR, and cleared with AND NOT. For example:

/* This code is a sample for bitwise operations.  */
#define BITFLAG1    (1)
#define BITFLAG2    (2)
#define BITFLAG3    (4) /* They are powers of 2 */

unsigned bitbucket = 0U;    /* Clear all */
bitbucket |= BITFLAG1;      /* Set bit flag 1 */
bitbucket &= ~BITFLAG2;     /* Clear bit flag 2 */
bitbucket ^= BITFLAG3;      /* Flip the state of bit flag 3 from off to on or
vice versa */
if (bitbucket & BITFLAG3) {
/* bit flag 3 is set */
} else {
/* bit flag 3 is not set */
}


### Logical operators

The logical operators are && (and), and || (or). Both of these operators produce 1 if the relationship is true and 0 for false. Both of these operators short-circuit; if the result of the expression can be determined from the first operand, the second is ignored. The && operator has higher precedence than the || operator.

&& is used to evaluate expressions left to right, and returns a 1 if both statements are true, 0 if either of them are false. If the first expression is false, the second is not evaluated.


int x = 7;
int y = 5;
if(x == 7 && y == 5) {
...
}


Here, the && operator checks the left-most expression, then the expression to its right. If there were more than two expressions chained (e.g. x && y && z), the operator would check x first, then y (if x is nonzero), then continue rightwards to z if neither x or y is zero. Since both statements return true, the && operator returns true, and the code block is executed.


if(x == 5 && y == 5) {
...
}


The && operator checks in the same way as before, and finds that the first expression is false. The && operator stops evaluating as soon as it finds a statement to be false, and returns a false.

|| is used to evaluate expressions left to right, and returns a 1 if either of the expressions are true, 0 if both are false. If the first expression is true, the second expression is not evaluated.


/* Use the same variables as before. */
if(x == 2 || y == 5) { // the || statement checks both expressions, finds that the latter is true, and returns true
...
}


The || operator here checks the left-most expression, finds it false, but continues to evaluate the next expression. It finds that the next expression returns true, stops, and returns a 1. Much how the && operator ceases when it finds an expression that returns false, the || operator ceases when it finds an expression that returns true.

It is worth noting that C does not have Boolean values (true and false) commonly found in other languages. It instead interprets a 0 as false, and any nonzero value as true.

### Conditional operators

The ternary ?: operator is the conditional operator. The expression (x ? y : z) has the value of y if x is nonzero, z otherwise.

Example:


int x = 0;
int y;
y = (x ? 10 : 6); /* The parentheses are technically not necessary as assignment
has a lower precedence than the conditional operator, but
it's there for clarity.  */


The expression x evaluates to 0. The ternary operator then looks for the "if-false" value, which in this case, is 6. It returns that, so y is equal to six. Had x been a non-zero, then the expression would have returned a 10.

### Assignment operators

The assignment operators are =, *=, /=, %=, +=, -=, <<=, >>=, &=, ^=, and |= . The = operator stores the value of the right operand into the location determined by the left operand, which must be an lvalue (a value that has an address, and therefore can be assigned to).

For the others, x op= y is shorthand for x = x op (y) . Hence, the following expressions are the same:

    1. x += y     -     x = x+y
2. x -= y     -     x = x-y
3. x *= y     -     x = x*y
4. x /= y     -     x = x/y
5. x %= y     -     x = x%y


The value of the assignment expression is the value of the left operand after the assignment. Thus, assignments can be chained; e.g. the expression a = b = c = 0; would assign the value zero to all three variables.

### Comma operator

The operator with the least precedence is the comma operator. The value of the expression x, y will evaluate both x and y, but provides the value of y.

This operator is useful for including multiple actions in one statement (e.g. within a for loop conditional).

Here is a small example of the comma operator:

int i, x;      /* Declares two ints, i and x, in one declaration.
Technically, this is not the comma operator. */

/* this loop initializes x and i to 0, then runs the loop */
for (x = 0, i = 0; i <= 6; i++) {
printf("x = %d, and i = %d\n", x, i);
}


## References

1. "Cleanups in ROTL/ROTR DAG combiner code" mentions that this code supports the "rotate" instruction in the CellSPU
2. "replace private copy of bit rotation routines" -- recommends including "bitops.h" and using its rol32 and ror32 rather than copy-and-paste into a new program.

# Arrays and Strings

Arrays in C act to store related data under a single variable name with an index, also known as a subscript. It is easiest to think of an array as simply a list or ordered grouping for variables of the same type. As such, arrays often help a programmer organize collections of data efficiently and intuitively.

Later we will consider the concept of a pointer, fundamental to C, which extends the nature of the array (array can be termed as a constant pointer). For now, we will consider just their declaration and their use.

## Arrays

C arrays are declared in the following form:

type name[number of elements];


For example, if we want an array of six integers (or whole numbers), we write in C:

int numbers[6];


For a six character array called letters,

char letters[6];


and so on.

You can also initialize as you declare. Just put the initial elements in curly brackets separated by commas as the initial value:

type name[number of elements]={comma-separated values}


For example, if we want to initialize an array with six integers, with 0, 0, 1, 0, 0, 0 as the initial values:

int point[6]={0,0,1,0,0,0};


Though when the array is initialized as in this case, the array dimension may be omitted, and the array will be automatically sized to hold the initial data:

int point[]={0,0,1,0,0,0};


This is very useful in that the size of the array can be controlled by simply adding or removing initializer elements from the definition without the need to adjust the dimension.

If the dimension is specified, but not all elements in the array are initialized, the remaining elements will contain a value of 0. This is very useful, especially when we have very large arrays.

int numbers[2000]={245};


The above example sets the first value of the array to 245, and the rest to 0.

If we want to access a variable stored in an array, for example with the above declaration, the following code will store a 1 in the variable x

int x;
x = point[2];


Arrays in C are indexed starting at 0, as opposed to starting at 1. The first element of the array above is point[0]. The index to the last value in the array is the array size minus one. In the example above the subscripts run from 0 through 5. C does not guarantee bounds checking on array accesses. The compiler may not complain about the following (though the best compilers do):

char y;
int z = 9;
char point[6] = { 1, 2, 3, 4, 5, 6 };
//examples of accessing outside the array. A compile error is not always raised
y = point[15];
y = point[-4];
y = point[z];


During program execution, an out of bounds array access does not always cause a run time error. Your program may happily continue after retrieving a value from point[-1]. To alleviate indexing problems, the sizeof() expression is commonly used when coding loops that process arrays.

Many people use a macro that in turn uses sizeof() to find the number of elements in an array, a macro variously named "lengthof()",[1] "MY_ARRAY_SIZE()" or "NUM_ELEM()",[2] "SIZEOF_STATIC_ARRAY()",[3] etc.

int ix;
short anArray[]= { 3, 6, 9, 12, 15 };

for (ix=0; ix< (sizeof(anArray)/sizeof(short)); ++ix) {
DoSomethingWith("%d", anArray[ix] );
}


Notice in the above example, the size of the array was not explicitly specified. The compiler knows to size it at 5 because of the five values in the initializer list. Adding an additional value to the list will cause it to be sized to six, and because of the sizeof expression in the for loop, the code automatically adjusts to this change. Good programming practice is to declare a variable size , and store the number of elements in the array in it.

size = sizeof(anArray)/sizeof(short)

C also supports multi dimensional arrays (or, rather, arrays of arrays). The simplest type is a two dimensional array. This creates a rectangular array - each row has the same number of columns. To get a char array with 3 rows and 5 columns we write in C

char two_d[3][5];


To access/modify a value in this array we need two subscripts:

char ch;
ch = two_d[2][4];


or

two_d[0][0] = 'x';


Similarly, a multi-dimensional array can be initialized like this:

int two_d[2][3] = {{ 5, 2, 1 },
{ 6, 7, 8 }};


The amount of columns must be explicitly stated; however, the compiler will find the appropriate amount of rows based on the initializer list.

There are also weird notations possible:

int a[100];
int i = 0;
if (a[i]==i[a])
{
printf("Hello world!\n");
}


a[i] and i[a] refer to the same location. (This is explained later in the next Chapter.)

## Strings

String "Merkkijono" stored in memory

C has no string handling facilities built in; consequently, strings are defined as arrays of characters. C allows a character array to be represented by a character string rather than a list of characters, with the null terminating character automatically added to the end. For example, to store the string "Merkkijono", we would write

char string[11] = "Merkkijono";


or

char string[11] = {'M', 'e', 'r', 'k', 'k', 'i', 'j', 'o', 'n', 'o', '\0'};


In the first example, the string will have a null character automatically appended to the end by the compiler; by convention, library functions expect strings to be terminated by a null character. The latter declaration indicates individual elements, and as such the null terminator needs to be added manually.

Strings do not always have to be linked to an explicit variable. As you have seen already, a string of characters can be created directly as an unnamed string that is used directly (as with the printf functions.)

To create an extra long string, you will have to split the string into multiple sections, by closing the first section with a quote, and recommencing the string on the next line (also starting and ending in a quote):

char string[58] = "This is a very, very long "
"string that requires two lines.";


While strings may also span multiple lines by putting the backslash character at the end of the line, this method is deprecated.

There is a useful library of string handling routines which you can use by including another header file.

#include <string.h>  //new header file


This standard string library will allow various tasks to be performed on strings, and is discussed in the Strings chapter.

# Program Flow Control

Very few programs follow exactly one control path and have each instruction stated explicitly. In order to program effectively, it is necessary to understand how one can alter the steps taken by a program due to user input or other conditions, how some steps can be executed many times with few lines of code, and how programs can appear to demonstrate a rudimentary grasp of logic. C constructs known as conditionals and loops grant this power.

From this point forward, it is necessary to understand what is usually meant by the word block. A block is a group of code statements that are associated and intended to be executed as a unit. In C, the beginning of a block of code is denoted with { (left curly), and the end of a block is denoted with }. It is not necessary to place a semicolon after the end of a block. Blocks can be empty, as in {}. Blocks can also be nested; i.e. there can be blocks of code within larger blocks.

## Conditionals

There is likely no meaningful program written in which a computer does not demonstrate basic decision-making skills. It can actually be argued that there is no meaningful human activity in which some sort of decision-making, instinctual or otherwise, does not take place. For example, when driving a car and approaching a traffic light, one does not think, "I will continue driving through the intersection." Rather, one thinks, "I will stop if the light is red, go if the light is green, and if yellow go only if I am traveling at a certain speed a certain distance from the intersection." These kinds of processes can be simulated in C using conditionals.

A conditional is a statement that instructs the computer to execute a certain block of code or alter certain data only if a specific condition has been met. The most common conditional is the If-Else statement, with conditional expressions and Switch-Case statements typically used as more shorthanded methods.

Before one can understand conditional statements, it is first necessary to understand how C expresses logical relations. C treats logic as being arithmetic. The value 0 (zero) represents false, and all other values represent true. If you chose some particular value to represent true and then compare values against it, sooner or later your code will fail when your assumed value (often 1) turns out to be incorrect. Code written by people uncomfortable with the C language can often be identified by the usage of #define to make a "TRUE" value. [1]

Because logic is arithmetic in C, arithmetic operators and logical operators are one and the same. Nevertheless, there are a number of operators that are typically associated with logic:

### Relational and Equivalence Expressions:

a < b
1 if a is less than b, 0 otherwise.
a > b
1 if a is greater than b, 0 otherwise.
a <= b
1 if a is less than or equal to b, 0 otherwise.
a >= b
1 if a is greater than or equal to b, 0 otherwise.
a == b
1 if a is equal to b, 0 otherwise.
a != b
1 if a is not equal to b, 0 otherwise

New programmers should take special note of the fact that the "equal to" operator is ==, not =. This is the cause of numerous coding mistakes and is often a difficult-to-find bug, as the expression (a = b) sets a equal to b and subsequently evaluates to b; but the expression (a == b), which is usually intended, checks if a is equal to b. It needs to be pointed out that, if you confuse = with ==, your mistake will often not be brought to your attention by the compiler. A statement such as  if (c = 20) {} is considered perfectly valid by the language, but will always assign 20 to c and evaluate as true. A simple technique to avoid this kind of bug (in many, not all cases) is to put the constant first. This will cause the compiler to issue an error, if == got misspelled with =.

Note that C does not have a dedicated boolean type as many other languages do. 0 means false and anything else true. So the following are equivalent:

 if (foo()) {
// do something
}


and

 if (foo() != 0) {
// do something
}


Often #define TRUE 1 and #define FALSE 0 are used to work around the lack of a boolean type. This is bad practice, since it makes assumptions that do not hold. It is a better idea to indicate what you are actually expecting as a result from a function call, as there are many different ways of indicating error conditions, depending on the situation.

 if (strstr("foo", bar) >= 0) {
// bar contains "foo"
}


Here, strstr returns the index where the substring foo is found and -1 if it was not found. Note that this would fail with the TRUE definition mentioned in the previous paragraph. It would also not produce the expected results if we omitted the  >= 0.

One other thing to note is that the relational expressions do not evaluate as they would in mathematical texts. That is, an expression myMin < value < myMax  does not evaluate as you probably think it might. Mathematically, this would test whether or not value is between myMin and myMax. But in C, what happens is that value is first compared with myMin. This produces either a 0 or a 1. It is this value that is compared against myMax. Example:

 int value = 20;
/* ... */
if (0 < value < 10) { // don't do this! it always evaluates to "true"!
/* do some stuff */
}


Because value is greater than 0, the first comparison produces a value of 1. Now 1 is compared to be less than 10, which is true, so the statements in the if are executed. This probably is not what the programmer expected. The appropriate code would be

 int value = 20;
/* ... */
if (0 < value && value < 10) {   // the && means "and"
/* do some stuff */
}


### Logical Expressions

a || b
when EITHER a or b is true (or both), the result is 1, otherwise the result is 0.
a && b
when BOTH a and b are true, the result is 1, otherwise the result is 0.
!a
when a is true, the result is 0, when a is 0, the result is 1.

Here's an example of a larger logical expression. In the statement:

  e = ((a && b) || (c > d));


e is set equal to 1 if a and b are non-zero, or if c is greater than d. In all other cases, e is set to 0.

C uses short circuit evaluation of logical expressions. That is to say, once it is able to determine the truth of a logical expression, it does no further evaluation. This is often useful as in the following:

int myArray[12];
....
if (i < 12 && myArray[i] > 3) {
....


In the snippet of code, the comparison of i with 12 is done first. If it evaluates to 0 (false), i would be out of bounds as an index to myArray. In this case, the program never attempts to access myArray[i] since the truth of the expression is known to be false. Hence we need not worry here about trying to access an out-of-bounds array element if it is already known that i is greater than or equal to zero. A similar thing happens with expressions involving the or || operator.

while (doThis() || doThat()) ...


doThat() is never called if doThis() returns a non-zero (true) value.

### The If-Else statement

If-Else provides a way to instruct the computer to execute a block of code only if certain conditions have been met. The syntax of an If-Else construct is:

 if (/* condition goes here */) {
/* if the condition is non-zero (true), this code will execute */
} else {
/* if the condition is 0 (false), this code will execute */
}


The first block of code executes if the condition in parentheses directly after the if evaluates to non-zero (true); otherwise, the second block executes.

The else and following block of code are completely optional. If there is no need to execute code if a condition is not true, leave it out.

Also, keep in mind that an if can directly follow an else statement. While this can occasionally be useful, chaining more than two or three if-elses in this fashion is considered bad programming practice. We can get around this with the Switch-Case construct described later.

Two other general syntax notes need to be made that you will also see in other control constructs: First, note that there is no semicolon after if or else. There could be, but the block (code enclosed in { and }) takes the place of that. Second, if you only intend to execute one statement as a result of an if or else, curly braces are not needed. However, many programmers believe that inserting curly braces anyway in this case is good coding practice.

The following code sets a variable c equal to the greater of two variables a and b, or 0 if a and b are equal.

 if (a > b) {
c = a;
} else if (b > a) {
c = b;
} else {
c = 0;
}


Consider this question: why can't you just forget about else and write the code like:

 if (a > b) {
c = a;
}

if (a < b) {
c = b;
}

if (a == b) {
c = 0;
}


There are several answers to this. Most importantly, if your conditionals are not mutually exclusive, two cases could execute instead of only one. If the code was different and the value of a or b changes somehow (e.g.: you reset the lesser of a and b to 0 after the comparison) during one of the blocks? You could end up with multiple if statements being invoked, which is not your intent. Also, evaluating if conditionals takes processor time. If you use else to handle these situations, in the case above assuming (a > b) is non-zero (true), the program is spared the expense of evaluating additional if statements. The bottom line is that it is usually best to insert an else clause for all cases in which a conditional will not evaluate to non-zero (true).

#### The conditional expression

A conditional expression is a way to set values conditionally in a more shorthand fashion than If-Else. The syntax is:

(/* logical expression goes here */) ? (/* if non-zero (true) */) : (/* if 0 (false) */)


The logical expression is evaluated. If it is non-zero (true), the overall conditional expression evaluates to the expression placed between the ? and :, otherwise, it evaluates to the expression after the :. Therefore, the above example (changing its function slightly such that c is set to b when a and b are equal) becomes:

c = (a > b) ? a : b;


Conditional expressions can sometimes clarify the intent of the code. Nesting the conditional operator should usually be avoided. It's best to use conditional expressions only when the expressions for a and b are simple. Also, contrary to a common beginner belief, conditional expressions do not make for faster code. As tempting as it is to assume that fewer lines of code result in faster execution times, there is no such correlation.

### The Switch-Case statement

Say you write a program where the user inputs a number 1-5 (corresponding to student grades, A(represented as 1)-D(4) and F(5)), stores it in a variable grade and the program responds by printing to the screen the associated letter grade. If you implemented this using If-Else, your code would look something like this:

 if (grade == 1) {
printf("A\n");
} else if (grade == 2) {
printf("B\n");
} else if /* etc. etc. */


Having a long chain of if-else-if-else-if-else can be a pain, both for the programmer and anyone reading the code. Fortunately, there's a solution: the Switch-Case construct, of which the basic syntax is:

 switch (/* integer or enum goes here */) {
case /* potential value of the aforementioned int or enum */:
/* code */
case /* a different potential value */:
/* different code */
/* insert additional cases as needed */
default:
/* more code */
}


The Switch-Case construct takes a variable, usually an int or an enum, placed after switch, and compares it to the value following the case keyword. If the variable is equal to the value specified after case, the construct "activates", or begins executing the code after the case statement. Once the construct has "activated", there will be no further evaluation of cases.

Switch-Case is syntactically "weird" in that no braces are required for code associated with a case.

Very important: Typically, the last statement for each case is a break statement. This causes program execution to jump to the statement following the closing bracket of the switch statement, which is what one would normally want to happen. However if the break statement is omitted, program execution continues with the first line of the next case, if any. This is called a fall-through. When a programmer desires this action, a comment should be placed at the end of the block of statements indicating the desire to fall through. Otherwise another programmer maintaining the code could consider the omission of the 'break' to be an error, and inadvertently 'correct' the problem. Here's an example:

 switch (someVariable) {
case 1:
printf("This code handles case 1\n");
break;
case 2:
printf("This prints when someVariable is 2, along with...\n");
/* FALL THROUGH */
case 3:
printf("This prints when someVariable is either 2 or 3.\n" );
break;
}


If a default case is specified, the associated statements are executed if none of the other cases match. A default case is optional. Here's a switch statement that corresponds to the sequence of if - else if statements above.

Back to our example above. Here's what it would look like as Switch-Case:

 switch (grade) {
case 1:
printf("A\n");
break;
case 2:
printf("B\n");
break;
case 3:
printf("C\n");
break;
case 4:
printf("D\n");
break;
default:
printf("F\n");
break;
}


A set of statements to execute can be grouped with more than one value of the variable as in the following example. (the fall-through comment is not necessary here because the intended behavior is obvious)

 switch (something) {
case 2:
case 3:
case 4:
/* some statements to execute for 2, 3 or 4 */
break;
case 1:
default:
/* some statements to execute for 1 or other than 2,3,and 4 */
break;
}


Switch-Case constructs are particularly useful when used in conjunction with user defined enum data types. Some compilers are capable of warning about an unhandled enum value, which may be helpful for avoiding bugs.

## Loops

Often in computer programming, it is necessary to perform a certain action a certain number of times or until a certain condition is met. It is impractical and tedious to simply type a certain statement or group of statements a large number of times, not to mention that this approach is too inflexible and unintuitive to be counted on to stop when a certain event has happened. As a real-world analogy, someone asks a dishwasher at a restaurant what he did all night. He will respond, "I washed dishes all night long." He is not likely to respond, "I washed a dish, then washed a dish, then washed a dish, then...". The constructs that enable computers to perform certain repetitive tasks are called loops.

### While loops

A while loop is the most basic type of loop. It will run as long as the condition is non-zero (true). For example, if you try the following, the program will appear to lock up and you will have to manually close the program down. A situation where the conditions for exiting the loop will never become true is called an infinite loop.

 int a = 1;
while (42) {
a = a * 2;
}


Here is another example of a while loop. It prints out all the powers of two less than 100.

 int a = 1;
while (a < 100) {
printf("a is %d \n", a);
a = a * 2;
}


The flow of all loops can also be controlled by break and continue statements. A break statement will immediately exit the enclosing loop. A continue statement will skip the remainder of the block and start at the controlling conditional statement again. For example:

 int a = 1;
while (42) { // loops until the break statement in the loop is executed
printf("a is %d ", a);
a = a * 2;
if (a > 100) {
break;
} else if (a == 64) {
continue;  // Immediately restarts at while, skips next step
}
printf("a is not 64\n");
}


In this example, the computer prints the value of a as usual, and prints a notice that a is not 64 (unless it was skipped by the continue statement).

Similar to If above, braces for the block of code associated with a While loop can be omitted if the code consists of only one statement, for example:

 int a = 1;
while (a < 100)
a = a * 2;


This will merely increase a until a is not less than 100.

When the computer reaches the end of the while loop, it always goes back to the while statement at the top of the loop, where it re-evaluates the controlling condition. If that condition is "true" at that instant -- even if it was temporarily 0 for a few statements inside the loop -- then the computer begins executing the statements inside the loop again; otherwise the computer exits the loop. The computer does not "continuously check" the controlling condition of a while loop during the execution of that loop. It only "peeks" at the controlling condition each time it reaches the while at the top of the loop.

It is very important to note, once the controlling condition of a While loop becomes 0 (false), the loop will not terminate until the block of code is finished and it is time to reevaluate the conditional. If you need to terminate a While loop immediately upon reaching a certain condition, consider using break.

A common idiom is to write:

 int i = 5;
while (i--) {
printf("java and c# can't do this\n");
}


This executes the code in the while loop 5 times, with i having values that range from 4 down to 0 (inside the loop). Conveniently, these are the values needed to access every item of an array containing 5 elements.

### For loops

For loops generally look something like this:

for (initialization; test; increment) {
/* code */
}


The initialization statement is executed exactly once - before the first evaluation of the test condition. Typically, it is used to assign an initial value to some variable, although this is not strictly necessary. The initialization statement can also be used to declare and initialize variables used in the loop.

The test expression is evaluated each time before the code in the for loop executes. If this expression evaluates as 0 (false) when it is checked (i.e. if the expression is not true), the loop is not (re)entered and execution continues normally at the code immediately following the FOR-loop. If the expression is non-zero (true), the code within the braces of the loop is executed.

After each iteration of the loop, the increment statement is executed. This often is used to increment the loop index for the loop, the variable initialized in the initialization expression and tested in the test expression. Following this statement execution, control returns to the top of the loop, where the test action occurs. If a continue statement is executed within the for loop, the increment statement would be the next one executed.

Each of these parts of the for statement is optional and may be omitted. Because of the free-form nature of the for statement, some fairly fancy things can be done with it. Often a for loop is used to loop through items in an array, processing each item at a time.

 int myArray[12];
int ix;
for (ix = 0; ix < 12; ix++) {
myArray[ix] = 5 * ix + 3;
}


The above for loop initializes each of the 12 elements of myArray. The loop index can start from any value. In the following case it starts from 1.

 for (ix = 1; ix <= 10; ix++) {
printf("%d ", ix);
}


which will print

1 2 3 4 5 6 7 8 9 10


You will most often use loop indexes that start from 0, since arrays are indexed at zero, but you will sometimes use other values to initialize a loop index as well.

The increment action can do other things, such as decrement. So this kind of loop is common:

 for (i = 5; i > 0; i--) {
printf("%d ", i);
}


which yields

5 4 3 2 1


Here's an example where the test condition is simply a variable. If the variable has a value of 0 or NULL, the loop exits, otherwise the statements in the body of the loop are executed.

 for (t = list_head; t; t = NextItem(t)) {
/* body of loop */
}


A WHILE loop can be used to do the same thing as a FOR loop, however a FOR loop is a more condensed way to perform a set number of repetitions since all of the necessary information is in a one line statement.

A FOR loop can also be given no conditions, for example:

 for (;;) {
/* block of statements */
}


This is called an infinite loop since it will loop forever unless there is a break statement within the statements of the for loop. The empty test condition effectively evaluates as true.

It is also common to use the comma operator in for loops to execute multiple statements.

 int i, j, n = 10;
for (i = 0, j = 0; i <= n; i++, j += 2) {
printf("i = %d , j = %d \n", i, j);
}


Special care should be taken when designing or refactoring the conditional part, especially whether using < or <= , whether start and stop should be corrected by 1, and in case of prefix- and postfix-notations. ( On a 100 yards promenade with a tree every 10 yards there are 11 trees. )

 int i, n = 10;
for (i = 0; i < n; i++)
printf("%d ", i); // processed n times => 0 1 2 3 ... (n-1)
printf("\n");
for (i = 0; i <= n; i++)
printf("%d ", i); // processed (n+1) times => 0 1 2 3 ... n
printf("\n");
for (i = n; i--;)
printf("%d ", i); // processed n times => (n-1) ...3 2 1 0
printf("\n");
for (i = n; --i;)
printf("%d ", i); // processed (n-1) times => (n-1) ...4 3 2 1
printf("\n");


### Do-While loops

A DO-WHILE loop is a post-check while loop, which means that it checks the condition after each run. As a result, even if the condition is zero (false), it will run at least once. It follows the form of:

 do {
/* do stuff */
} while (condition);


Note the terminating semicolon. This is required for correct syntax. Since this is also a type of while loop, break and continue statements within the loop function accordingly. A continue statement causes a jump to the test of the condition and a break statement exits the loop.

It is worth noting that Do-While and While are functionally almost identical, with one important difference: Do-While loops are always guaranteed to execute at least once, but While loops will not execute at all if their condition is 0 (false) on the first evaluation.

## One last thing: goto

goto is a very simple and traditional control mechanism. It is a statement used to immediately and unconditionally jump to another line of code. To use goto, you must place a label at a point in your program. A label consists of a name followed by a colon (:) on a line by itself. Then, you can type "goto label;" at the desired point in your program. The code will then continue executing beginning with label. This looks like:

 MyLabel:
/* some code */
goto MyLabel;


The ability to transfer the flow of control enabled by gotos is so powerful that, in addition to the simple if, all other control constructs can be written using gotos instead. Here, we can let "S" and "T" be any arbitrary statements:

 if (''cond'') {
S;
} else {
T;
}
/* ... */


The same statement could be accomplished using two gotos and two labels:

 if (''cond'') goto Label1;
T;
goto Label2;
Label1:
S;
Label2:
/* ... */


Here, the first goto is conditional on the value of "cond". The second goto is unconditional. We can perform the same translation on a loop:

 while (''cond1'') {
S;
if (''cond2'')
break;
T;
}
/* ... */


Which can be written as:

 Start:
if (!''cond1'') goto End;
S;
if (''cond2'') goto End;
T;
goto Start;
End:
/* ... */


As these cases demonstrate, often the structure of what your program is doing can usually be expressed without using gotos. Undisciplined use of gotos can create unreadable, unmaintainable code when more idiomatic alternatives (such as if-elses, or for loops) can better express your structure. Theoretically, the goto construct does not ever have to be used, but there are cases when it can increase readability, avoid code duplication, or make control variables unnecessary. You should consider first mastering the idiomatic solutions, and use goto only when necessary. Keep in mind that many, if not most, C style guidelines strictly forbid use of goto, with the only common exceptions being the following examples.

One use of goto is to break out of a deeply nested loop. Since break will not work (it can only escape one loop), goto can be used to jump completely outside the loop. Breaking outside of deeply nested loops without the use of the goto is always possible, but often involves the creation and testing of extra variables that may make the resulting code far less readable than it would be with goto. The use of goto makes it easy to undo actions in an orderly fashion, typically to avoid failing to free memory that had been allocated.

Another accepted use is the creation of a state machine. This is a fairly advanced topic though, and not commonly needed.

## Examples

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>

int main(void)
{
int years;

printf("Enter your age in years : ");
fflush(stdout);
errno = 0;
if (scanf("%d", &years) != 1 || errno)
return EXIT_FAILURE;
printf("Your age in days is %d\n", years * 365);
return 0;
}


# Standard Libraries

The C standard library is a standardized collection of header files and library routines used to implement common operations, such as input/output and character string handling. Unlike other languages (such as COBOL, Fortran, and PL/I) C does not include builtin keywords for these tasks, so nearly all C programs rely on the standard library to operate.

## History

The C programming language previously did not provide any elementary functions, such as I/O operations. Over time, user communities of C shared ideas and implementations to provide those functions. These ideas became common, and were eventually incorporated into the definition of the standardized C programming language in 1989. These are now called the C standard libraries.

Both Unix and C were created at AT&T's Bell Laboratories in the late 1960s and early 1970s. During the 1970s the C programming language became increasingly popular, with many universities and organizations beginning to create their own variations of the language for their own projects. By the start of the 1980s compatibility problems between the various C implementations became apparent. In 1983 the American National Standards Institute (ANSI) formed a committee to establish a standard specification of C known as "ANSI C". This work culminated in the creation of the so-called C89 standard in 1989. Part of the resulting standard was a set of software libraries called the ANSI C standard library.

Later revisions of the C standard have added several new required header files to the library. Support for these new extensions varies between implementations.

The headers <iso646.h>, <wchar.h>, and <wctype.h> were added with Normative Addendum 1 (hereafter abbreviated as NA1), an addition to the C Standard ratified in 1995.

The headers <complex.h>, <fenv.h>, <inttypes.h>, <stdbool.h>, <stdint.h>, and <tgmath.h> were added with C99, a revision to the C Standard published in 1999.

 Note: The C++ programming language includes the functions of the ANSI C 89 standard library, but has made several modifications, such as placing all identifiers into the std namespace and changing the names of the header files from to (however, the C-style names are still available, although deprecated).

## Design

The declaration of each function is kept in a header file, while the actual implementation of functions are separated into a library file. The naming and scope of headers have become common but the organization of libraries still remains diverse. The standard library is usually shipped along with a compiler. Since C compilers often provide extra functions that are not specified in ANSI C, a standard library with a particular compiler is mostly incompatible with standard libraries of other compilers.

Much of the C standard library has been shown to have been well-designed. A few parts, with the benefit of hindsight, are regarded as mistakes. The string input functions gets() (and the use of scanf() to read string input) are the source of many buffer overflows, and most programming guides recommend avoiding this usage. Another oddity is strtok(), a function that is designed as a primitive lexical analyser but is highly "fragile" and difficult to use.

## ANSI Standard

The ANSI C standard library consists of 24 C header files which can be included into a programmer's project with a single directive. Each header file contains one or more function declarations, data type definitions and macros. The contents of these header files follows.

In comparison to some other languages (for example Java) the standard library is minuscule. The library provides a basic set of mathematical functions, string manipulation, type conversions, and file and console-based I/O. It does not include a standard set of "container types" like the C++ Standard Template Library, let alone the complete graphical user interface (GUI) toolkits, networking tools, and profusion of other functions that Java provides as standard. The main advantage of the small standard library is that providing a working ANSI C environment is much easier than it is with other languages, and consequently porting C to a new platform is relatively easy.

Many other libraries have been developed to supply equivalent functions to that provided by other languages in their standard library. For instance, the GNOME desktop environment project has developed the GTK+ graphics toolkit and GLib, a library of container data structures, and there are many other well-known examples. The variety of libraries available has meant that some superior toolkits have proven themselves through history. The considerable downside is that they often do not work particularly well together, programmers are often familiar with different sets of libraries, and a different set of them may be available on any particular platform.

### ANSI C library header files

 Contains the assert macro, used to assist with detecting logical errors and other types of bug in debugging versions of a program. A set of functions for manipulating complex numbers. (New with C99) This header file contains functions used to classify characters by their types or to convert between upper and lower case in a way that is independent of the used character set (typically ASCII or one of its extensions, although implementations utilizing EBCDIC are also known). For testing error codes reported by library functions. For controlling floating-point environment. (New with C99) Contains defined constants specifying the implementation-specific properties of the floating-point library, such as the minimum difference between two different floating-point numbers (_EPSILON), the maximum number of digits of accuracy (_DIG) and the range of numbers which can be represented (_MIN, _MAX). For precise conversion between integer types. (New with C99) For programming in ISO 646 variant character sets. (New with NA1) Contains defined constants specifying the implementation-specific properties of the integer types, such as the range of numbers which can be represented (_MIN, _MAX). For setlocale() and related constants. This is used to choose an appropriate locale. For computing common mathematical functions -- see Further math or C++ Programming/Code/Standard C Library/Math for details. setjmp and longjmp, which are used for non-local exits For controlling various exceptional conditions For accessing a varying number of arguments passed to functions. For a boolean data type. (New with C99) For defining various integer types. (New with C99) For defining several useful types and macros. Provides the core input and output capabilities of the C language. This file includes the venerable printf function. For performing a variety of operations, including conversion, pseudo-random numbers, memory allocation, process control, environment, signalling, searching, and sorting. For manipulating several kinds of strings. For type-generic mathematical functions. (New with C99) For converting between various time and date formats. For manipulating wide streams and several kinds of strings using wide characters - key to supporting a range of languages. (New with NA1) For classifying wide characters. (New with NA1)

## Common support libraries

While not standardized, C programs may depend on a runtime library of routines which contain code the compiler uses at runtime. The code that initializes the process for the operating system, for example, before calling main(), is implemented in the C Run-Time Library for a given vendor's compiler. The Run-Time Library code might help with other language feature implementations, like handling uncaught exceptions or implementing floating point code.

The C standard library only documents that the specific routines mentioned in this article are available, and how they behave. Because the compiler implementation might depend on these additional implementation-level functions to be available, it is likely the vendor-specific routines are packaged with the C Standard Library in the same module, because they're both likely to be needed by any program built with their toolset.

Though often confused with the C Standard Library because of this packaging, the C Runtime Library is not a standardized part of the language and is vendor-specific.

## Compiler built-in functions

Some compilers (for example, GCC) provide built-in versions of many of the functions in the C standard library; that is, the implementations of the functions are written into the compiled object file, and the program calls the built-in versions instead of the functions in the C library shared object file. This reduces function call overhead, especially if function calls are replaced with inline variants, and allows other forms of optimization (as the compiler knows the control-flow characteristics of the built-in variants), but may cause confusion when debugging (for example, the built-in versions cannot be replaced with instrumented variants).

## POSIX standard library

POSIX, (along with the Single Unix Specification), specifies a number of routines that should be available over and above those in the C standard library proper; these are often implemented alongside the C standard library functions, with varying degrees of closeness. For example, glibc implements functions such as fork within libc.so, but before NPTL was merged into glibc it constituted a separate library with its own linker flag. Often, this POSIX-specified function will be regarded as part of the library; the C library proper may be identified as the ANSI or ISO C library.

The following libraries are recognized by POSIX:

# Beginning exercises

## Variables

### Naming

2. Can a variable name start with a typographical symbol(e.g. #, *, _)?
3. Give an example of a C variable name that would not work. Why doesn't it work?

### Data Types

1. List at least three data types in C
1. On your computer, how much memory does each require?
2. Which ones can be used in place of another? Why?
1. Are there any limitations on these uses?
2. If so, what are they?
3. Is it necessary to do anything special to use the alternative?
2. Can the name we use for a data type (e.g. 'int', 'float') be used as a variable?

### Assignment

1. How would you assign the value 3.14 to a variable called pi?
2. Is it possible to assign an int to a double?
1. Is the reverse possible?

### Referencing

1. A common mistake for new students is reversing the assignment statement. Suppose you want to assign the value stored in the variable "pi" to another variable, say "pi2":
1. What is the correct statement?
2. What is the reverse? Is this a valid C statement (even if it gives incorrect results)?
3. What if you wanted to assign a constant value (like 3.1415) to "pi2":
a. What would the correct statement look like?
b. Would the reverse be a valid or invalid C statement?

## Simple I/O

### String manipulation

1. Write a program that prompts the user for a string, and prints its reverse.

2. Write a program that prompts the user for a sentence, and prints each word on its own line.

### Loops

1. Write a function that outputs a right isosceles triangle of height and width n, so n = 3 would look like

*
**
***


2. Write a function that outputs a sideways triangle of height 2n-1 and width n, so the output for n = 4 would be:

*
**
***
****
***
**
*


3. Write a function that outputs a right-side-up triangle of height n and width 2n-1; the output for n = 6 would be:

     *
***
*****
*******
*********
***********


## Program Flow

1. Build a program where control passes from main to four different functions with 4 calls.

2. Now make a while loop in main with the function calls inside it. Ask for input at the beginning of the loop. End the while loop if the user hits Q

3. Next add conditionals to call the functions when the user enters numbers, so 1 goes to function1, 2 goes to function 2, etc.

4. Have function 1 call function a, which calls function b, which calls function c

5. Draw out a diagram of program flow, with arrows to indicate where control goes

## Functions

1. Write a function to check if an integer is negative; the declaration should look like bool is_positive(int i);

2. Write a function to raise a floating point number to an integer power, so for example to when you use it

float a = raise_to_power(2, 3);    //a gets 8
float b = raise_to_power(9, 2);    //b gets 81
float raise_to_power(float f, int power);    //make this your declaration


## Math

1. Write a function to calculate if a number is prime. Return 1 if it is prime and 0 if it is not a prime.

2. Write a function to determine the number of prime numbers below n.

3. Write a function to find the square root by using Newton's method.

4. Write functions to evaluate the trigonometric functions.

5. Try to write a random number generator.

6. Write a function to determine the prime number(s) between 2 and 100.

## Recursion

#### Merge sort

1. Write a C program to generate a random integer array with a given length n , and sort it recursively using the Merge sort algorithm.

• The merge sort algorithm is a recursive algorithm .

- sorting a one element array is easy.

- sorting two one-element arrays, requires the merge operation. The merge operation looks at two sorted arrays as lists, and compares the head of the list , and which ever head is smaller, this element is put on the sorted list and the head of that list is ticked off, so the next element becomes the head of that list. This is done until one of the lists is exhausted, and the other list is then copied onto the end of the sorted list.

- the recursion occurs, because merging two one-element arrays produces one two-element sorted array, which can be merged with another two-element sorted array produced the same way. This produces a sorted 4 element array, and the same applies for another 4 element sorted array.

- so the basic merge sort, is to check the size of list to be sorted, and if it is greater than one, divide the array into two, and call merge sort again on the two halves. After wards, merge the two halves in a temporary space of equal size, and then copy back the final sorted array onto the original array.

#### Binary heaps

2. Binary heaps :

• A binary max-heap or min-heap, is an ordered structure where some nodes are guaranteed greater than other nodes, e.g. the parent vs two children. A binary heap can be stored in an array , where ,

- given a position i (the parent) , i*2 is the left child, and i*2+1 is the right child.

- ( C arrays begin at position 0, but 0 * 2 = 0, and 0 *2 + 1= 1, which is incorrect , so start the heap at position 1, or add 1 for parent-to-child calculations, and subtract 1 for child-to-parent calculations ).

• try to model this using with a pencil and paper, using 10 random unsorted numbers, and inserting each of them into a "heapsort" array of 10 elements.
• To insert into a heap, end-add and swap-parent if higher, until parent higher.
• To delete the top of a heap, move end-to-top, and defer-higher-child or sift-down , until no child is higher.
• try it on a pen and paper the numbers 10, 4, 6 ,3 ,5 , 11.
• the answer was 11, 5, 10, 3, 4 , 6.
• EXERCISE: Now try removing each top element of 11, 5, 10, 3, 4, 6 , using end-to-top and sift-down ( or swap-higher-child) to get the numbers

in descending order.

• a priority queue allows elements to be inserted with a priority , and extracted according to priority. ( This can happen usefully, if the element has a paired structure, one part is the key, and the other part the data. Otherwise, it is just a mechanism for sorting ).
• EXERCISE: Using the above technique of insert-back/challenge-parent, and delete-front/last-to-front/defer-higher-child, implement either heap sort or a priority queue.

#### Dijkstra's algorithm

Dijkstra's algorithm is a searching algorithm using a priority queue. It begins with inserting the start node with a priority value of 0. All other nodes are inserted with priority values of large N. Each node has an adjacency list of other nodes, a current distance to start node, and previous pointer to previous node used to calculate current node. Alternative to an adjacency list, is an adjacency matrix, which needs n x n boolean adjacencies.

The algorithm basically iterates over the priority queue, removing the front node, examining the adjacent nodes, and updating with a distance equal to the sum of the front nodes distance for each adjacent node , and the distance given by the adjacency information for an adjacent node.

After each node's update, the extra operation "update priority" is used on that node :

while the node's distance is less than it's parents node ( for this priority queue, parents have lesser distances than the children), the node is swapped with the parent.

After this, while the node is greater distance than one or more of its children, it is swapped with the least distant child, so the least distant child becomes parent of its greater distant sibling, and parent to the greater distant current node.

With updating the priority, the adjacent node to the current node has a back pointer changed to the current node.

The algorithm ends when the target node becomes the current node removed, and the path to the start node can be recorded in an array by following back pointers, and then doing something like a quick sort partition to reverse the order of the array , to give the shortest path to target node from the start node.

#### Quick sort

3. Write a C program to recursively sort using the Quick sort partition exchange algorithm.

• you can use the "driver", or the random number test data from Q1. on mergesort. This is "re-use", which is encouraged in general.

- an advantage of reuse is less writing time, debugging time, testing time.

• the concept of partition exchange is that a partition element is (randomly) selected, and every thing that needs sorted is put into 3 equivalance

classes : those elements less than the partition value, the partition element, and everything above (and equal to ) the partition element.

• this can be done without allocating more space than one temporary element space for swapping two elements. e.g a temporary integer for integer data.
• However, where the partition element should be using the original array space is not known.
• This is usually implemented with putting the partition on the end of the array to be sorted, and then putting two pointers , one at the start of the array,

and one at the element next to the partition element , and repeatedly scanning the left pointer right, and the right pointer left.

• the left scan stops when an element equal to or greater than the partition is found, and the right scan stops for a smaller element than the partition value,

and these are swapped, which uses the temporary extra space.

• the left scan will always stop if it reaches the partition element , which is the last element; this means the entire array is less than partition value.
• the right scan could reach the first element, if the entire array is greater than the partition , and this needs to be tested for, else the scan doesn't stop.
• the outer loop exits when then left and right pointers cross. Testing for pointer crossing and outer loop exit

should occur before swapping, otherwise the right pointer may be swapping a less-than-partition element previously scanned by the left pointer.

• finally, the partition element needs to be put between the left and right partitions, once the pointers cross.
• At pointer crossing, the left pointer may be stopped at the partition element's last position in the array, and the right pointer not progressed past the

element just before the last element. This happens when all the elements are less than the partition.

- if the right pointer is chosen to swap with the partition, then an incorrect state results where the last element of the left array becomes less than the partition element value.

- if the left pointer is chosen to swap with the partition, then the left array will be less than the partition, and partition will have swapped with an element with value greater than the partition or the partition itself.

• The corner case of quicksorting a 2 element out-of-order array has to be examined.

- The left pointer stops on the first out of order element. The right pointer begins on the first out-of-order element, but the outer loop exits because this is the leftmost element. The partition element is then swapped with the left pointer's first element, and the two elements are now in order.

- In the case of a 2 element in order array, the leftmost pointer skips the first element which is less than the partition, and stops on the partition. The right pointer begins on the first element and exits because it is the first position. The pointers have crossed so the outer loop exits. The partition swaps with itself, so the in-ordering is preserved.

• After doing a swap, the left pointer should be incremented and right pointer decremented, so the same positions aren't scanned again, because an endless loop can result ( possibly when the left pointer exits when the element is equal to or greater than the partition, and the right element is equal to the partition value). One implementation, Sedgewick, starts the pointers with the left pointer minus one and right pointer

the plus one the intended initial scan positions, and use the pre-increment and pre-decrement operators e.g. ( ++i, --i) .

# Intermediate C

 To do:Add information and a section about recursive and cyclic mutually-inclusive data structures.

In the chapter Variables we looked at the primitive data types. However advanced data types allow us greater flexibility in managing data in our program.

## Structs

Structs are data types made of variables of other data types (possibly including other structs). They are used to group pieces of information into meaningful units, and also permit some constructs not possible otherwise. The variables declared in a struct are called "members". One defines a struct using the struct keyword. For example:

struct mystruct {
int int_member;
double double_member;
char string_member[25];
} struct_var;


struct_var is a variable of type struct mystruct, which we declared along with the definition of the new struct mystruct data type. More commonly, struct variables are declared after the definition of the struct, using the form:

struct mystruct struct_var;


It is often common practice to make a type synonym so we don't have to type "struct mystruct" all the time. C allows us the possibility to do so using a typedef statement, which aliases a type:

typedef struct {
// ...
} Mystruct;


The struct itself is an incomplete type (by the absence of a name on the first line), but it is aliased as Mystruct. Then the following may be used:

Mystruct struct_var;


The members of a struct variable may be accessed using the member access operator . (a dot) or the indirect member access operator -> (an arrow) if the struct variable is a pointer:

struct_var.int_member = 0;
struct_var->int_number = 0; // this statement is equivalent to: *struct_var.int_number = 0;


Structs may not only contain variables of other structs, but also their own variables. This allows a recursive definition, which is very powerful when used with pointers:

struct restaurant_order {
char description[100];
double price;
struct restaurant_order *next_order;
};


This is an implementation of the linked list data structure. Each node (a restaurant order) is pointing to one other node. The linked list is terminated on the last node (in our example, this would be the last order) whose next_order variable would be assigned to NULL.

A recursive struct definition can be tricky when used with typedef. It is not possible to declare a struct variable inside its own type by using its aliased definition, since the aliased definition by typedef does not exist before the typedef statement is evaluated:

typedef struct Mystruct {
// ...
struct Mystruct *pointer; // Mystruct *pointer; would cause a compile-time error
} Mystruct;


The size of a struct type equals the sum of the sizes of all its members.

## Unions

The definition of a union is similar to that of a struct. The difference between the two is that in a struct, the members occupy different areas of memory, but in a union, the members occupy the same area of memory. Thus, in the following type, for example:

union {
int i;
double d;
} u;


The programmer can access either u.i or u.d, but not both at the same time. Since u.i and u.d occupy the same area of memory, modifying one modifies the value of the other, sometimes in unpredictable ways.

The size of a union is the size of its largest member.

## Enumerations

Enumerations are artificial data types representing associations between labels and integers. Unlike structs or unions, they are not composed of other data types. An example declaration:

enum color {
red,
orange,
yellow,
green,
cyan,
blue,
purple,
} crayon_color;


In the example above, red equals 0, orange equals 1, ... and so each subsequent label is by 1 larger that the previous one. It is possible to assign values to labels within the integer range, but they must be a literal.

Similar declaration syntax that applies for structs and unions also applies for enums. Also, one normally doesn't need to be concerned with the integers that labels represent:

enum weather weather_outside = rain;


This peculiar property makes enums especially convenient in switch-case statements:

enum weather {
sunny,
windy,
cloudy,
rain,
} weather_outside;

// ...

switch (weather_outside) {
case sunny:
wear_sunglasses();
break;
case windy:
wear_windbreaker();
break;
case cloudy:
get_umbrella();
break;
case rain:
get_umbrella();
wear_raincoat();
break;
}


# Pointers and Relationship to Arrays

Pointer a pointing to variable b. Note that b stores a number, whereas a stores the address of b in memory (1462)

A pointer is a value that designates the address (i.e., the location in memory), of some value. Pointers are variables that hold a memory location.

There are four fundamental things you need to know about pointers:

• How to declare them (with the address operator '&': int *pointer = &variable;)
• How to assign to them (pointer = NULL;)
• How to reference the value to which the pointer points (known as dereferencing, by using the dereferencing operator '*': value = *pointer;)
• How they relate to arrays (the vast majority of arrays in C are simple lists, also called "1 dimensional arrays", but we will briefly cover multi-dimensional arrays with some pointers in a later chapter).

Pointers can reference any data type, even functions. We'll also discuss the relationship of pointers with text strings and the more advanced concept of function pointers.

## Declaring pointers

Consider the following snippet of code which declares two pointers:

 struct MyStruct {
int   m_aNumber;
float num2;
};

int main()
{
int *pJ2;
struct MyStruct *pAnItem;
}


Lines 1-4 define a structure. Line 8 declares a variable which points to an int, and line 9 declares a variable which points to something with structure MyStruct. So to declare a variable as something which points to some type, rather than contains some type, the asterisk (*) is placed before the variable name.

In the following, line 1 declares var1 as a pointer to a long and var2 as a long and not a pointer to a long. In line 2, p3 is declared as a pointer to a pointer to an int.

long  *  var1, var2;
int   ** p3;


Pointer types are often used as parameters to function calls. The following shows how to declare a function which uses a pointer as an argument. Since C passes function arguments by value, in order to allow a function to modify a value from the calling routine, a pointer to the value must be passed. Pointers to structures are also used as function arguments even when nothing in the struct will be modified in the function. This is done to avoid copying the complete contents of the structure onto the stack. More about pointers as function arguments later.

int MyFunction( struct MyStruct *pStruct );


## Assigning values to pointers

So far we've discussed how to declare pointers. The process of assigning values to pointers is next. To assign the address of a variable to a pointer, the & or 'address of' operator is used.

 int   myInt;
int  *pPointer;
struct MyStruct   dvorak;
struct MyStruct  *pKeyboard;

pPointer = &myInt;
pKeyboard = &dvorak;


Here, pPointer will now reference myInt and pKeyboard will reference dvorak.

Pointers can also be assigned to reference dynamically allocated memory. The malloc() and calloc() functions are often used to do this.

 #include <stdlib.h>
/* ... */
struct MyStruct *pKeyboard;
/* ... */
pKeyboard = malloc(sizeof *pKeyboard);


The malloc function returns a pointer to dynamically allocated memory (or NULL if unsuccessful). The size of this memory will be appropriately sized to contain the MyStruct structure.

The following is an example showing one pointer being assigned to another and of a pointer being assigned a return value from a function.

 static struct MyStruct val1, val2, val3, val4;

struct MyStruct *ASillyFunction( int b )
{
struct MyStruct *myReturn;

if (b == 1) myReturn = &val1;
else if (b==2) myReturn = &val2;
else if (b==3) myReturn = &val3;
else myReturn = &val4;

return myReturn;
}

struct MyStruct *strPointer;
int     *c, *d;
int     j;

c = &j;                           /* pointer assigned using & operator */
d = c;                            /* assign one pointer to another     */
strPointer = ASillyFunction( 3 ); /* pointer returned from a function. */


When returning a pointer from a function, do not return a pointer that points to a value that is local to the function or that is a pointer to a function argument. Pointers to local variables become invalid when the function exits. In the above function, the value returned points to a static variable. Returning a pointer to dynamically allocated memory is also valid.

## Pointer dereferencing

The pointer p points to the variable a.

To access a value to which a pointer points, the * operator is used. Another operator, the -> operator is used in conjunction with pointers to structures. Here's a short example.

 int   c, d;
int   *pj;
struct MyStruct astruct;
struct MyStruct *bb;

c   = 10;
pj  = &c;             /* pj points to c */
d   = *pj;            /* d is assigned the value to which pj points, 10 */
pj  = &d;             /* now points to d */
*pj = 12;             /* d is now 12 */

bb = &astruct;
(*bb).m_aNumber = 3;  /* assigns 3 to the m_aNumber member of astruct */
bb->num2 = 44.3;      /* assigns 44.3 to the num2 member of astruct   */
*pj = bb->m_aNumber;  /* equivalent to d = astruct.m_aNumber;          */


The expression bb->m_aNumber is entirely equivalent to (*bb).m_aNumber. They both access the m_aNumber element of the structure pointed to by bb. There is one more way of dereferencing a pointer, which will be discussed in the following section.

When dereferencing a pointer that points to an invalid memory location, an error often occurs which results in the program terminating. The error is often reported as a segmentation error. A common cause of this is failure to initialize a pointer before trying to dereference it.

C is known for giving you just enough rope to hang yourself, and pointer dereferencing is a prime example. You are quite free to write code that accesses memory outside that which you have explicitly requested from the system. And many times, that memory may appear as available to your program due to the vagaries of system memory allocation. However, even if 99 executions allow your program to run without fault, that 100th execution may be the time when your "memory pilfering" is caught by the system and the program fails. Be careful to ensure that your pointer offsets are within the bounds of allocated memory!

The declaration void *somePointer; is used to declare a pointer of some nonspecified type. You can assign a value to a void pointer, but you must cast the variable to point to some specified type before you can dereference it. Pointer arithmetic is also not valid with void * pointers.

## Pointers and Arrays

Up to now, we've carefully been avoiding discussing arrays in the context of pointers. The interaction of pointers and arrays can be confusing but here are two fundamental statements about it:

• A variable declared as an array of some type acts as a pointer to that type. When used by itself, it points to the first element of the array.
• A pointer can be indexed like an array name.

The first case often is seen to occur when an array is passed as an argument to a function. The function declares the parameter as a pointer, but the actual argument may be the name of an array. The second case often occurs when accessing dynamically allocated memory. Let's look at examples of each. In the following code, the call to calloc() effectively allocates an array of struct MyStruct items.

 float KrazyFunction( struct MyStruct *parm1, int p1size, int bb )
{
int ix; //declaring an integer variable//
for (ix=0; ix<p1size; ix++) {
if (parm1[ix].m_aNumber == bb )
return parm1[ix].num2;
}
return 0.0f;
}

/* ... */
struct MyStruct myArray[4];
#define MY_ARRAY_SIZE (sizeof(myArray)/sizeof(*myArray))
float v3;
struct MyStruct *secondArray;
int   someSize;
int   ix;
/* initialization of myArray ... */
v3 = KrazyFunction( myArray, MY_ARRAY_SIZE, 4 );
/* ... */
secondArray = calloc( someSize, sizeof(MyStruct) );
for (ix=0; ix<someSize; ix++) {
secondArray[ix].m_aNumber = ix *2;
secondArray[ix].num2 = .304 * ix * ix;
}


Pointers and array names can pretty much be used interchangeably. There are exceptions. You cannot assign a new pointer value to an array name. The array name will always point to the first element of the array. In the function KrazyFunction above, you could however assign a new value to parm1, as it is just a pointer to the first element of myArray. It is also valid for a function to return a pointer to one of the array elements from an array passed as an argument to a function. A function should never return a pointer to a local variable, even though the compiler will probably not complain.

When declaring parameters to functions, declaring an array variable without a size is equivalent to declaring a pointer. Often this is done to emphasize the fact that the pointer variable will be used in a manner equivalent to an array.


/* two equivalent function definitions */
int LittleFunction( int *paramN );
int LittleFunction( int paramN[] );


Now we're ready to discuss pointer arithmetic. You can add and subtract integer values to/from pointers. If myArray is declared to be some type of array, the expression *(myArray+j), where j is an integer, is equivalent to myArray[j]. So for instance in the above example where we had the expression secondArray[i].num2, we could have written that as *(secondArray+i).num2 or more simply (secondArray+i)->num2.

Note that for addition and subtraction of integers and pointers, the value of the pointer is not adjusted by the integer amount, but is adjusted by the amount multiplied by the size (in bytes) of the type to which the pointer refers. One pointer may also be subtracted from another, provided they point to elements of the same array (or the position just beyond the end of the array). If you have a pointer that points to an element of an array, the index of the element is the result when the array name is subtracted from the pointer. Here's an example.

 struct MyStruct someArray[20];
struct MyStruct *p2;
int idx;

.
/* array initialization .. */
.
for (p2 = someArray; p2 < someArray+20;  ++p2) {
if (p2->num2 > testValue) break;
}
idx = p2 - someArray;


You may be wondering how pointers and multidimensional arrays interact. Let's look at this a bit in detail. Suppose A is declared as a two dimensional array of floats (float A[D1][D2];) and that pf is declared a pointer to a float. If pf is initialized to point to A[0][0], then *(pf+1) is equivalent to A[0][1] and *(pf+D2) is equivalent to A[1][0]. The elements of the array are stored in row-major order.


float A[6][8];
float *pf;
pf = &A[0][0];
*(pf+1) = 1.3;   /* assigns 1.3 to A[0][1] */
*(pf+8) = 2.3;   /* assigns 2.3 to A[1][0] */


Let's look at a slightly different problem. We want to have a two dimensional array, but we don't need to have all the rows the same length. What we do is declare an array of pointers. The second line below declares A as an array of pointers. Each pointer points to a float. Here's some applicable code:

 float  linearA[30];
float *A[6];

A[0] = linearA;              /*  5 - 0 = 5 elements in row  */
A[1] = linearA + 5;          /* 11 - 5 = 6 elements in row  */
A[2] = linearA + 11;         /* 15 - 11 = 4 elements in row */
A[3] = linearA + 15;         /* 21 - 15 = 6 elements        */
A[4] = linearA + 21;         /* 25 - 21 = 4 elements        */
A[5] = linearA + 25;         /* 30 - 25 = 5 elements        */

*A[3][2] = 3.66;          /* assigns 3.66 to linearA[17];     */
*A[3][-3] = 1.44;         /* refers to linearA[12];
negative indices are sometimes useful. But avoid using them as much as possible. */


We also note here something curious about array indexing. Suppose myArray is an array and idx is an integer value. The expression myArray[idx] is equivalent to idx[myArray]. The first is equivalent to *(myArray+idx), and the second is equivalent to *(idx+myArray). These turn out to be the same, since the addition is commutative.

Pointers can be used with preincrement or post decrement, which is sometimes done within a loop, as in the following example. The increment and decrement applies to the pointer, not to the object to which the pointer refers. In other words, *pArray++ is equivalent to *(pArray++).

 long  myArray[20];
long  *pArray;
int  i;

/* Assign values to the entries of myArray */
pArray = myArray;
for (i=0; i<10; ++i) {
*pArray++ = 5 + 3*i + 12*i*i;
*pArray++ = 6 + 2*i + 7*i*i;
}


## Pointers in Function Arguments

Often we need to invoke a function with an argument that is itself a pointer. In many instances, the variable is itself a parameter for the current function and may be a pointer to some type of structure. The ampersand character is not needed in this circumstance to obtain a pointer value, as the variable is itself a pointer. In the example below, the variable pStruct, a pointer, is a parameter to function FunctTwo, and is passed as an argument to FunctOne. The second parameter to FunctOne is an int. Since in function FunctTwo, mValue is a pointer to an int, the pointer must first be dereferenced using the * operator, hence the second argument in the call is *mValue. The third parameter to function FunctOne is a pointer to a long. Since pAA is itself a pointer to a long, no ampersand is needed when it is used as the third argument to the function.

 int FunctOne( struct SomeStruct *pValue, int iValue, long *lValue )
{
/*  do some stuff ... */
return 0;
}
int FunctTwo( struct someStruct *pStruct, int *mValue )
{
int j;
long  AnArray[25];
long *pAA;

pAA = &AnArray[13];
j = FunctOne( pStruct, *mValue, pAA );
return j;
}


## Pointers and Text Strings

Historically, text strings in C have been implemented as arrays of characters, with the last byte in the string being a zero, or the null character '\0'. Most C implementations come with a standard library of functions for manipulating strings. Many of the more commonly used functions expect the strings to be null terminated strings of characters. To use these functions requires the inclusion of the standard C header file "string.h".

A statically declared, initialized string would look similar to the following:

static const char *myFormat = "Total Amount Due: %d";


The variable myFormat can be viewed as an array of 21 characters. There is an implied null character ('\0') tacked on to the end of the string after the 'd' as the 21st item in the array. You can also initialize the individual characters of the array as follows:

static const char myFlower[] = { 'P', 'e', 't', 'u', 'n', 'i', 'a', '\0' };


An initialized array of strings would typically be done as follows:

static const char *myColors[] = {
"Red", "Orange", "Yellow", "Green", "Blue", "Violet" };


The initialization of an especially long string can be split across lines of source code as follows.

static char *longString = "Hello. My name is Rudolph and I work as a reindeer "
"around Christmas time up at the North Pole.  My boss is a really swell guy."
" He likes to give everybody gifts.";


The library functions that are used with strings are discussed in a later chapter.

## Pointers to Functions

C also allows you to create pointers to functions. Pointers to functions can get rather messy. Declaring a typedef to a function pointer generally clarifies the code. Here's an example that uses a function pointer, and a void * pointer to implement what's known as a callback. The DoSomethingNice function invokes a caller supplied function TalkJive with caller data. Note that DoSomethingNice really doesn't know anything about what dataPointer refers to.

 typedef  int (*MyFunctionType)( int, void *);      /* a typedef for a function pointer */

#define THE_BIGGEST 100

int DoSomethingNice( int aVariable, MyFunctionType aFunction, void *dataPointer )
{
int rv = 0;
if (aVariable < THE_BIGGEST) {
/* invoke function through function pointer (old style) */
rv = (*aFunction)(aVariable, dataPointer );
} else {
/* invoke function through function pointer (new style) */
rv = aFunction(aVariable, dataPointer );
};
return rv;
}

typedef struct {
int    colorSpec;
char   *phrase;
} DataINeed;

int TalkJive( int myNumber, void *someStuff )
{
/* recast void * to pointer type specifically needed for this function */
DataINeed *myData = someStuff;
/* talk jive. */
return 5;
}

static DataINeed  sillyStuff = { BLUE, "Whatcha talkin 'bout Willis?" };

/* ... */
DoSomethingNice( 41, &TalkJive,  &sillyStuff );


Some versions of C may not require an ampersand preceding the TalkJive argument in the DoSomethingNice call. Some implementations may require specifically casting the argument to the MyFunctionType type, even though the function signature exacly matches that of the typedef.

Function pointers can be useful for implementing a form of polymorphism in C. First one declares a structure having as elements function pointers for the various operations to that can be specified polymorphically. A second base object structure containing a pointer to the previous structure is also declared. A class is defined by extending the second structure with the data specific for the class, and static variable of the type of the first structure, containing the addresses of the functions that are associated with the class. This type of polymorphism is used in the standard library when file I/O functions are called.

A similar mechanism can also be used for implementing a state machine in C. A structure is defined which contains function pointers for handling events that may occur within state, and for functions to be invoked upon entry to and exit from the state. An instance of this structure corresponds to a state. Each state is initialized with pointers to functions appropriate for the state. The current state of the state machine is in effect a pointer to one of these states. Changing the value of the current state pointer effectively changes the current state. When some event occurs, the appropriate function is called through a function pointer in the current state.

## Practical use of function pointers in C

Function pointers are mainly used to reduce the complexity of switch statement. Example with switch statement:

#include <stdio.h>
int sub(int a, int b);
int mul(int a, int b);
int div(int a, int b);
int main()
{
int i, result;
int a=10;
int b=5;
printf("Enter the value between 0 and 3 : ");
scanf("%d",&i);
switch(i)
{
case 0:  result = add(a,b); break;
case 1:  result = sub(a,b); break;
case 2:  result = mul(a,b); break;
case 3:  result = div(a,b); break;
}
}
{
return (i+j);
}
int sub(int i, int j)
{
return (i-j);
}
int mul(int i, int j)
{
return (i*j);
}
int div(int i, int j)
{
return (i/j);
}


Without using a switch statement:

#include <stdio.h>
int sub(int a, int b);
int mul(int a, int b);
int div(int a, int b);
int (*oper[4])(int a, int b) = {add, sub, mul, div};
int main()
{
int i,result;
int a=10;
int b=5;
printf("Enter the value between 0 and 3 : ");
scanf("%d",&i);
result = oper[i](a,b);
}
{
return (i+j);
}
int sub(int i, int j)
{
return (i-j);
}
int mul(int i, int j)
{
return (i*j);
}
int div(int i, int j)
{
return (i/j);
}


Function pointers may be used to create a struct member function:

typedef struct
{
int (*open)(void);
void (*close)(void);
int (*reg)(void);
} device;

int my_device_open(void)
{
/* ... */
}

void my_device_close(void)
{
/* ... */
}

void register_device(void)
{
/* ... */
}

device create(void)
{
device my_device;
my_device.open = my_device_open;
my_device.close = my_device_close;
my_device.reg = register_device;
my_device.reg();
return my_device;
}


Use to implement this pointer (following code must be placed in library).

static struct device_data
{
/* ... here goes data of structure ... */
};

static struct device_data obj;

typedef struct
{
int (*open)(void);
void (*close)(void);
int (*reg)(void);
} device;

static struct device_data create_device_data(void)
{
struct device_data my_device_data;
/* ... here goes constructor ... */
return my_device_data;
}

/* here I omit the my_device_open, my_device_close and register_device functions */

device create_device(void)
{
device my_device;
my_device.open = my_device_open;
my_device.close = my_device_close;
my_device.reg = register_device;
my_device.reg();
return my_device;
}


## Examples of pointer constructs

Below are some example constructs which may aid in creating your pointer.

int i;          // integer variable 'i'
int *p;         // pointer 'p' to an integer
int a[];        // array 'a' of integers
int f();        // function 'f' with return value of type integer
int **pp;       // pointer 'pp' to a pointer to an integer
int (*pa)[];    // pointer 'pa' to an array of integer
int (*pf)();    // pointer 'pf' to a function with return value integer
int *ap[];      // array 'ap' of pointers to an integer
int *fp();      // function 'fp' which returns a pointer to an integer
int ***ppp;     // pointer 'ppp' to a pointer to a pointer to an integer
int (**ppa)[];  // pointer 'ppa' to a pointer to an array of integers
int (**ppf)();  // pointer 'ppf' to a pointer to a function with return value of type integer
int *(*pap)[];  // pointer 'pap' to an array of pointers to an integer
int *(*pfp)();  // pointer 'pfp' to function with return value of type pointer to an integer
int **app[];    // array of pointers 'app' that point to pointers to integer values
int (*apa[])[]; // array of pointers 'apa' to arrays of integers
int (*apf[])(); // array of pointers 'apf' to functions with return values of type integer
int ***fpp();   // function 'fpp' which returns a pointer to a pointer to a pointer to an int
int (*fpa())[]; // function 'fpa' with return value of a pointer to array of integers
int (*fpf())(); // function 'fpf' with return value of a pointer to function which returns an integer


## sizeof

The sizeof operator is often used to refer to the size of a static array declared earlier in the same function.

To find the end of an array (example from wikipedia:Buffer overflow):

/* better.c - demonstrates one method of fixing the problem */

#include <stdio.h>
#include <string.h>

int main(int argc, char *argv[])
{
char buffer[10];
if (argc < 2)
{
fprintf(stderr, "USAGE: %s string\n", argv[0]);
return 1;
}
strncpy(buffer, argv[1], sizeof(buffer));
buffer[sizeof(buffer) - 1] = '\0';
return 0;
}


To iterate over every element of an array, use

 #define NUM_ELEM(x) (sizeof (x) / sizeof (*(x)))

for( i = 0; i < NUM_ELEM(array); i++ )
{
/* do something with array[i] */
;
}


Note that the sizeof operator only works on things defined earlier in the same function. The compiler replaces it with some fixed constant number. In this case, the buffer was declared as an array of 10 char's earlier in the same function, and the compiler replaces sizeof(buffer) with the number 10 at compile time (equivalent to us hard-coding 10 into the code in place of sizeof(buffer)). The information about the length of buffer is not actually stored anywhere in memory (unless we keep track of it separately) and cannot be programmatically obtained at run time from the array/pointer itself.

Often a function needs to know the size of an array it was given -- an array defined in some other function. For example,

/* broken.c - demonstrates a flaw */

#include <stdio.h>
#include <string.h>
#define NUM_ELEM(x) (sizeof (x) / sizeof (*(x)))

int sum( int input_array[] ){
int sum_so_far = 0;
int i;
for( i = 0; i < NUM_ELEM(input_array); i++ ) // WON'T WORK -- input_array wasn't defined in this function.
{
sum_so_far += input_array[i];
};
return( sum_so_far );
}

int main(int argc, char *argv[])
{
int left_array[] = { 1, 2, 3 };
int right_array[] = { 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 };
int the_sum = sum( left_array );
printf( "the sum of left_array is: %d", the_sum );
the_sum = sum( right_array );
printf( "the sum of right_array is: %d", the_sum );

return 0;
}


Unfortunately, (in C and C++) the length of the array cannot be obtained from an array passed in at run time, because (as mentioned above) the size of an array is not stored anywhere. The compiler always replaces sizeof with a constant. This sum() routine needs to handle more than just one constant length of an array.

There are some common ways to work around this fact:

• Write the function to require, for each array parameter, a "length" parameter (which has type "size_t"). (Typically we use sizeof at the point where this function is called).
• Use of a convention, such as a null-terminated string to mark the end of the array.
• Instead of passing raw arrays, pass a structure that includes the length of the array (such as ".length") as well as the array (or a pointer to the first element); similar to the string or vector classes in C++.
/* fixed.c - demonstrates one work-around */

#include <stdio.h>
#include <string.h>
#define NUM_ELEM(x) (sizeof (x) / sizeof (*(x)))

int sum( int input_array[], size_t length ){
int sum_so_far = 0;
int i;
for( i = 0; i < length; i++ )
{
sum_so_far += input_array[i];
};
return( sum_so_far );
}

int main(int argc, char *argv[])
{
int left_array[] = { 1, 2, 3, 4 };
int right_array[] = { 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 };
int the_sum = sum( left_array, NUM_ELEM(left_array) ); // works here, because left_array is defined in this function
printf( "the sum of left_array is: %d", the_sum );
the_sum = sum( right_array, NUM_ELEM(right_array) ); // works here, because right_array is defined in this function
printf( "the sum of right_array is: %d", the_sum );

return 0;
}


It's worth mentioning that sizeof operator has two variations: sizeof (type) (for instance: sizeof (int) or sizeof (struct some_structure)) and sizeof expression (for instance: sizeof some_variable.some_field or sizeof 1).