Posted in C programming Language

Solution of C question paper MCA-1(2017)

Q1. What are the four components that make up the computer system? Discuss with the help of Diagram.

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COMPUTER SYSTEM

COMPUTER SYSTEM
Definition: Is a collection of entities(hardware,software and liveware) that are designed to receive, process, manage and present information in a meaningful format.COMPONENTS OF COMPUTER SYSTEM

  • Computer hardware – Are physical parts/ intangible parts of a computer. eg Input devices, output devices, central processing unit and storage devices
  • Computer software – also known as programs or applications. They are classified into two classes namely – sytem software and application software
  • Liveware – is the computer user. Also kwon as orgwareor the humanware. The user commands the computer system to execute on instructions.

a) COMPUTER HARDWARE
Hardware refers to the physical, tangible computer equipment and devices, which provide support for major functions such as input, processing (internal storage, computation and control), output, secondary storage (for data and programs), and communication.

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HARDWARE CATEGORIES (Functional Parts)

A computer system is a set of integrated devices that input, output, process, and store data and information. Computer systems are currently built around at least one digital processing device. There are five main hardware components in a computer system: Input, Processing, Storage, Output and Communication devices.

  1. INPUT DEVICES

Are devices used for entering data or instructions to the central processing unit. Are classifie according to the method they use to enter data.

a) KEYING DEVICES
Are devices used to enter data into the computer using a set of Keys eg Keyboard, key-to- storage and keypad.

i) The keyboard

Keyboard (similar to a typewriter) is the main input device of a computer . It contains three types of keys– alphanumeric keys, special keys and function keys. Alphanumeric keys are used to type all alphabets, numbers and special symbols like $, %, @, A etc. Special keys such as <Shift>, <Ctrl>, <Alt>, <Home>, <Scroll Lock> etc. are used for special functions. Function keys such as <Fl>, <F2>, <F3> etc. are used to give special commands depending upon the software used e.g.F5 reloads a page of an internet browser. The function of each and every key can be well understood only after working on a PC. When any key is pressed, an electronic signal is produced. This signal is detected by a keyboard encoder that sends a binary code corresponding to the key pressed to the CPU. There are many types of keyboards but 101 keys keyboard is the most popular one.

How the keys are organized

The keys on your keyboard can be divided into several groups based on function:

  • Typing (alphanumeric) keys. These keys include the same letter, number, punctuation, and symbol keys found on a traditional typewriter.
  • Special (Control) keys. These keys are used alone or in combination with other keys to perform certain actions. The most frequently used control keys are CTRL, ALT, the Windows key, and ESC.
  • Function keys. The function keys are used to perform specific tasks. They are labelled as F1, F2, F3, and so on, up to F12. The functionality of these keys differs from program to program.
  • Cursor Movement (Navigation) keys. These keys are used for moving around in documents or WebPages and editing text. They include the arrow keys, HOME, END, PAGE UP, PAGE DOWN, DELETE, and INSERT and ARROW KEYS.
  • Numeric keypad. The numeric keypad is handy for entering numbers quickly. The keys are grouped together in a block like a conventional calculator or adding machine.


B. POINTING DEVICES
Are devices that enter data and instructions into the computer using a pointer that appears on the screen. The items to be entered are selected by either pointing to or clicking on them.e.g mice, joystick, touch sensitive screen, trackballs

i) THE MOUSE
A mouse is a small device used to point to and select items on your computer screen. Although mice come in many shapes, the typical mouse does look a bit like an actual mouse. It’s small, oblong, and connected to the system unit by a long wire that resembles a tail and the connector which can either be PS/2 or USB. Some newer mice are wireless.

A mouse usually has two buttons: a primary button (usually the left button) and a secondary button. Many mice also have a wheel between the two buttons, which allows you to scroll smoothly through screens of information.

When you move the mouse with your hand, a pointer on your screen moves in the same direction. (The pointer’s appearance might change depending on where it’s positioned on your screen.) When you want to select an item, you point to the item and then click (press and release) the primary button. Pointing and clicking with your mouse is the main way to interact with your computer. There are several types of mice: Mechanical mouse, optical mouse, optical-mechanical mouse and laser mouse.

Basic parts

A mouse typically has two buttons: a primary button (usually the left button) and a secondary button (usually the right button). The primary button is the one you will use most often. Most mice also include a scroll wheel between the buttons to help you scroll through documents and WebPages more easily. On some mice, the scroll wheel can be pressed to act as a third button. Advanced mice might have additional buttons that can perform other functions.

Image result for computer mouse parts

Holding and moving the mouse

Place your mouse beside your keyboard on a clean, smooth surface, such as a mouse pad. Hold the mouse gently with your index finger resting on the primary button and you thumb resting on the side. To move the mouse, slide it slowly in any direction. Don’t twist it—keep the front of the mouse aimed away from you. As you move the mouse, a pointer (see picture) on your screen moves in the same direction. If you run out of room to move your mouse on your desk or mouse pad, just pick up the mouse and bring it back closer to you.
Image result for how to hold a mouse
Pointing to an object often reveals a descriptive message about it.The pointer can change depending on what you’re pointing at. For example, when you point to a link in your web browser, the pointer changes from an arrow to a hand with a pointing finger .

Most mouse actions combine pointing with pressing one of the mouse buttons. There are four basic ways to use your mouse buttons: clicking, double-clicking, right-clicking, and dragging.

Clicking (single-clicking)

To click an item, point to the item on the screen, and then press and release the primary button (usually the left button).

Clicking is most often used to select (mark) an item or open a menu. This is sometimes called single-clicking or left-clicking.

Double-clicking

To double-click an item, point to the item on the screen, and then click twice quickly. If the two clicks are spaced too far apart, they might be interpreted as two individual clicks rather than as one double-click.

Double-clicking is most often used to open items on your desktop. For example, you can start a program or open a folder by double-clicking its icon on the desktop.

Right-clicking

To right-click an item, point to the item on the screen, and then press and release the secondary button (usually the right button).

Right-clicking an item usually displays a list of things you can do with the item. For example, when you right-click the Recycle Bin on your desktop, Windows displays a menu allowing you to open it, empty it, delete it, or see its properties. If you are unsure of what to do with something, right-click it.

C) SCANNING DEVICES
Are devices that capture an object or a document directly from the source. They are classifie according to the technology used to capture data e.g. Scanners and Document readers.
i) Scanners
Used to capture a source document and converts it into an electronic form.
Example are – FlatBed and HandHeld scanners.


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Image result for type of scanners

ii) Document readers
Are documents that reads data directly from source document and convey them as input in the form of electronic signal. e
Types of Document Readers
i) Optical Mar Reader (OMR)Image result for optical mark reader

ii) Barcode readers
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iii) Optical Character ReadersImage result for optical character reader

b) Magnetic Readers
Reads data using magnetic ink.t uses principle of magnetism to sense data which have been written using magnetised ink.

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THE CENTRAL PROCESSING UNIT ( C P U)

Is the brain or the heart of a computer. Is also known as processor and consist of three units namely –
i) Control Unit ( C U)
ii) Arithmetic logic Unit ( A L U)
iii) Main Memory unit ( M M U)

BLOCK DIAGRAM OF THE COMPUTER.JPG

The system unit is the core of a computer system. Usually it’s a rectangular box placed on or underneath your desk. Inside this box are many electronic components that process data. The most important of these components is the central processing unit (CPU), or microprocessor, which acts as the “brain” of your computer. Another component is random access memory (RAM), which temporarily stores information that the CPU uses while the computer is on. The information stored in RAM is erased when the computer is turned off.

Almost every other part of your computer connects to the system unit using cables. The cables plug into specific ports (openings), typically on the back of the system unit. Hardware that is not part of the system unit is sometimes called a peripheral device. Peripheral devices can be external such as a mouse, keyboard, printer, monitor, external Zip drive or scanner or internal, such as a CD-ROM drive, CD-R drive or internal modem. Internal peripheral devices are often referred to as integrated peripherals. There are two types according to shape: tower and desktop.
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Tower System Unit Desktop System Unit

motherboard (mainboardsystem boardplanar board or logic board) is the main printed circuit board found in computers and other expandable systems. It holds many of the crucial electronic components of the system, such as the central processing unit (CPU) and memory, and provides connectors for other peripherals.

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Motherboard

TYPES OF PROCESSORS
I) Comples Instruction Set Computers (CISC)
ii) Reduced Instruction Set Computers (RISC)

FUNCTIONS OF CENTRAL PROCESSING UNIT
– Process data
– Control sequence of operaions within the computers
– It gives command to all parts of a computer
– It control the use of the main memory in storing of data and instructions
– it provides temporary storage (RAM) and permanent storage(ROM) of data

THE CONTROL UNIT
Is the center of operations for the computer system, it directs the activities of the computer system.
Functions of Control Unit.

Q2. What are Computer Virus? What are the various types of threat that can be faced by computer system?

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A computer virus is a malicious program that self-replicates by copying itself to another program. In other words, the computer virus spreads by itself into other executable code or documents. The purpose of creating a computer virus is to infect vulnerable systems, gain admin control and steal user sensitive data. Hackers design computer viruses with malicious intent and prey on online users by tricking them.

One of the ideal methods by which viruses spread is through emails – opening the attachment in the email, visiting an infected website, clicking on an executable file, or viewing an infected advertisement can cause the virus to spread to your system. Besides that, infections also spread while connecting with already infected removable storage devices, such as USB drives.

It is quite easy and simple for the viruses to sneak into a computer by dodging the defense systems. A successful breach can cause serious issues for the user such as infecting other resources or system software, modifying or deleting key functions or applications and copy/delete or encrypt data.

There are two types of ways in viruses operate, as soon as they land on a new device they begin replicating, while the second type plays dead until a particular trigger makes the malicious code to be executed. Thereby, it is highly important to stay protected by installing a robust antivirus program.


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Presently, the sophisticated ones come with evasion capabilities that help in bypassing antivirus software and other advanced levels of defenses. Subsequently, the polymorphic malware development in the recent times enables the viruses to dynamically change its code as it spreads. This has made the virus detection and identification very challenging.

Types of Threats

Malware – Malicious software that infects your computer, such as computer viruses, worms, Trojan horses, spyware, and adware.

Virus – It is a malicious program where it replicates itself and aim to only destroy a computer. The ultimate goal is the computer will never be able to operate properly. First computer virus name is “Brian”.

Trojan – It has the ability to hide itself from antivirus detection & and steal important data.

Worms – program designed only to spread, it will use up your computer hard disk space due to the replication. “Morris was one of the first computer worm distributed via the internet.

Spyware – designed to spy on the victim’s computer. Such as if you browse on football for a week every day, the attacker will try to come out with a football scam to cheat on your money.

Adware – Is a form of threat it will start popping out a lot of advertisement. It gathers and transfer to its distributor personal information of the user.

Scareware – programs designed to trick a user into buying and downloading unnecessary and potentially dangerous software.

Botnet – something which is installed by a BotMaster to take control of the system via botnet infection.

C&C – A command and control server (C&C server) is the centralized computer that issues commands to a botnet.

Ransomware – Ransomware is a type of malware that restricts access to your computer. It displays an image that prevents you from accessing your computer or encrypts files on your system’s.

Key logger – keeps a record of every keystroke you made on your keyboard. It’s mainly used to steal people’s login credential such as username and password.

Backdoor – attacker will be able to bypass all the regular authentication service.

Dropper – designed to drop into a computer and install something useful to the attacker such as Malware or Backdoor. It immediately drops and install to avoid Antivirus detection. Another is, it will only drop a small file, it will auto trigger a download process to download the Malware.

Phishing – A fake website or email which is designed to look almost like the actual website is a form of phishing attack. purpose is stealing the identity of the victim.

Spam – more common methods of both sending information out and collecting it from unsuspecting people.

Spoofing – An email address may even include your own name, or the name of someone you know.

Posted in C programming Language

Storage classes in C

Storage Classes are used to describe about the features of a variable/function. These features basically include the scope, visibility and life-time which help us to trace the existence of a particular variable during the runtime of a program.

C language uses 4 storage classes, namely:

auto: This is the default storage class for all the variables declared inside a function or a block. Hence, the keyword auto is rarely used while writing programs in C language. Auto variables can be only accessed within the block/function they have been declared and not outside them (which defines their scope). Of course, these can be accessed within nested blocks within the parent block/function in which the auto variable was declared. However, they can be accessed outside their scope as well using the concept of pointers given here by pointing to the very exact memory location where the variables resides. They are assigned a garbage value by default whenever they are declared.

extern: Extern storage class simply tells us that the variable is defined elsewhere and not within the same block where it is used. Basically, the value is assigned to it in a different block and this can be overwritten/changed in a different block as well. So an extern variable is nothing but a global variable initialized with a legal value where it is declared in order to be used elsewhere. It can be accessed within any function/block. Also, a normal global variable can be made extern as well by placing the ‘extern’ keyword before its declaration/definition in any function/block. This basically signifies that we are not initializing a new variable but instead we are using/accessing the global variable only. The main purpose of using extern variables is that they can be accessed between two different files which are part of a large program. For more information on how extern variables work, have a look at this link.

 

static: This storage class is used to declare static variables which are popularly used while writing programs in C language. Static variables have a property of preserving their value even after they are out of their scope! Hence, static variables preserve the value of their last use in their scope. So we can say that they are initialized only once and exist till the termination of the program. Thus, no new memory is allocated because they are not re-declared. Their scope is local to the function to which they were defined. Global static variables can be accessed anywhere in the program. By default, they are assigned the value 0 by the compiler.

register: This storage class declares register variables which have the same functionality as that of the auto variables. The only difference is that the compiler tries to store these variables in the register of the microprocessor if a free register is available. This makes the use of register variables to be much faster than that of the variables stored in the memory during the runtime of the program. If a free register is not available, these are then stored in the memory only. Usually few variables which are to be accessed very frequently in a program are declared with the register keyword which improves the running time of the program. An important and interesting point to be noted here is that we cannot obtain the address of a register variable using pointers.

To specify the storage class for a variable, the following syntax is to be followed:

Syntax:

storage_class var_data_type var_name;

Functions follow the same syntax as given above for variables. Have a look at the following C example for further clarification:

// A C program to demonstrate different storage
// classes
#include <stdio.h>
 
// declaring and initializing an extern variable
extern int x = 9;
 
// declaring and initialing a global variable z
// simply int z; would have initialized z with
// the default value of a global variable which is 0
int z = 10;
 
int main()
{
    // declaring an auto variable (simply
    // writing "int a=32;" works as well)
    auto int a = 32;
 
    // declaring a register variable
    register char b = 'G';
 
    // telling the compiler that the variable
    // z is an extern variable and has been
    // defined elsewhere (above the main
    // function)
    extern int z;
 
    printf("Hello World!\n");
 
    // printing the auto variable 'a'
    printf("\nThis is the value of the auto "
           " integer 'a': %d\n",a);
 
    // printing the extern variables 'x'
    // and 'z'
    printf("\nThese are the values of the"
           " extern integers 'x' and 'z'"
           " respectively: %d and %d\n", x, z);
 
    // printing the register variable 'b'
    printf("\nThis is the value of the "
           "register character 'b': %c\n",b);
 
    // value of extern variable x modified
    x = 2;
 
    // value of extern variable z modified
    z = 5;
 
    // printing the modified values of
    // extern variables 'x' and 'z'
    printf("\nThese are the modified values "
           "of the extern integers 'x' and "
           "'z' respectively: %d and %d\n",x,z);
 
    // using a static variable 'y'
    printf("\n'y' is a static variable and its "
           "value is NOT initialized to 5 after"
           " the first iteration! See for"
           " yourself :)\n");
 
    while (x > 0)
    {
        static int y = 5;
        y++;
 
        // printing value of y at each iteration
        printf("The value of y is %d\n",y);
        x--;
    }
 
    // exiting
    printf("\nBye! See you soon. :)\n");
 
    return 0;
}

Output:

Hello World!

This is the value of the auto  integer 'a': 32

These are the values of the extern integers 'x' and 'z' 
respectively: 9 and 10

This is the value of the register character 'b': G

These are the modified values of the extern integers 'x' 
and 'z' respectively: 2 and 5

'y' is a static variable and its value is NOT initialized 
to 5 after the first iteration! See for yourself :)
The value of y is 6
The value of y is 7

Bye! See you soon. :)
Posted in C programming Language

Method of Compilation in C

Compiling a C program:- Behind the Scenes

C is a high level language and it needs a compiler to convert it into an executable code so that the program can be run on our machine.

How do we compile and run a C program?

Below are the steps we use on an Ubuntu machine with gcc compiler.

  • compilationWe first create a C program using an editor and save the file as filename.c
     $ vi filename.c

    The diagram on right shows a simple program to add two numbers.

  • compil31Then compile it using below command.
     $ gcc –Wall filename.c –o filename

    The option -Wall enables all compiler’s warning messages. This option is recommended to generate better code.
    The option -o is used to specify output file name. If we do not use this option, then an output file with name a.out is generated.

  • compil21After compilation executable is generated and we run the generated executable using below command.
     $ ./filename

 

What goes inside the compilation process?

Compiler converts a C program into an executable. There are four phases for a C program to become an executable:

  1. Pre-processing
  2. Compilation
  3. Assembly
  4. Linking

By executing below command, We get the all intermediate files in the current directory along with the executable.

 $gcc –Wall –save-temps filename.c –o filename

The following screenshot shows all generated intermediate files.
compil4

Let us one by one see what these intermediate files contain.

Pre-processingThis is the first phase through which source code is passed. This phase include:

  • Removal of Comments
  • Expansion of Macros
  • Expansion of the included files.

The preprocessed output is stored in the filename.i. Let’s see what’s inside filename.i: using $vi filename.i

compil5compile6

In the above output, source file is filled with lots and lots of info, but at the end our code is preserved.
Analysis:

  • printf contains now a + b rather than add(a, b) that’s because macros have expanded.
  • Comments are stripped off.
  • #include<stdio.h> is missing instead we see lots of code. So header files has been expanded and included in our source file.

CompilingThe next step is to compile filename.i and produce an; intermediate compiled output file filename.s. This file is in assembly level instructions. Let’s see through this file using $vi filename.s

image

The snapshot shows that it is in assembly language, which assembler can understand.

AssemblyIn this phase the filename.s is taken as input and turned into filename.o by assembler. This file contain machine level instructions. At this phase, only existing code is converted into machine language, the function calls like printf() are not resolved. Let’s view this file using $vi filename.o

compil7

LinkingThis is the final phase in which all the linking of function calls with their definitions are done. Linker knows where all these functions are implemented. Linker does some extra work also, it adds some extra code to our program which is required when the program starts and ends. For example, there is a code which is required for setting up the environment like passing command line arguments. This task can be easily verified by using $size filename.o and $size filename. Through these commands, we know that how output file increases from an object file to an executable file. This is because of the extra code that linker adds with our program.
compil8
Note that GCC by default does dynamic linking, so printf() is dynamically linked in above program. Refer thisthis and this for more details on static and dynamic linkings.

Posted in C programming Language

History of C language

  Early developments

Year C Standard
1972 Birth
1978 K&R C
19(89/90) ANSI C and ISO C
1999 C99
2011 C11

 

The origin of C is closely tied to the development of the Unix operating system, originally implemented in assembly language on a PDP-7 by Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was developed in assembly language. The developers were considering rewriting the system using the B language, Thompson’s simplified version of BCPL. However B’s inability to take advantage of some of the PDP-11’s features, notably byte addressability, led to C. The name of C was chosen simply as the next after B.

The development of C started in 1972 on the PDP-11 Unix system and first appeared in Version 2 Unix. The language was not initially designed with portability in mind, but soon ran on different platforms as well: a compiler for the Honeywell 6000 was written within the first year of C’s history, while an IBM System/370 port followed soon.

Also in 1972, a large part of Unix was rewritten in C. By 1973, with the addition of struct types, the C language had become powerful enough that most of the Unix kernel was now in C.

Unix was one of the first operating system kernels implemented in a language other than assembly. Earlier instances include the Multics system (which was written in PL/I) and Master Control Program (MCP) for the Burroughs B5000 (which was written in ALGOL) in 1961. In around 1977, Ritchie and Stephen C. Johnson made further changes to the language to facilitate portability of the Unix operating system. Johnson’s Portable C Compiler served as the basis for several implementations of C on new platforms.

K&R C

The cover of the book, The C Programming Language, first edition by Brian Kernighan and Dennis Ritchie

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language. This book, known to C programmers as “K&R”, served for many years as an informal specification of the language. The version of C that it describes is commonly referred to as K&R C. The second edition of the book covers the later ANSI C standard, described below.

K&R introduced several language features:

  • Standard I/O library
  • long int data type
  • unsigned int data type
  • Compound assignment operators of the form =op (such as =-) were changed to the form op= (that is, -=) to remove the semantic ambiguity created by constructs such as i=-10, which had been interpreted as i =- 10 (decrement i by 10) instead of the possibly intended i = -10 (let i be -10).

Even after the publication of the 1989 ANSI standard, for many years K&R C was still considered the “lowest common denominator” to which C programmers restricted themselves when maximum portability was desired, since many older compilers were still in use, and because carefully written K&R C code can be legal Standard C as well.

In early versions of C, only functions that return types other than int must be declared if used before the function definition; functions used without prior declaration were presumed to return type int.

For example:

long some_function();
/* int */ other_function();

/* int */ calling_function()
{
    long test1;
    register /* int */ test2;

    test1 = some_function();
    if (test1 > 0)
          test2 = 0;
    else
          test2 = other_function();
    return test2;
}

The int type specifiers which are commented out could be omitted in K&R C, but are required in later standards.

Since K&R function declarations did not include any information about function arguments, function parameter type checks were not performed, although some compilers would issue a warning message if a local function was called with the wrong number of arguments, or if multiple calls to an external function used different numbers or types of arguments. Separate tools such as Unix’s lint utility were developed that (among other things) could check for consistency of function use across multiple source files.

In the years following the publication of K&R C, several features were added to the language, supported by compilers from AT&T (in particular PCC) and some other vendors. These included:

  • void functions (i.e., functions with no return value)
  • functions returning struct or union types (rather than pointers)
  • assignment for struct data types
  • enumerated types

The large number of extensions and lack of agreement on a standard library, together with the language popularity and the fact that not even the Unix compilers precisely implemented the K&R specification, led to the necessity of standardization.

ANSI C and ISO C

During the late 1970s and 1980s, versions of C were implemented for a wide variety of mainframe computersminicomputers, and microcomputers, including the IBM PC, as its popularity began to increase significantly.

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C standard was ratified as ANSI X3.159-1989 “Programming Language C”. This version of the language is often referred to as ANSI C, Standard C, or sometimes C89.

In 1990, the ANSI C standard (with formatting changes) was adopted by the International Organization for Standardization (ISO) as ISO/IEC 9899:1990, which is sometimes called C90. Therefore, the terms “C89” and “C90” refer to the same programming language.

ANSI, like other national standards bodies, no longer develops the C standard independently, but defers to the international C standard, maintained by the working group ISO/IEC JTC1/SC22/WG14. National adoption of an update to the international standard typically occurs within a year of ISO publication.

One of the aims of the C standardization process was to produce a superset of K&R C, incorporating many of the subsequently introduced unofficial features. The standards committee also included several additional features such as function prototypes (borrowed from C++), void pointers, support for international character sets and locales, and preprocessor enhancements. Although the syntax for parameter declarations was augmented to include the style used in C++, the K&R interface continued to be permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and most C code being written today is based on it. Any program written only in Standard C and without any hardware-dependent assumptions will run correctly on any platform with a conforming C implementation, within its resource limits. Without such precautions, programs may compile only on a certain platform or with a particular compiler, due, for example, to the use of non-standard libraries, such as GUI libraries, or to a reliance on compiler- or platform-specific attributes such as the exact size of data types and byte endianness.

In cases where code must be compilable by either standard-conforming or K&R C-based compilers, the __STDC__ macro can be used to split the code into Standard and K&R sections to prevent the use on a K&R C-based compiler of features available only in Standard C.

After the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add more extensive support for international character sets.[18]

C99

The C standard was further revised in the late 1990s, leading to the publication of ISO/IEC 9899:1999 in 1999, which is commonly referred to as “C99“. It has since been amended three times by Technical Corrigenda.[19]

C99 introduced several new features, including inline functions, several new data types (including long long int and a complex type to represent complex numbers), variable-length arrays and flexible array members, improved support for IEEE 754 floating point, support for variadic macros (macros of variable arity), and support for one-line comments beginning with //, as in BCPL or C++. Many of these had already been implemented as extensions in several C compilers.

C99 is for the most part backward compatible with C90, but is stricter in some ways; in particular, a declaration that lacks a type specifier no longer has int implicitly assumed. A standard macro __STDC_VERSION__ is defined with value 199901L to indicate that C99 support is available. GCCSolaris Studio, and other C compilers now support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.[20]

C11

In 2007, work began on another revision of the C standard, informally called “C1X” until its official publication on 2011-12-08. The C standards committee adopted guidelines to limit the adoption of new features that had not been tested by existing implementations.

The C11 standard adds numerous new features to C and the library, including type generic macros, anonymous structures, improved Unicode support, atomic operations, multi-threading, and bounds-checked functions. It also makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is defined as 201112L to indicate that C11 support is available.

Embedded C

Historically, embedded C programming requires nonstandard extensions to the C language in order to support exotic features such as fixed-point arithmetic, multiple distinct memory banks, and basic I/O operations.

In 2008, the C Standards Committee published a technical report extending the C language[21] to address these issues by providing a common standard for all implementations to adhere to. It includes a number of features not available in normal C, such as fixed-point arithmetic, named address spaces, and basic I/O hardware addressing.

Syntax

C has a formal grammar specified by the C standard.[22] Line endings are generally not significant in C; however, line boundaries do have significance during the preprocessing phase. Comments may appear either between the delimiters /* and */, or (since C99) following // until the end of the line. Comments delimited by /* and */ do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear inside string or character literals.[23]

C source files contain declarations and function definitions. Function definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as structunion, and enum, or assign types to and perhaps reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes called “curly brackets”) to limit the scope of declarations and to act as a single statement for control structures.

As an imperative language, C uses statements to specify actions. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming is supported by if(-else) conditional execution and by dowhilewhile, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression.

Expressions can use a variety of built-in operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the next “sequence point“; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&||?: and the comma operator). This permits a high degree of object code optimization by the compiler, but requires C programmers to take more care to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Language: “C, like any other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better.”[24] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Character set

The basic C source character set includes the following characters:

Newline indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it as one.

Additional multi-byte encoded characters may be used in string literals, but they are not entirely portable. The latest C standard (C11) allows multi-national Unicode characters to be embedded portably within C source text by using \uXXXX or \UXXXXXXXX encoding (where the X denotes a hexadecimal character), although this feature is not yet widely implemented.

The basic C execution character set contains the same characters, along with representations for alertbackspace, and carriage returnRun-time support for extended character sets has increased with each revision of the C standard.

Reserved words

C89 has 32 reserved words, also known as keywords, which are the words that cannot be used for any purposes other than those for which they are predefined:

C99 reserved five more words:

C11 reserved seven more words:[25]

Most of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. The language previously included a reserved word called entry, but this was seldom implemented, and has now been removed as a reserved word.[26]

Operators

C supports a rich set of operators, which are symbols used within an expression to specify the manipulations to be performed while evaluating that expression. C has operators for:

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (assignment and equality) may result in the accidental use of one in place of the other, and in many cases, the mistake does not produce an error message (although some compilers produce warnings). For example, the conditional expression if(a==b+1) might mistakenly be written as if(a=b+1), which will be evaluated as true if a is not zero after the assignment.[27]

The C operator precedence is not always intuitive. For example, the operator == binds more tightly than (is executed prior to) the operators & (bitwise AND) and | (bitwise OR) in expressions such as x & 1 == 0, which must be written as (x & 1) == 0 if that is the coder’s intent.[28]

“Hello, world” example

The “hello, world” example, which appeared in the first edition of K&R, has become the model for an introductory program in most programming textbooks, regardless of programming language. The program prints “hello, world” to the standard output, which is usually a terminal or screen display.

The original version was:[29]

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

A standard-conforming “hello, world” program is:[a]

#include <stdio.h>

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

The first line of the program contains a preprocessing directive, indicated by #include. This causes the compiler to replace that line with the entire text of the stdio.h standard header, which contains declarations for standard input and output functions such as printf. The angle brackets surrounding stdio.h indicate that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same name, as opposed to double quotes which typically include local or project-specific header files.

The next line indicates that a function named main is being defined. The main function serves a special purpose in C programs; the run-time environment calls the main function to begin program execution. The type specifier int indicates that the value that is returned to the invoker (in this case the run-time environment) as a result of evaluating the mainfunction, is an integer. The keyword void as a parameter list indicates that this function takes no arguments.[b]

The opening curly brace indicates the beginning of the definition of the main function.

The next line calls (diverts execution to) a function named printf, which in this case is supplied from a system library. In this call, the printf function is passed (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamed array with elements of type char, set up automatically by the compiler with a final 0-valued character to mark the end of the array (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The return value of the printf function is of type int, but it is silently discarded since it is not used. (A more careful program might test the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the end of the code for the main function. According to the C99 specification and newer, the main function, unlike any other function, will implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit return 0; statement was required.) This is interpreted by the run-time system as an exit code indicating successful execution.[30]

Data types

C is often used in low-level systems programming where escapes from the type system may be necessary. The compiler attempts to ensure type correctness of most expressions, but the programmer can override the checks in various ways, either by using a type cast to explicitly convert a value from one type to another, or by using pointers or unions to reinterpret the underlying bits of a data object in some other way.The type system in C is static and weakly typed, which makes it similar to the type system of ALGOL descendants such as Pascal.[31] There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, and enumerated types (enum). Integer type char is often used for single-byte characters. C99 added a boolean datatype. There are also derived types including arrayspointersrecords (struct), and unions (union).

Some find C’s declaration syntax unintuitive, particularly for function pointers. (Ritchie’s idea was to declare identifiers in contexts resembling their use: “declaration reflects use“.)[32]

C’s usual arithmetic conversions allow for efficient code to be generated, but can sometimes produce unexpected results. For example, a comparison of signed and unsigned integers of equal width requires a conversion of the signed value to unsigned. This can generate unexpected results if the signed value is negative.

Pointers

C supports the use of pointers, a type of reference that records the address or location of an object or function in memory. Pointers can be dereferenced to access data stored at the address pointed to, or to invoke a pointed-to function. Pointers can be manipulated using assignment or pointer arithmetic. The run-time representation of a pointer value is typically a raw memory address (perhaps augmented by an offset-within-word field), but since a pointer’s type includes the type of the thing pointed to, expressions including pointers can be type-checked at compile time. Pointer arithmetic is automatically scaled by the size of the pointed-to data type. Pointers are used for many purposes in C. Text strings are commonly manipulated using pointers into arrays of characters. Dynamic memory allocation is performed using pointers. Many data types, such as trees, are commonly implemented as dynamically allocated struct objects linked together using pointers. Pointers to functions are useful for passing functions as arguments to higher-order functions (such as qsort or bsearch) or as callbacks to be invoked by event handlers.[30]

null pointer value explicitly points to no valid location. Dereferencing a null pointer value is undefined, often resulting in a segmentation fault. Null pointer values are useful for indicating special cases such as no “next” pointer in the final node of a linked list, or as an error indication from functions returning pointers. In appropriate contexts in source code, such as for assigning to a pointer variable, a null pointer constant can be written as 0, with or without explicit casting to a pointer type, or as the NULL macro defined by several standard headers. In conditional contexts, null pointer values evaluate to false, while all other pointer values evaluate to true.

Void pointers (void *) point to objects of unspecified type, and can therefore be used as “generic” data pointers. Since the size and type of the pointed-to object is not known, void pointers cannot be dereferenced, nor is pointer arithmetic on them allowed, although they can easily be (and in many contexts implicitly are) converted to and from any other object pointer type.[30]

Careless use of pointers is potentially dangerous. Because they are typically unchecked, a pointer variable can be made to point to any arbitrary location, which can cause undesirable effects. Although properly used pointers point to safe places, they can be made to point to unsafe places by using invalid pointer arithmetic; the objects they point to may continue to be used after deallocation (dangling pointers); they may be used without having been initialized (wild pointers); or they may be directly assigned an unsafe value using a cast, union, or through another corrupt pointer. In general, C is permissive in allowing manipulation of and conversion between pointer types, although compilers typically provide options for various levels of checking. Some other programming languages address these problems by using more restrictive reference types.

Arrays

Array types in C are traditionally of a fixed, static size specified at compile time. (The more recent C99 standard also allows a form of variable-length arrays.) However, it is also possible to allocate a block of memory (of arbitrary size) at run-time, using the standard library’s malloc function, and treat it as an array. C’s unification of arrays and pointers means that declared arrays and these dynamically allocated simulated arrays are virtually interchangeable.

Since arrays are always accessed (in effect) via pointers, array accesses are typically not checked against the underlying array size, although some compilers may provide bounds checking as an option.[33][34] Array bounds violations are therefore possible and rather common in carelessly written code, and can lead to various repercussions, including illegal memory accesses, corruption of data, buffer overruns, and run-time exceptions. If bounds checking is desired, it must be done manually.

C does not have a special provision for declaring multi-dimensional arrays, but rather relies on recursion within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting “multi-dimensional array” can be thought of as increasing in row-major order.

Multi-dimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices. The structure of the C array is well suited to this particular task. However, since arrays are passed merely as pointers, the bounds of the array must be known fixed values or else explicitly passed to any subroutine that requires them, and dynamically sized arrays of arrays cannot be accessed using double indexing. (A workaround for this is to allocate the array with an additional “row vector” of pointers to the columns.)

C99 introduced “variable-length arrays” which address some, but not all, of the issues with ordinary C arrays.

Array–pointer interchangeability

The subscript notation x[i] (where x designates a pointer) is syntactic sugar for *(x+i).[35] Taking advantage of the compiler’s knowledge of the pointer type, the address that x + i points to is not the base address (pointed to by x) incremented by i bytes, but rather is defined to be the base address incremented by i multiplied by the size of an element that x points to. Thus, x[i] designates the i+1th element of the array.

Furthermore, in most expression contexts (a notable exception is as operand of sizeof), the name of an array is automatically converted to a pointer to the array’s first element. This implies that an array is never copied as a whole when named as an argument to a function, but rather only the address of its first element is passed. Therefore, although function calls in C use pass-by-value semantics, arrays are in effect passed by reference.

The size of an element can be determined by applying the operator sizeof to any dereferenced element of x, as in n = sizeof *x or n = sizeof x[0], and the number of elements in a declared array A can be determined as sizeof A / sizeof A[0]. The latter only applies to array names: variables declared with subscripts (int A[20]). Due to the semantics of C, it is not possible to determine the entire size of arrays through pointers to arrays or those created by dynamic allocation (malloc); code such as sizeof arr / sizeof arr[0] (where arr designates a pointer) will not work since the compiler assumes the size of the pointer itself is being requested.[36][37] Since array name arguments to sizeof are not converted to pointers, they do not exhibit such ambiguity. However, arrays created by dynamic allocation are accessed by pointers rather than true array variables, so they suffer from the same sizeof issues as array pointers.

Thus, despite this apparent equivalence between array and pointer variables, there is still a distinction to be made between them. Even though the name of an array is, in most expression contexts, converted into a pointer (to its first element), this pointer does not itself occupy any storage; the array name is not an l-value, and its address is a constant, unlike a pointer variable. Consequently, what an array “points to” cannot be changed, and it is impossible to assign a new address to an array name. Array contents may be copied, however, by using the memcpy function, or by accessing the individual elements.

Memory management

One of the most important functions of a programming language is to provide facilities for managing memory and the objects that are stored in memory. C provides three distinct ways to allocate memory for objects:[30]

  • Static memory allocation: space for the object is provided in the binary at compile-time; these objects have an extent (or lifetime) as long as the binary which contains them is loaded into memory.
  • Automatic memory allocation: temporary objects can be stored on the stack, and this space is automatically freed and reusable after the block in which they are declared is exited.
  • Dynamic memory allocation: blocks of memory of arbitrary size can be requested at run-time using library functions such as malloc from a region of memory called the heap; these blocks persist until subsequently freed for reuse by calling the library function realloc or free

These three approaches are appropriate in different situations and have various trade-offs. For example, static memory allocation has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across function calls, automatic allocation is easy to use but stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows convenient allocation of objects whose size is known only at run-time. Most C programs make extensive use of all three.

Where possible, automatic or static allocation is usually simplest because the storage is managed by the compiler, freeing the programmer of the potentially error-prone chore of manually allocating and releasing storage. However, many data structures can change in size at runtime, and since static allocations (and automatic allocations before C99) must have a fixed size at compile-time, there are many situations in which dynamic allocation is necessary.[30] Prior to the C99 standard, variable-sized arrays were a common example of this. (See the article on malloc for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the form of a null pointer value) when the required storage cannot be allocated. (Static allocation that is too large is usually detected by the linker or loader, before the program can even begin execution.)

Unless otherwise specified, static objects contain zero or null pointer values upon program startup. Automatically and dynamically allocated objects are initialized only if an initial value is explicitly specified; otherwise they initially have indeterminate values (typically, whatever bit pattern happens to be present in the storage, which might not even represent a valid value for that type). If the program attempts to access an uninitialized value, the results are undefined. Many modern compilers try to detect and warn about this problem, but both false positives and false negatives can occur.

Another issue is that heap memory allocation has to be synchronized with its actual usage in any program in order for it to be reused as much as possible. For example, if the only pointer to a heap memory allocation goes out of scope or has its value overwritten before free() is called, then that memory cannot be recovered for later reuse and is essentially lost to the program, a phenomenon known as a memory leak. Conversely, it is possible for memory to be freed but continue to be referenced, leading to unpredictable results. Typically, the symptoms will appear in a portion of the program far removed from the actual error, making it difficult to track down the problem. (Such issues are ameliorated in languages with automatic garbage collection.)

Libraries

The C programming language uses libraries as its primary method of extension. In C, a library is a set of functions contained within a single “archive” file. Each library typically has a header file, which contains the prototypes of the functions contained within the library that may be used by a program, and declarations of special data types and macro symbols used with these functions. In order for a program to use a library, it must include the library’s header file, and the library must be linked with the program, which in many cases requires compiler flags (e.g., -lm, shorthand for “link the math library”).[30]

The most common C library is the C standard library, which is specified by the ISO and ANSI C standards and comes with every C implementation (implementations which target limited environments such as embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and time values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

Another common set of C library functions are those used by applications specifically targeted for Unix and Unix-like systems, especially functions which provide an interface to the kernel. These functions are detailed in various standards such as POSIX and the Single UNIX Specification.

Since many programs have been written in C, there are a wide variety of other libraries available. Libraries are often written in C because C compilers generate efficient object code; programmers then create interfaces to the library so that the routines can be used from higher-level languages like JavaPerl, and Python.[30]

Language tools

Automated source code checking and auditing are beneficial in any language, and for C many such tools exist, such as Lint. A common practice is to use Lint to detect questionable code when a program is first written. Once a program passes Lint, it is then compiled using the C compiler. Also, many compilers can optionally warn about syntactically valid constructs that are likely to actually be errors. MISRA C is a proprietary set of guidelines to avoid such questionable code, developed for embedded systems.[38]A number of tools have been developed to help C programmers find and fix statements with undefined behavior or possibly erroneous expressions, with greater rigor than that provided by the compiler. The tool lint was the first such, leading to many others.

There are also compilers, libraries, and operating system level mechanisms for performing actions that are not a standard part of C, such as bounds checking for arrays, detection of buffer overflowserializationdynamic memory tracking, and automatic garbage collection.

Tools such as Purify or Valgrind and linking with libraries containing special versions of the memory allocation functions can help uncover runtime errors in memory usage.

Uses

The TIOBE index graph, showing a comparison of the popularity of various programming languages[39]

C is widely used for system programming in implementing operating systems and embedded systemapplications,[40] because C code, when written for portability, can be used for most purposes, yet when needed, system-specific code can be used to access specific hardware addresses and to perform type punning to match externally imposed interface requirements, with a low run-time demand on system resources.

C can also be used for website programming using CGI as a “gateway” for information between the Web application, the server, and the browser.[41] C is often chosen over interpreted languages because of its speed, stability, and near-universal availability.[42]

One consequence of C’s wide availability and efficiency is that compilers, libraries and interpreters of other programming languages are often implemented in C. The reference implementations of PythonPerl and PHP, for example, are all written in C.

Because the layer of abstraction is thin and the overhead is low, C enables programmers to create efficient implementations of algorithms and data structures, useful for computationally intense programs. For example, the GNU Multiple Precision Arithmetic Library, the GNU Scientific LibraryMathematica, and MATLAB are completely or partially written in C.

C is sometimes used as an intermediate language by implementations of other languages. This approach may be used for portability or convenience; by using C as an intermediate language, additional machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that support compilation of generated code. However, some of C’s shortcomings have prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C–.

C has also been widely used to implement end-user applications. However, such applications can also be written in newer, higher-level languages.

Related languages

C has both directly and indirectly influenced many later languages such as C#DGoJavaJavaScriptLimboLPCPerlPHPPython, and Unix’s C shell.[43] The most pervasive influence has been syntactical, all of the languages mentioned combine the statement and (more or less recognizably) expression syntax of C with type systems, data models and/or large-scale program structures that differ from those of C, sometimes radically.

Several C or near-C interpreters exist, including Ch and CINT, which can also be used for scripting.

When object-oriented languages became popular, C++ and Objective-C were two different extensions of C that provided object-oriented capabilities. Both languages were originally implemented as source-to-source compilers; source code was translated into C, and then compiled with a C compiler.[44]

The C++ programming language was devised by Bjarne Stroustrup as an approach to providing object-oriented functionality with a C-like syntax.[45] C++ adds greater typing strength, scoping, and other tools useful in object-oriented programming, and permits generic programming via templates. Nearly a superset of C, C++ now supports most of C, with a few exceptions.

Objective-C was originally a very “thin” layer on top of C, and remains a strict superset of C that permits object-oriented programming using a hybrid dynamic/static typing paradigm. Objective-C derives its syntax from both C and Smalltalk: syntax that involves preprocessing, expressions, function declarations, and function calls is inherited from C, while the syntax for object-oriented features was originally taken from Smalltalk.

In addition to C++ and Objective-CChCilk and Unified Parallel C are nearly supersets of C.

Notes

  1. Jump up^ The original example code will compile on most modern compilers that are not in strict standard compliance mode, but it does not fully conform to the requirements of either C89 or C99. In fact, C99 requires that a diagnostic message be produced.
  2. Jump up^ The main function actually has two arguments, int argc and char *argv[], respectively, which can be used to handle command line arguments. The ISO C standard (section 5.1.2.2.1) requires both forms of main to be supported, which is special treatment not afforded to any other function.

References

  1. Jump up to:a b c d e Kernighan, Brian W.Ritchie, Dennis M. (February 1978). The C Programming Language (1st ed.). Englewood Cliffs, NJPrentice HallISBN 0-13-110163-3. Regarded by many to be the authoritative reference on C.
  2. Jump up^ Ritchie (1993): “Thompson had made a brief attempt to produce a system coded in an early version of C—before structures—in 1972, but gave up the effort.”
  3. Jump up^ Ritchie (1993): “The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol’s adherents would approve of.”
  4. Jump up^ Ring Team (5 December 2017). “Ring language and other languages”ring-lang.netring-lang.
  5. Jump up to:a b “Verilog HDL (and C)” (PDF). The Research School of Computer Science at the Australian National University. 2010-06-03. Archived from the original (PDF) on 2013-11-06. Retrieved 2013-08-191980s: ; Verilog first introduced ; Verilog inspired by the C programming language
  6. Jump up^ Ritchie (1993)
  7. Jump up^ Lawlis, Patricia K. (August 1997). “Guidelines for Choosing a Computer Language: Support for the Visionary Organization”. Ada Information Clearinghouse. Retrieved 18 July 2006.
  8. Jump up^ “Programming Language Popularity”. 2009. Archived from the original on 13 December 2007. Retrieved 16 January 2009.
  9. Jump up^ “TIOBE Programming Community Index”. 2009. Retrieved 6 May 2009.
  10. Jump up^ “History of C – cppreference.com”en.cppreference.com.
  11. Jump up^ Ritchie, Dennis M. (March 1993). “The Development of the C Language”ACM SIGPLAN Notices28 (3): 201–208. doi:10.1145/155360.155580.
  12. Jump up^ Ulf Bilting & Jan Skansholm “Vägen till C” (Swedish) meaning “The Road to C”, third edition, Studentlitteratur, year 2000, page 3. ISBN 91-44-01468-6.
  13. Jump up to:a b c Johnson, S. C.Ritchie, D. M. (1978). “Portability of C Programs and the UNIX System”Bell System Tech. J57 (6): 2021–2048. doi:10.1002/j.1538-7305.1978.tb02141.x. Retrieved 16 December 2012. (Note: this reference is an OCR scan of the original, and contains an OCR glitch rendering “IBM 370” as “IBM 310”.)
  14. Jump up^ McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer’s Manual, 1971–1986 (PDF) (Technical report). CSTR. Bell Labs. p. 10. 139.
  15. Jump up^ Stallings, William. “Operating Systems: Internals and Design Principles” 5th ed, page 91. Pearson Education, Inc. 2005.
  16. Jump up to:a b Kernighan, Brian W.Ritchie, Dennis M. (March 1988). The C Programming Language(2nd ed.). Englewood Cliffs, NJPrentice HallISBN 0-13-110362-8.
  17. Jump up^ Stroustrup, Bjarne (2002). Sibling rivalry: C and C++ (PDF) (Report). AT&T Labs.
  18. Jump up^ C Integrity. International Organization for Standardization. 1995-03-30.
  19. Jump up^ “JTC1/SC22/WG14 – C”Home page. ISO/IEC. Retrieved 2 June 2011.
  20. Jump up^ Andrew Binstock (October 12, 2011). “Interview with Herb Sutter”Dr. Dobbs. Retrieved September 7, 2013.
  21. Jump up^ “TR 18037: Embedded C” (PDF). ISO / IEC. Retrieved 26 July 2011.
  22. Jump up^ Harbison, Samuel P.; Steele, Guy L. (2002). C: A Reference Manual (5th ed.). Englewood Cliffs, NJPrentice HallISBN 0-13-089592-X. Contains a BNF grammar for C.
  23. Jump up^ Kernighan, Brian W.Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. p. 192. ISBN 7 302 02412 X.
  24. Jump up^ Page 3 of the original K&R[1]
  25. Jump up^ ISO/IEC 9899:201x (ISO C11) Committee Draft
  26. Jump up^ Kernighan, Brian W.Ritchie, Dennis M. (1996). The C Programming Language (2nd ed.). Prentice Hall. pp. 192, 259. ISBN 7 302 02412 X.
  27. Jump up^ “10 Common Programming Mistakes in C++”. Cs.ucr.edu. Retrieved 26 June 2009.
  28. Jump up^ Schultz, Thomas (2004). C and the 8051 (3rd ed.). Otsego, MI: PageFree Publishing Inc. p. 20. ISBN 1-58961-237-X. Retrieved 10 February 2012.
  29. Jump up^ Page 6 of the original K&R[1]
  30. Jump up to:a b c d e f g Klemens, Ben (2013). 21st Century CO’Reilly MediaISBN 1-4493-2714-1.
  31. Jump up^ Feuer, Alan R.; Gehani, Narain H. (March 1982). “Comparison of the Programming Languages C and Pascal”. ACM Computing Surveys14 (1): 73–92. doi:10.1145/356869.356872. (Subscription required (help)).
  32. Jump up^ Page 122 of K&R2[16]
  33. Jump up^ For example, gcc provides _FORTIFY_SOURCE. “Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE)”. fedoraproject.org. Retrieved 2012-08-05.
  34. Jump up^ เอี่ยมสิริวงศ์, โอภาศ (2016). Programming with C. Bangkok, Thailand: SE-EDUCATION PUBLIC COMPANY LIMITED. pp. 225–230. ISBN 978-616-08-2740-4.
  35. Jump up^ Raymond, Eric S. (11 October 1996). The New Hacker’s Dictionary (3rd ed.). MIT Press. p. 432. ISBN 978-0-262-68092-9. Retrieved 5 August 2012.
  36. Jump up^ Summit, Steve. “comp.lang.c Frequently Asked Questions 6.23”. Retrieved March 6,2013.
  37. Jump up^ Summit, Steve. “comp.lang.c Frequently Asked Questions 7.28”. Retrieved March 6,2013.
  38. Jump up^ “Man Page for lint (freebsd Section 1)”unix.com. 2001-05-24. Retrieved 2014-07-15.
  39. Jump up^ McMillan, Robert (2013-08-01). “Is Java Losing Its Mojo?”Wired.
  40. Jump up^ Chip., Weems, (2014). Programming and problem solving with C++ : brief, sixth edition. Jones & Bartlett Learning. ISBN 1449694284OCLC 894992484.
  41. Jump up^ Dr. Dobb’s Sourcebook. U.S.A.: Miller Freeman, Inc. November–December 1995.
  42. Jump up^ “Using C for CGI Programming”. linuxjournal.com. 1 March 2005. Retrieved 4 January2010.
  43. Jump up^ Gerard),, O’Regan, Gerard (Cornelius. Pillars of computing : a compendium of select, pivotal technology firmsISBN 3319214640OCLC 922324121.
  44. Jump up^ Lawrence., Rauchwerger, (2004). Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, USA, October 2-4, 2003 : revised papers. Springer. ISBN 3540246444OCLC 57965544.
  45. Jump up^ Stroustrup, Bjarne (1993). “A History of C++: 1979−1991” (PDF). Retrieved 9 June
Posted in C programming Language

Syntax of C

Syntax

C has a formal grammar specified by the C standard. Line endings are generally not significant in C; however, line boundaries do have significance during the preprocessing phase. Comments may appear either between the delimiters /* and */, or (since C99) following // until the end of the line. Comments delimited by /* and */ do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear inside string or character literals.

C source files contain declarations and function definitions. Function definitions, in turn, contain declarations and statements. Declarations either define new types using keywords such as structunion, and enum, or assign types to and perhaps reserve storage for new variables, usually by writing the type followed by the variable name. Keywords such as char and int specify built-in types. Sections of code are enclosed in braces ({ and }, sometimes called “curly brackets”) to limit the scope of declarations and to act as a single statement for control structures.

As an imperative language, C uses statements to specify actions. The most common statement is an expression statement, consisting of an expression to be evaluated, followed by a semicolon; as a side effect of the evaluation, functions may be called and variables may be assigned new values. To modify the normal sequential execution of statements, C provides several control-flow statements identified by reserved keywords. Structured programming is supported by if(-else) conditional execution and by dowhilewhile, and for iterative execution (looping). The for statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. break and continue can be used to leave the innermost enclosing loop statement or skip to its reinitialization. There is also a non-structured goto statement which branches directly to the designated label within the function. switch selects a case to be executed based on the value of an integer expression.

Expressions can use a variety of built-in operators and may contain function calls. The order in which arguments to functions and operands to most operators are evaluated is unspecified. The evaluations may even be interleaved. However, all side effects (including storage to variables) will occur before the next “sequence point”; sequence points include the end of each expression statement, and the entry to and return from each function call. Sequence points also occur during evaluation of expressions containing certain operators (&&||?: and the comma operator). This permits a high degree of object code optimization by the compiler, but requires C programmers to take more care to obtain reliable results than is needed for other programming languages.

Kernighan and Ritchie say in the Introduction of The C Programming Language: “C, like any other language, has its blemishes. Some of the operators have the wrong precedence; some parts of the syntax could be better.”[24] The C standard did not attempt to correct many of these blemishes, because of the impact of such changes on already existing software.

Character set

The basic C source character set includes the following characters:

  • Lowercase and uppercase letters of ISO Basic Latin Alphabet: az AZ
  • Decimal digits: 09
  • Graphic characters: ! " # % & ' ( ) * + , - . / : ; < = > ? [ \ ] ^ _ { | } ~
  • Whitespace characters: spacehorizontal tabvertical tabform feednewline

Newline indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it as one.

Additional multi-byte encoded characters may be used in string literals, but they are not entirely portable. The latest C standard (C11) allows multi-national Unicode characters to be embedded portably within C source text by using \uXXXX or \UXXXXXXXX encoding (where the X denotes a hexadecimal character), although this feature is not yet widely implemented.

The basic C execution character set contains the same characters, along with representations for alert, backspace, and carriage return. Run-time support for extended character sets has increased with each revision of the C standard.

Reserved words

C89 has 32 reserved words, also known as keywords, which are the words that cannot be used for any purposes other than those for which they are predefined:

C99 reserved five more words:

C11 reserved seven more words:[25]

Most of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. The language previously included a reserved word called entry, but this was seldom implemented, and has now been removed as a reserved word.

Operators

C supports a rich set of operators, which are symbols used within an expression to specify the manipulations to be performed while evaluating that expression. C has operators for:

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between these two operators (assignment and equality) may result in the accidental use of one in place of the other, and in many cases, the mistake does not produce an error message (although some compilers produce warnings). For example, the conditional expression if(a==b+1) might mistakenly be written as if(a=b+1), which will be evaluated as true if a is not zero after the assignment.[27]

The C operator precedence is not always intuitive. For example, the operator == binds more tightly than (is executed prior to) the operators & (bitwise AND) and | (bitwise OR) in expressions such as x & 1 == 0, which must be written as (x & 1) == 0 if that is the coder’s intent.

“Hello, world” example

The “hello, world” example, which appeared in the first edition of K&R, has become the model for an introductory program in most programming textbooks, regardless of programming language. The program prints “hello, world” to the standard output, which is usually a terminal or screen display.

The original version was:

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

A standard-conforming “hello, world” program is:

#include <stdio.h>

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

The first line of the program contains a preprocessing directive, indicated by #include. This causes the compiler to replace that line with the entire text of the stdio.h standard header, which contains declarations for standard input and output functions such as printf. The angle brackets surrounding stdio.h indicate that stdio.h is located using a search strategy that prefers headers provided with the compiler to other headers having the same name, as opposed to double quotes which typically include local or project-specific header files.

The next line indicates that a function named main is being defined. The main function serves a special purpose in C programs; the run-time environment calls the main function to begin program execution. The type specifier int indicates that the value that is returned to the invoker (in this case the run-time environment) as a result of evaluating the mainfunction, is an integer. The keyword void as a parameter list indicates that this function takes no arguments.[b]

The opening curly brace indicates the beginning of the definition of the main function.

The next line calls (diverts execution to) a function named printf, which in this case is supplied from a system library. In this call, the printf function is passed (provided with) a single argument, the address of the first character in the string literal "hello, world\n". The string literal is an unnamed array with elements of type char, set up automatically by the compiler with a final 0-valued character to mark the end of the array (printf needs to know this). The \n is an escape sequence that C translates to a newline character, which on output signifies the end of the current line. The return value of the printf function is of type int, but it is silently discarded since it is not used. (A more careful program might test the return value to determine whether or not the printf function succeeded.) The semicolon ; terminates the statement.

The closing curly brace indicates the end of the code for the main function. According to the C99 specification and newer, the main function, unlike any other function, will implicitly return a value of 0 upon reaching the } that terminates the function. (Formerly an explicit return 0; statement was required.) This is interpreted by the run-time system as an exit code indicating successful execution.[30]

Posted in C programming Language

Topics in C language

 

C language MCA

Posted in C programming Language

C programming language

  1. C Language Introduction

C is a procedural programming language. It was initially developed by Dennis Ritchie between 1969 and 1973. It was mainly developed as a system programming language to write operating system. The main features of C language include low-level access to memory, simple set of keywords, and clean style, these features make C language suitable for system programming like operating system or compiler development.
Many later languages have borrowed syntax/features directly or indirectly from C language. Like syntax of Java, PHP, JavaScript and many other languages is mainly based on C language. C++ is nearly a superset of C language (There are few programs that may compile in C, but not in C++).

Beginning with C programming:

1) Finding a Compiler:
Before we start C programming, we need to have a compiler to compile and run our programs. There are certain online compilers.

Windows: There are many compilers available freely for compilation of C programs like Code Blocks  and Dev-CPP.   We strongly recommend Code Blocks.

Linux: For Linux, gcc comes bundled with the linux,  Code Blocks can also be used with Linux.

2) Writing first program:
Following is first program in C

 #include <stdio.h>
int main(void)
{
printf(“This is my first testing”);
return 0;
}

Let us analyze the program line by line.
Line 1: [ #include <stdio.h> ] In a C program, all lines that start with are processed by preprocessor which is a program invoked by the compiler. In a very basic term, preprocessor takes a C program and produces another C program. The produced program has no lines starting with #, all such lines are processed by the preprocessor. In the above example, preprocessor copies the preprocessed code of stdio.h to our file. The .h files are called header files in C. These header files generally contain declaration of functions. We need stdio.h for the function printf() used in the program.

Line 2 [ int main(void) ] There must to be starting point from where execution of compiled C program begins. In C, the execution typically begins with first line of main(). The void written in brackets indicates that the main doesn’t take any parameter. main() can be written to take parameters also.
The int written before main indicates return type of main(). The value returned by main indicates status of program termination.

Line 3 and 6: [ { and } ] In C language, a pair of curly brackets define a scope and mainly used in functions and control statements like if, else, loops. All functions must start and end with curly brackets.

Line 4 [ printf(“online_study”); ] printf() is a standard library function to print something on standard output. The semicolon at the end of printf indicates line termination. In C, semicolon is always used to indicate end of statement.

Line 5 [ return 0; ] The return statement returns the value from main(). The returned value may be used by operating system to know termination status of your program. The value 0 typically means successful termination.

 

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