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C programming language

C is a programming language developed by Dennis Ritchie, in the early 1970s, for use on the UNIX operating system. It is now used on practically every operating system, and is the most popular language for writing system software, though it is also used for writing applications. It is also commonly used in computer science education. The popular C++ programming language is based on C.

Table of contents
1 Features
2 History
3 "Hello, World!" in C
4 Anatomy of a C Program
5 Undefined behaviors
6 Links and references

Features

C is a relatively minimalist programming language. It is significantly lower-level than most other programming languages. Even though it is sometimes referred to as a "high level language", it is only really higher-level than the various assembly languages.

C has two important advantages over assembly. Firstly, code is generally easier to read and much less burdensome to write, especially for lengthy programs. Secondly, assembly code is usually applicable only to a specific computer architecture, whereas a C program can be ported to any architecture on which a C compiler and certain required libraries exist. (C code is almost always compiled, rather than interpreted.) On the other hand, the efficiency of C code is somewhat dependent on the ability of the compiler to optimize the resulting machine language, which is largely out of the programmer's control. In contrast, the efficiency of assembly code is precisely determined, since assembly is just human-readable notation for a machine language. For this reason, programs such as operating system kernelss, though mostly written in C, may contain "hand-tuned" fragments of assembly language where performance is especially crucial.

Similar advantages and disadvantages distinguish C from higher-level languages: the efficiency of C code can be more closely controlled, at the cost of being generally more troublesome to read and write. Note, however, that C is at least as portable as higher-level languages, because nowadays most computer architectures are equipped with a C compiler and libraries; in fact, the compilers, libraries, and interpreters of higher-level languages are often implemented in C!

One of the most notable features of C is that it is up to the programmer to manage the contents of computer memory. Standard C provides no facilities for array bounds checking or automatic garbage collection. In contrast, the Java and C# languages, both descendants of C, provide automatic memory management, including garbage collection. While manual memory management provides the programmer with greater leeway in tuning the performance of a program, it also makes it easy to produce bugs involving erroneous memory operations, such as buffer overflows. Bugs of these sort have gained notoriety for their effects on computer insecurity. Some tools have been created to help C programmers avoid memory errors, including libraries for performing array bounds checking and automatic garbage collection, and automated source code checkers such as Lint.

Some of the specific features of C are:

History

Early developments

The initial development of C occurred at AT&T Bell Labs between 1969 and 1973; according to Ritchie, the most creative period occurred in 1972. It was named "C" because many of its features were derived from an earlier language called "B". Accounts differ regarding the origins of the name "B": Ken Thompson credits the BCPL programming language, but he had also created a language called Bon in honor of his wife Bonnie.

By 1973, the C language had become powerful enough that most of the UNIX kernel, originally written in PDP-11/20 assembly language, was rewritten in C. This was one of the first operating system kernels implemented in a language other than assembly, earlier instances being the Multics system (written in PL/I) and Tripos (written in BCPL.)

K&R C

In 1978, Ritchie and Brian Kernighan 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 the following features to the language:

K&R C is often considered the most basic part of the language that is necessary for a C compiler to support. For many years, even after the introduction of ANSI C, it was considered the "lowest common denominator" that C programmers stuck to when maximum portability was desired, since not all compilers were updated to fully support ANSI C, and reasonably well-written K&R C code is also legal ANSI C.

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

ANSI C and ISO C

During the late 1970s, C began to replace BASIC as the leading microcomputer programming language. During the 1980s, it was adopted for use with the IBM PC, and its popularity began to increase significantly. At the same time, Bjarne Stroustrup and others at Bell Labs began work on adding object-oriented programming language constructs to C. The language they produced, called C++, is now the most common application programming language on the Microsoft Windows operating system; C remains more popular in the Unix world.

In 1983, the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. After a long and arduous process, the standard was completed in 1989 (one year after the first ANSI standard for C++!) and ratified as ANSI X3.159-1989 "Programming Language C". This version of the language is often referred to as ANSI C. In 1990, the ANSI C standard (with a few minor modifications) was adopted by the International Standards Organization (ISO) as ISO/IEC 9899:1990.

One of the aims of the ANSI C standardization process was to produce a superset of K&R C, incorporating many of the unofficial features subsequently introduced. However, the standards committee also included several new features, such as function prototypes (borrowed from C++), and a more capable preprocessor.

ANSI C is now supported by almost all the widely used compilers. Most of the C code being written nowadays is based on ANSI C. Any program written only in standard C is guaranteed to perform correctly on any platform with a conforming C implementation. However, many programs have been written that will only compile on a certain platform, or with a certain compiler, due to (i) the use of non-standard libraries, e.g. for graphical displays, and (ii) some compilers not adhering to the ANSI C standard, or its successor, in their default mode.

C99

After the ANSI standardization process, the C language specification remained relatively static for some time, whereas C++ continued to evolve. (Normative Amendment 1 created a new version of the C language in 1995, but this version is rarely acknowledged.) However, the standard underwent revision in the late 1990s, leading to the publication of ISO 9899:1999 in 1999. This standard is commonly referred to as "C99". It was adopted as an ANSI standard in March 2000.

The new features in C99 include:

Interest in supporting the new C99 features appears to be mixed. Whereas GCC and several other compilers now support most of the new features of C99, the compilers maintained by Microsoft and Borland do not, and these two companies do not seem to be interested in adding such support.

"Hello, World!" in C

The following simple application prints out "Hello, World" to the standard output file (which is usually the screen, but might be a file or some other hardware device). A version of this program appeared for the first time in K&R.

\r\n#include \r\n\r\nint main(void)\r\n{\r\n    printf("Hello, World!\\n");\r\n    return 0;\r\n}\r\n

Anatomy of a C Program

A C program consists of functions and variables. C functions are like the subroutines and functions of Fortran or the procedures and functions of Pascal. The function main() is special in that a C program always begins executing at the beginning of this function. This means that every C program must have a main() function.

The main() function will usually call other functions to help perform its job. Functions may be written by the programmer, or provided by existing libraries; the latter are accessed by including "standard headers" via the #include preprocessing directive. Certain library functions, such as printf() in the above example, are defined by the C standards; these are referred to as the standard library. (An implementation of C providing all of the standard library functions is called a "hosted implementation"; some implementations are not hosted, usually because they are not intended to be used with an operating system.) Other libraries can provide extra functionality, such as a graphical interface, advanced mathematical operations, or access to platform-specific features.

A function may return a value to the environment which called it. This is usually another C function. The main() function's calling environment is the operating system. Hence, in the "Hello, world!" example above, the operating system receives a value of 0 when the program terminates. (The printf function above returns how many characters were printed -- in the case above, 14 -- but its value is effectively ignored.)

A C function consists of a return type (void if no value is returned), a unique name, a list of parameters in parentheses (void if there are none) and a function body delimited by braces. The syntax of the function body is equivalent to that of a compound statement.

Control structures

free-form

Note: bracing style varies from programmer to programmer and can be the subject of great debate ("religious wars"). See Indent style for more details.

Compound statements

Compound statements in C have the form

  {   }
and are used as the body of a function or anywhere that a single statement is expected.

Expression statements

A statement of the form

   ;
is an expression statement. If the expression is missing, the statement is called a null statement.

Selection statements

C has three types of selection statements: two kinds of if and the switch statement.

The two kinds of if statement are

  if ()
and

  if () 
     
  else
In the if statement, if the expression in parentheses is nonzero or true, control passes to the statement following the if. If the else clause is present, control will pass to the statement following the else clause if the expression in parentheses is zero or false. The two are disambiguated by matching an else to the next previous unmatched if at the same nesting level. Braces may be used to override this or for clarity.

The switch statement causes control to be transferred to one of several statements depending on the value of an expression, which must have integral type. The substatement controlled by a switch is typically compound. Any statement within the substatement may be labeled with one or more case labels, which consist of the keyword case followed by a constant expression and then a colon (:). No two of the case constants associated with the same switch may have the same value. There may be at most one default label associated with a switch; control passes to the default label if none of the case labels are equal to the expression in the parentheses following switch. Switches may be nested; a case or default label is associated with the smallest switch that contains it. Switch statements can "fall-through", that is, when one case section has completed its execution, statements will continue to be executed downward until a break statement is encountered. This may prove useful in certain circumstances, newer programming languages forbid case statements to "fall-through". In the below example, if is reached, the statements are executed and nothing more inside the braces. However if is reached, both and are executed since there is no break to separate the two case statements.

  switch () {
     case  :
        
     case  :
        
        break;
     default :
        
  }

Iteration statements

C has three forms of iteration statement:

  do 
     
  while ();

  while ()
     

  for ( ;  ; )
In the while and do statements, the substatement is executed repeatedly so long as the value of the expression remains nonzero or true. With while, the test, including all side effects from the expression, occurs before each execution of the statement; with do, the test follows each iteration.

If all three expressions are present in a for, the statement

  for (e1; e2; e3)
     s;
is equivalent to

  e1;
  while (e2) {
     s;
     e3;
  }
Any of the three expressions in the for loop may be omitted. A missing second expression makes the while test nonzero, creating an infinite loop.

Jump statements

Jump statements transfer control unconditionally. There are four types of jump statements in C: goto, continue, break, and return.

The goto statement looks like this:

  goto <identifier>;
The identifier must be a label located in the current function. Control transfers to the labeled statement.

A continue statement may appear only within an iteration statement and causes control to pass to the loop-continuation portion of the smallest enclosing such statement. That is, within each of the statements

  while (expression) {
     /* ... */
     cont: ;
  }

  do {
     /* ... */
     cont: ;
  } while (expression);

  for (optional-expr; optexp2; optexp3) {
     /* ... */
     cont: ;
  }
a continue not contained within a nested iteration statement is the same as goto cont.

The break statement is used to get out of a for loop, while loop, do loop, or switch statement. Control passes to the statement following the terminated statement.

A function returns to its caller by the return statement. When return is followed by an expression, the value is returned to the caller of the function. Flowing off the end of the function is equivalent to a return with no expression. In either case, the returned value is undefined.

Operator precedence in C89

     () [] -> . ++ -- (cast)     postfix operators
     ++ -- * & ~ ! + - sizeof    unary operators
     * / %                       multiplicative operators
     + -                         additive operators
     << >>                       shift operators
     <  <=  >  >=                relational operators
     == !=                       equality operators
     &                           bitwise and
     ^                           bitwise exclusive or
     |                           bitwise inclusive or
     &&                          logical and
     ||                          logical or
     ?:                          conditional operator
     = += -= *= /= %= <<= >>=
         &= |= ^=                assignment operators 
     ,                           comma operator

Data declaration

Elementary data types

The values in the <limits.h> and <float.h> headers determine the ranges of the fundamental data types. The ranges of the float, double, and long double types are typically those mentioned in the IEEE 754 Standard.

name minimum range
char -127..127 or 0..255
unsigned char 0..255
signed char -127..127
int -32767..32767
short int -32767..32767
long int -2147483647..2147483647
float 1e-37..1e+37 (positive range)
double 1e-37..1e+37 (positive range)
long double 1e-37..1e+37 (positive range)

Arrays

If a declaration is suffixed by a number in square brackets ([]), the declaration is said to be an array declaration. Strings are just character arrays. They are terminated by a character zero (represented in C by '\\0', the null character). Array bounds are not checked, and if a memory location beyond the array is written to, it may result in a segmentation fault.

Examples: 

   int myvector [100];
   char mystring [80]; 
   float mymatrix [3] [2] = {2.0 , 10.0, 20.0, 123.0, 1.0, 1.0}
   char lexicon  [10000] [300] ;  /* 10000 entries with max 300 chars each. */
   int a[3][4];
The last example above creates an array of arrays, but can be thought of as a multidimensional array for most purposes. The 12 int values created could be accessed as follows:

a[0][0] a[0][1] a[0][2] a[0][3]
a[1][0] a[1][1] a[1][2] a[1][3]
a[2][0] a[2][1] a[2][2] a[2][3]

Pointers

If a variable has an asterisk (*) in its declaration it is said to be a pointer.

Examples: 

   int *pi; /* pointer to int */
   int *api[3]; /* array of 3 pointers to int */
   char **argv; /* pointer to pointer to char */
The value at the address stored in a pointer variable can then be accessed in the program with an asterisk. For example, given the first example declaration above, *pi is an int. This is called "dereferencing" a pointer.

Another operator, the & (ampersand), called the address-of operator, returns the address of variable, array, or function. Thus, given the following

  int i, *pi; /* int and pointer to int */
  pi = &i;
i and *pi could be used interchangeably (at least until pi is set to something else).

Strings

Strings may be manipulated without using the standard library. However, the library contains many useful functions for working with both zero-terminated strings and unterminated arrays of char.

The most commonly used string functions are:

The less important string functions are:

File Input / Output

In C, input and output are performed via a group of functions in the standard library. In ANSI/ISO C, those functions are defined in the <stdio.h> header.

Standard I/O

Three standard I/O streams are predefined:

These streams are automatically opened and closed by the runtime environment, they need not and should not be opened explicitly.

The following example demonstrates how a filter program is typically structured:

\r\n#include \r\n\r\nint main()\r\n{\r\n   int c;\r\n\r\n   while (( c = getchar()) != EOF ) {\r\n         /* do various things \r\n            to the characters */\r\n\r\n          if (anErrorOccurs) {\r\n              fputs("an error eee occurred\\n", stderr);\r\n              break;\r\n          }\r\n\r\n         /* ... */\r\n         putchar(c);\r\n         /* ... */\r\n\r\n    }\r\n    return 0;\r\n}\r\n

Passing command line arguments

The parameters given on a command line are passed to a C program with two predefined variables - the count of the command line arguments in argc and the individual arguments as character arrays in the pointer array argv. So the command 

 myFilt p1 p2 p3
results in something like

(Note: there is no guarantee that the individual strings are contiguous.)

The individual values of the parameters may be accessed with argv[1], argv[2], and argv[3].

Undefined behaviors

An interesting (though certainly not unique) aspect of the C standards is that the behavior of certain code is said to be "undefined". In practice, this means that the program produced from this code can do anything, from (accidentally) working as intended to crashing every time it is run

For example, the following code produces undefined behavior, because the variable b is operated on more than once in the expression a = b + b++;:

\r\n#include \r\n\r\nint main (void)\r\n{\r\n  int a, b = 1;\r\n  a = b + b++;\r\n  printf ("%d\\n", a);\r\n  return 0;\r\n}\r\n

Links and references

See also:

External links

References

An early version of this article contained material from FOLDOC, used with permission.