C programming language
From Wikipedia, the free encyclopedia.
A C program that prints "Hello world!"
| Table of contents |
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2 History 3 "Hello, World!" in C 4 Relation to C++ 5 Programming tools 6 See also 7 References 8 External links |
Features
C is a relatively minimalist programming language and is more low-level than most others. It is often referred to as a low level language or "high level assembler". 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 significant differences from assembly. C code is generally easier to read and much less burdensome to write, especially for lengthy programs. Assembly code is usually applicable only to a specific computer architecture, whereas C code can generally be easily ported to any architecture on which a C compiler and any required libraries exist. 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. It should be noted, however, that for complex modern processors the machine code generated by C compilers is usually faster than hand-written assembly.
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. C is typically at least as portable as higher-level languages, because 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! Data storage in C is handled in 3 basic ways, by static memory allocation (essentially at compile time), by automatic allocation on the program stack, and by dynamic allocation through library calls from an area of memory called the heap. There is also a data type called a pointer that can hold a reference to allocated memory, for example, the memory that allocates a variable. A pointer is, roughly speaking, an abstraction of what assembly programmers call an index register. Pointers have been criticized because the simplicity of the implementation allows the introduction of malfunctions and vulnerabilities in C programs. The Java and C# languages, both descendants of C, use safer ways of referring to variables that make it much harder to write incorrect programs. These languages also have run-time systems that can detect most of the remaining problems. Run-time checking introduces overhead, but in almost all applications is worthwhile because of the greater guarantee of correct operation it provides. Pointer variables exist independently from the variables they refer to, leading to various wild pointer problems. The programmer is responsible for deallocation of dynamic memory, so it is easy to produce the kind of problem known as a memory leak, where dynamically allocated memory is never deallocated during the lifetime of the executing program. Some languages reduce this problem with automatic garbage collection. They do not completely prevent it because, for instance, allocations can not be garbage collected while static references exist to them.
In C, memory can be referenced by pointer arithmetic (adding or subtracting an integer to a pointer), but there is generally no check on whether the result is valid. Array elements in C are accessed using pointer arithmetic, so it is possible to refer to elements in an array that were never allocated. Languages that use run-time array bounds checking are protected from the buffer overflow vulnerabilities that result from allocating fixed size buffers on the stack. Tools have been created to help C programmers avoid memory errors, including libraries for performing array bounds checking and automatic garbage collection, but they are not a standard part of C. Automated source code checking and auditing is fruitful in any language, and for C many such tools exist, for example Lint.
Some of the specific features of C are:
- An extremely simple core language, with non-essential functionality provided by a standardized set of library routines.
- Focus on the procedural programming paradigm, with facilities for programming in a structured style.
- Use of a preprocessor language, the C preprocessor, for tasks such as defining macros and including multiple source code files.
- Low-level access to computer memory via the use of pointers. (Semantically incorrect operations are not prevented by the compiler.)
- Parameterss are always passed to functions by value, never by reference.
- Lexical variable scoping (but no support for closures or functions defined within functions).
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:
-
structdata types -
long intdata type -
unsigned intdata type - The
=+operator was changed to+=, and so forth (=+was confusing the C compiler's lexical analyzer).
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:
-
voidfunctions andvoid *data type - functions returning
structoruniontypes -
structfield names in a separate name space for each struct type - assignment for
structdata types -
constqualifier to make an object read-only - a standard library incorporating most of the functionality implemented by various vendors
- enumerations
- the single-precision
floattype
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 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:
- inline functions
- freeing of restrictions on the location of variable declarations (as in C++)
- addition of several new data types, including
long long int(to reduce the pain of the looming 32-bit to 64-bit transition), an explicit boolean data type, and acomplextype representing complex numbers - variable-length arrays
- official support for one-line comments beginning with
//, borrowed from C++ - several new library functions, such as
snprintf() - several new header files, such as
stdint.h
"Hello, World!" in C
The following simple application prints out "Hello, World" to standard output (which is usually the screen, but might be a file or some other hardware device or perhaps even the bit bucket depending on how standard output is mapped at the time the program is executed). A version of this program appeared for the first time in K&R.
#include <stdio.h>
int main(void)
{
printf("Hello, World!\\n");
return 0;
}
Relation to C++
The C++ programming language was originally derived from C. As C and C++ have evolved independently, the division between the two has widened, however.
C99 created a number of conflicting features. Today, the primary differences between the two languages are:
- inline - in C++, inline functions are in the global scope while in C being in a file scope.
- To indicate a boolean type, C has _Bool whereas, under C++ the keyword is bool. (In C89, any integral type was a boolean type; nonzero is true, zero is false. The bool keyword in C99 is in its own header,
.)
- Prototyping.
- Line comments. (Indicated by //. Line comments end with a newline character.)
- inline keyword.
- void type.
- Stronger typing.
Programming tools
- GNU Compiler Collection
- C99 Parser
- DJGPP
- g++
- Digital Mars™
- Bloodshed
- make
- linker
- SPlint
- Objective C
See also
- Anatomy of a C program
- C library
- C standard library (ANSI C standard library)
- Symbol table
- Abstract syntax tree (AST)
- List of C-programs
- Operators in C and C Plus Plus.
References
- Brian Kernighan, Dennis Ritchie: The C Programming Language. Also known as K&R - The original book on C.
- 1st, Prentice Hall 1978; ISBN 0-131-10163-3. Pre-ANSI C.
- 2nd, Prentice Hall 1988; ISBN 0-131-10362-8. ANSI C.
- British Standard Institute: The C Standard, John Wiley & Sons, ISBN 0-470-84573-2. The official ISO standard (C99) in book form.
- Samuel P. Harbison, Guy L. Steele: C: A Reference Manual. This book is excellent as a definitive reference manual, and for those working on C compiler and processors. The book contains a BNF grammar for C.
- 4th, Prentice Hall 1994; ISBN 0-133-26224-3.
- 5th, Prentice Hall 2002; ISBN 0-130-89592-X.
- Stephen Prata: C Primer Plus, Sams, ISBN 0-672-32222-6
- Steve Oualline: Practical C Programming, O'Reilly & Associates, ISBN 1-56592-306-5
- David M. Collopy: Introduction to C Programming - A Modular Approach, Prentice Hall, ISBN 0-1-3060855-6
- Harvey M. Deitel, Paul J. Deitel: How to program C, Prentice Hall, ISBN 0-1-3142644-3
- Peter G. Aitken, Bradley L. Jones: C in 21 Days, Sams, ISBN 0-672-31767-2
- Ivor Horton: Beginning C, Wrox Press, ISBN 1861001142
- Al Kelley, Ira Pohl: C by Dissection - The Essentials of C Programming, Addison-Wesley, ISBN 0805331492
- Dave Mark: Learn C on the Macintosh, Addison-Wesley, ISBN 0-201-48406-4
- Kyle Loudon: Mastering Algorithms with C, O'Reilly & Associates, ISBN 1-56592-453-3
- Robert Sedgewick: Algorithms in C, Addison-Wesley, ISBN 0-201-31452-5 (Part 1-4) and ISBN 0-201-31663-3 (Part 5)
- Peter A. Darnell, Philip E. Margolis: C - A Software Engineering Approach, Springer-Verlag, ISBN 0-387-94675-6
- David R. Brooks: C Programming: The Essentials for Engineers and Scientists, Springer-Verlag, ISBN 0-387-98632-4
- Mark Austin, David Chancogne: Introduction to Engineering Programming in C, Matlab and Java, John Wiley & Sons, ISBN 0-471-00116-3
- Jeri H. Hanly, Elliot B. Koffman: Problem Solving and Program Design in C, Addison-Wesley, ISBN 0-201-35748-8
- John Viega, Matt Messier: Secure Programming Cookbook for C and C++, O'Reilly & Associates, ISBN 0-596-00394-3
- Michael Barr: Programming Embedded Systems in C and C++, O'Reilly & Associates, ISBN 1-56592-354-5
- Miro Samek: Practical Statecharts in C/C++, CMP Books, ISBN 1-57820-110-1
- Michael Welschenbach: Cryptography in C and C++, Apress, ISBN 1-893115-95-X
- James D. Foley, Steven K. Feiner, Andries van Dam: Computer Graphics: Principles and Practice, Addison-Wesley, ISBN 0-201-84840-6
- Michael J. Donahoo, Kenneth L. Calvert: TCP/IP Sockets in C, Morgan Kaufmann Publishers, ISBN 1-55860-826-5
- Lincoln D. Stein, Doug McEachern: Writing Apache Modules with Perl and C, O'Reilly & Associates, ISBN 1-56592-567-X
- Alan R. Feuer: The C Puzzle Book, Addison-Wesley, ISBN 0-201-60461-2
External links
- The Development of the C Language by Dennis M. Ritchie
- Programming in C: A Tutorial by Brian W. Kernighan
- Lysator collection of C language resources
- comp.lang.c Answers to Frequently Asked Questions (FAQ List) by Steve Summit
- Cprogramming.com - informative tutorials, FAQ
- Description from the developer's encyclopedia
- Status of C99 features in GCC
- The FreeBSD C99 & POSIX® Conformance Project
- The Dinkum® C99 Library
- Tech Talk About C99 - Comeau C99 FAQ
- Are you Ready For C99?
- Article "Open source development using C99 - Is your C code up to standard?" by Peter Seebach
- Article "The New C: Declarations and Initializations" by Randy Meyers
- Article "The 'restrict' feature in C99" by Christian Bau
- Forum
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