C is a general-purpose computer programming language developed in 1972 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system.
Although C was designed for implementing system software, it is also widely used for developing portable application software.
C is one of the most popular programming languages and there are very few computer architectures for which a C compiler does not exist. C has greatly influenced many other popular programming languages, most notably C++, which originally began as an extension to C.
Design
C is an imperative systems implementation language. It was designed to be compiled using a relatively straightforward compiler, to provide low-level access to memory, to provide language constructs that map efficiently to machine instructions, and to require minimal run-time support. C was therefore useful for many applications that had formerly been coded in assembly language.
Despite its low-level capabilities, the language was designed to encourage machine-independent programming. A standards-compliant and portably written C program can be compiled for a very wide variety of computer platforms and operating systems with little or no change to its source code. The language has become available on a very wide range of platforms, from embedded microcontrollers to supercomputers.
Minimalism
C's design is tied to its intended use as a portable systems implementation language. It provides simple, direct access to any addressable object , and its source-code expressions can be translated in a straightforward manner to primitive machine operations in the executable code. Some early C compilers were comfortably implemented on PDP-11 processors having only 16 address bits. C compilers for several common 8-bit platforms have been implemented as well.
Characteristics
Like most imperative languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion, while a static type system prevents many unintended operations. In C, all executable code is contained within functions. Function parameters are always passed by value. Pass-by-reference is simulated in C by explicitly passing pointer values. Heterogeneous aggregate data types (struct) allow related data elements to be combined and manipulated as a unit. C program source text is free-format, using the semicolon as a statement terminator .
C also exhibits the following more specific characteristics:
variables may be hidden in nested blocks
partially weak typing; for instance, characters can be used as integers
low-level access to computer memory by converting machine addresses to typed pointers
function and data pointers supporting ad hoc run-time polymorphism
array indexing as a secondary notion, defined in terms of pointer arithmetic
a preprocessor for macro definition, source code file inclusion, and conditional compilation
complex functionality such as I/O, string manipulation, and mathematical functions consistently delegated to library routines
A relatively small set of reserved keywords
A lexical structure that resembles B more than ALGOL, for example:
{ ... } rather than either of ALGOL 60's begin ... end or ALGOL 68's ( ... )
= is used for assignment (copying), like Fortran, rather than ALGOL's :=
== is used to test for equality (rather than .EQ. in Fortran, or = in BASIC and ALGOL)
Logical "and" and "or" are represented with && and || in place of ALGOL's and ; note that the doubled-up operators will never evaluate the right operand if the result can be determined from the left alone (this is called short-circuit evaluation), and are semantically distinct from the bit-wise operators & and |
However Unix Version 6 & 7 versions of C indeed did use ALGOL's and ASCII operators, but for determining the infimum and supremum respectively.
a large number of compound operators, such as +=, ++, etc. (Equivalent to ALGOL 68's +:= and +:=1 operators)
Absent features
The relatively low-level nature of the language affords the programmer close control over what the computer does, while allowing special tailoring and aggressive optimization for a particular platform. This allows the code to run efficiently on very limited hardware, such as embedded systems.
C does not have some features that are available in some other programming languages:
No nested function definitions
No direct assignment of arrays or strings (copying can be done via standard functions; assignment of objects having struct or union type is supported)
No automatic garbage collection
No requirement for bounds checking of arrays
No operations on whole arrays
No syntax for ranges, such as the A..B notation used in several languages
Prior to C99, no separate Boolean type (zero/nonzero is used instead)
No formal closures or functions as parameters (only function and variable pointers)
No generators or coroutines; intra-thread control flow consists of nested function calls, except for the use of the longjmp or setcontext library functions
No exception handling; standard library functions signify error conditions with the global errno variable and/or special return values, and library functions provide non-local gotos
Only rudimentary support for modular programming
No compile-time polymorphism in the form of function or operator overloading
Very limited support for object-oriented programming with regard to polymorphism and inheritance
Limited support for encapsulation
No native support for multithreading and networking
No standard libraries for computer graphics and several other application programming needs
A number of these features are available as extensions in some compilers, or are provided in some operating environments (e.g., POSIX), or are supplied by third-party libraries, or can be simulated by adopting certain coding disciplines.
Undefined behavior
Many operations in C that have undefined behavior are not required to be diagnosed at compile time. In the case of C, "undefined behavior" means that the exact behavior which arises is not specified by the standard, and exactly what will happen does not have to be documented by the C implementation. A famous, although misleading, expression in the newsgroups comp.std.c and comp.lang.c is that the program could cause "demons to fly out of your nose". Sometimes in practice what happens for an instance of undefined behavior is a bug that is hard to track down and which may corrupt the contents of memory. Sometimes a particular compiler generates reasonable and well-behaved actions that are completely different from those that would be obtained using a different C compiler. The reason some behavior has been left undefined is to allow compilers for a wide variety of instruction set architectures to generate more efficient executable code for well-defined behavior, which was deemed important for C's primary role as a systems implementation language; thus C makes it the programmer's responsibility to avoid undefined behavior, possibly using tools to find parts of a program whose behavior is undefined. Examples of undefined behavior are:
*accessing outside the bounds of an array
*overflowing a signed integer
*reaching the end of a non-void function without finding a return statement, when the return value is used
*reading the value of a variable before initializing it
These operations are all programming errors that could occur using many programming languages; C draws criticism because its standard explicitly identifies numerous cases of undefined behavior, including some where the behavior could have been made well defined, and does not specify any run-time error handling mechanism.
Invoking fflush() on a stream opened for input is an example of a different kind of undefined behavior, not necessarily a programming error but a case for which some conforming implementations may provide well-defined, useful semantics as an allowed extension. Use of such nonstandard extensions generally limits software portability.
Related languages in C
C has directly or indirectly influenced many later languages such as Java, Perl, PHP, JavaScript, LPC, C# and Unix's C Shell. The most pervasive influence has been syntactical: all of the languages mentioned combine the statement and expression syntax of C with type systems, data models and/or large-scale program structures that differ from those of C, sometimes radically.
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.
Bjarne Stroustrup devised the C++ programming language as one approach to providing object-oriented functionality with C-like syntax. 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 .
Unlike C++, which maintains nearly complete backwards compatibility with C, the D language makes a clean break with C while maintaining the same general syntax. It abandons a number of features of C which Walter Bright considered undesirable, including the C preprocessor and trigraphs. Some, but not all, of D's extensions to C overlap with those of C++.
Objective-C was originally a very "thin" layer on top of, 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.
Limbo is a language developed by the same team at Bell Labs that was responsible for C and Unix, and while retaining some of the syntax and the general style, introduced garbage collection, CSP based concurrency and other major innovations.
Python has a different sort of C heritage. While the syntax and semantics of Python are radically different from C, the most widely used Python implementation, CPython, is an open source C program. This allows C users to extend Python with C, or embed Python into C programs. This close relationship is one of the key factors leading to Python's success as a general-use dynamic language.
Perl is another example of a popular programming language rooted in C. However, unlike Python, Perl's syntax does closely follow C syntax. The standard Perl implementation is written in C and supports extensions written in C.
Although C was designed for implementing system software, it is also widely used for developing portable application software.
C is one of the most popular programming languages and there are very few computer architectures for which a C compiler does not exist. C has greatly influenced many other popular programming languages, most notably C++, which originally began as an extension to C.
Design
C is an imperative systems implementation language. It was designed to be compiled using a relatively straightforward compiler, to provide low-level access to memory, to provide language constructs that map efficiently to machine instructions, and to require minimal run-time support. C was therefore useful for many applications that had formerly been coded in assembly language.
Despite its low-level capabilities, the language was designed to encourage machine-independent programming. A standards-compliant and portably written C program can be compiled for a very wide variety of computer platforms and operating systems with little or no change to its source code. The language has become available on a very wide range of platforms, from embedded microcontrollers to supercomputers.
Minimalism
C's design is tied to its intended use as a portable systems implementation language. It provides simple, direct access to any addressable object , and its source-code expressions can be translated in a straightforward manner to primitive machine operations in the executable code. Some early C compilers were comfortably implemented on PDP-11 processors having only 16 address bits. C compilers for several common 8-bit platforms have been implemented as well.
Characteristics
Like most imperative languages in the ALGOL tradition, C has facilities for structured programming and allows lexical variable scope and recursion, while a static type system prevents many unintended operations. In C, all executable code is contained within functions. Function parameters are always passed by value. Pass-by-reference is simulated in C by explicitly passing pointer values. Heterogeneous aggregate data types (struct) allow related data elements to be combined and manipulated as a unit. C program source text is free-format, using the semicolon as a statement terminator .
C also exhibits the following more specific characteristics:
variables may be hidden in nested blocks
partially weak typing; for instance, characters can be used as integers
low-level access to computer memory by converting machine addresses to typed pointers
function and data pointers supporting ad hoc run-time polymorphism
array indexing as a secondary notion, defined in terms of pointer arithmetic
a preprocessor for macro definition, source code file inclusion, and conditional compilation
complex functionality such as I/O, string manipulation, and mathematical functions consistently delegated to library routines
A relatively small set of reserved keywords
A lexical structure that resembles B more than ALGOL, for example:
{ ... } rather than either of ALGOL 60's begin ... end or ALGOL 68's ( ... )
= is used for assignment (copying), like Fortran, rather than ALGOL's :=
== is used to test for equality (rather than .EQ. in Fortran, or = in BASIC and ALGOL)
Logical "and" and "or" are represented with && and || in place of ALGOL's and ; note that the doubled-up operators will never evaluate the right operand if the result can be determined from the left alone (this is called short-circuit evaluation), and are semantically distinct from the bit-wise operators & and |
However Unix Version 6 & 7 versions of C indeed did use ALGOL's and ASCII operators, but for determining the infimum and supremum respectively.
a large number of compound operators, such as +=, ++, etc. (Equivalent to ALGOL 68's +:= and +:=1 operators)
Absent features
The relatively low-level nature of the language affords the programmer close control over what the computer does, while allowing special tailoring and aggressive optimization for a particular platform. This allows the code to run efficiently on very limited hardware, such as embedded systems.
C does not have some features that are available in some other programming languages:
No nested function definitions
No direct assignment of arrays or strings (copying can be done via standard functions; assignment of objects having struct or union type is supported)
No automatic garbage collection
No requirement for bounds checking of arrays
No operations on whole arrays
No syntax for ranges, such as the A..B notation used in several languages
Prior to C99, no separate Boolean type (zero/nonzero is used instead)
No formal closures or functions as parameters (only function and variable pointers)
No generators or coroutines; intra-thread control flow consists of nested function calls, except for the use of the longjmp or setcontext library functions
No exception handling; standard library functions signify error conditions with the global errno variable and/or special return values, and library functions provide non-local gotos
Only rudimentary support for modular programming
No compile-time polymorphism in the form of function or operator overloading
Very limited support for object-oriented programming with regard to polymorphism and inheritance
Limited support for encapsulation
No native support for multithreading and networking
No standard libraries for computer graphics and several other application programming needs
A number of these features are available as extensions in some compilers, or are provided in some operating environments (e.g., POSIX), or are supplied by third-party libraries, or can be simulated by adopting certain coding disciplines.
Undefined behavior
Many operations in C that have undefined behavior are not required to be diagnosed at compile time. In the case of C, "undefined behavior" means that the exact behavior which arises is not specified by the standard, and exactly what will happen does not have to be documented by the C implementation. A famous, although misleading, expression in the newsgroups comp.std.c and comp.lang.c is that the program could cause "demons to fly out of your nose". Sometimes in practice what happens for an instance of undefined behavior is a bug that is hard to track down and which may corrupt the contents of memory. Sometimes a particular compiler generates reasonable and well-behaved actions that are completely different from those that would be obtained using a different C compiler. The reason some behavior has been left undefined is to allow compilers for a wide variety of instruction set architectures to generate more efficient executable code for well-defined behavior, which was deemed important for C's primary role as a systems implementation language; thus C makes it the programmer's responsibility to avoid undefined behavior, possibly using tools to find parts of a program whose behavior is undefined. Examples of undefined behavior are:
*accessing outside the bounds of an array
*overflowing a signed integer
*reaching the end of a non-void function without finding a return statement, when the return value is used
*reading the value of a variable before initializing it
These operations are all programming errors that could occur using many programming languages; C draws criticism because its standard explicitly identifies numerous cases of undefined behavior, including some where the behavior could have been made well defined, and does not specify any run-time error handling mechanism.
Invoking fflush() on a stream opened for input is an example of a different kind of undefined behavior, not necessarily a programming error but a case for which some conforming implementations may provide well-defined, useful semantics as an allowed extension. Use of such nonstandard extensions generally limits software portability.
Related languages in C
C has directly or indirectly influenced many later languages such as Java, Perl, PHP, JavaScript, LPC, C# and Unix's C Shell. The most pervasive influence has been syntactical: all of the languages mentioned combine the statement and expression syntax of C with type systems, data models and/or large-scale program structures that differ from those of C, sometimes radically.
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.
Bjarne Stroustrup devised the C++ programming language as one approach to providing object-oriented functionality with C-like syntax. 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 .
Unlike C++, which maintains nearly complete backwards compatibility with C, the D language makes a clean break with C while maintaining the same general syntax. It abandons a number of features of C which Walter Bright considered undesirable, including the C preprocessor and trigraphs. Some, but not all, of D's extensions to C overlap with those of C++.
Objective-C was originally a very "thin" layer on top of, 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.
Limbo is a language developed by the same team at Bell Labs that was responsible for C and Unix, and while retaining some of the syntax and the general style, introduced garbage collection, CSP based concurrency and other major innovations.
Python has a different sort of C heritage. While the syntax and semantics of Python are radically different from C, the most widely used Python implementation, CPython, is an open source C program. This allows C users to extend Python with C, or embed Python into C programs. This close relationship is one of the key factors leading to Python's success as a general-use dynamic language.
Perl is another example of a popular programming language rooted in C. However, unlike Python, Perl's syntax does closely follow C syntax. The standard Perl implementation is written in C and supports extensions written in C.
No comments:
Post a Comment