C language knowledge (2) — History and Version (C语言 基本知识2)

C language knowledge (2) — History and Version

 

C
(programming language)

From
Wikipedia, the free encyclopedia

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," which according to Ken Thompson was a stripped down
version of the BCPL
programming language.

There are many legends as to the origin of C and
the closely related Unix
operating system, including these:

  • The development of Unix
    was the result of programmers’ desire to play the Space
    Travel
    computer game.[1]
    They had been playing it on their company’s mainframe, but as it was underpowered and
    had to support about 100 users, Thompson and Ritchie found they did not
    have sufficient control over the spaceship to avoid collisions with the
    wandering space
    rocks
    . This led to the decision to port the game to an idle PDP-7 in the
    office. As this machine lacked an operating system, the two set out to develop
    one, based on several ideas from colleagues. Eventually it was decided to
    port the operating system to the office’s PDP-11, but
    faced with the daunting task of translating a large body of custom-written
    assembly language code, the programmers
    began considering using a portable, high-level language so that the OS
    could be ported easily from one computer to another. They looked at using
    B, but it lacked functionality to take advantage of some of the PDP-11’s
    advanced features. This led to the development of an early version of the
    C programming language.
  • The justification for
    obtaining the original computer to be used in developing the Unix
    operating system was to create a system to automate the filing of patents.
    The original version of the Unix system was developed in assembly
    language. Later, nearly all of the operating system was rewritten in C, an
    unprecedented move at a time when nearly all operating systems were
    written in assembly.

By 1973, the C language had become powerful enough
that most of the Unix
kernel, originally written in PDP-11
assembly language, was rewritten in C. This was one of the first operating
system kernels implemented in a language other than assembly. (Earlier
instances include the Multics system (written in PL/I), and MCP (Master Control Program) for the Burroughs B5000 written in ALGOL in 1961.)

Version

  •     
    K&R C
  •     
    ANSI CISO C
  •     
    C99

K&R C

In 1978, Dennis
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.

K&R introduced several language features:

  • struct data types
  • long
    int
    data type
  • unsigned
    int
    data type
  • The =- operator was changed to -= to remove the semantic ambiguity
    created by the construct
    i=-10, which could be interpreted as either i =- 10 or i = -10

For many years after the introduction of ANSI C,
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 ANSI C as well.

In early versions of C, only functions that
returned a non-integer value needed to be declared if used before the function
definition; a function used without any previous declaration was assumed to
return an integer.

For example:

long int SomeFunction();
int OtherFunction();
 
int CallingFunction()
{
    long int test1;
    int test2;
 
    test1 = SomeFunction();
    if (test1 > 0) 
          test2 = 0;
    else 
          test2 = OtherFunction();
 
    return test2;
}

In the example, both SomeFunction and
OtherFunction were declared before use. In K&R, OtherFunction
declaration could be omitted.

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 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 unofficial features were added to the language (since there was no
standard), supported by compilers from AT&T and some other vendors. These
included:

The large number of extensions and lack of 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, 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, resulting in the language now called C++.

In 1983, the American National Standards
Institute
(ANSI) formed a committee, X3J11, to establish a standard
specification of C. In 1989, the 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 a few minor
modifications) was adopted by the International
Organization for Standardization
(ISO) as ISO/IEC 9899:1990. This version
is sometimes called C90. Therefore, the terms "C89" and
"C90" refer to essentially the same language.

One of the aims of the 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++), void
pointers, support for international character sets and locales, and a more
capable preprocessor. The syntax for parameter declarations was also augmented
to include the C++ style:

int main(int argc, char **argv)
{
...
}

although the K&R interface

int main(argc, argv)
    int argc;
    char **argv;
{
...
}

continued to be permitted, for compatibility with
existing source code.

C89 is supported by current C compilers, and most
C code being written nowadays 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, in
order to take advantage of features available only in Standard C.

#ifdef __STDC__
extern int getopt(int,char * const *,const char *);
#else
extern int getopt();
#endif

In the above example, a compiler which has defined
the __STDC__ macro (as mandated by the C standard) only interprets
the line following the ifdef command. In other, nonstandard
compilers which don’t define the macro, only the line following the else
command is interpreted.

C99

Note: C99 is also the name
of a C compiler for the Texas Instruments TI-99/4A home
computer. Aside from being a C compiler, it is otherwise unrelated.

After the ANSI standardization process, the C
language specification remained relatively static for some time, whereas C++ continued to
evolve, largely during its own standardization effort. Normative Amendment 1
created a new standard for the C language in 1995, but only to correct some
details of the C89 standard and to add more extensive support for international
character sets. However, the standard underwent further 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.

New features

C99 introduced several new features, many of which
had already been implemented as extensions in several compilers:

  • Inline
    functions
  • Variables
    can be declared anywhere (as in C++), rather than only after another
    declaration or the start of a compound statement
  • Several new data types,
    including
    long long int, optional extended integer types, an explicit boolean data type, and a complex type to represent complex
    numbers
  • Variable-length arrays
  • Support for one-line
    comments beginning with
    //, as in BCPL
    or C++
  • New library functions,
    such as
    snprintf
  • New header
    files
    , such as
    stdbool.h and inttypes.h
  • Type-generic math
    functions (
    tgmath.h)
  • Improved support for IEEE floating point
  • Designated initializers
  • Compound literals
  • Support for variadic
    macros (macros of variable arity)
  • restrict qualification to allow more
    aggressive code optimization

Details

C99 is for the most part upward-compatible with
C90, but is stricter in some ways; in particular, a declaration that lacks a
type specifier no longer has int implicitly assumed. The C
standards committee decided that it was of more value for compilers to diagnose
inadvertent omission of the type specifier than to silently process legacy code
that relied on implicit int. In practice, compilers are likely to
generate a warning.

Support by major compilers

GCC and other C compilers now support many
of the new features of C99. However, there has been less support from vendors
such as Microsoft
and Borland
that have mainly focused on C++, since C++ provides similar functionality
improvement.

GCC, despite its extensive C99 support, is still
not a completely compliant implementation; several key features are missing or
don’t work correctly.[2]

Version detection

A standard macro __STDC_VERSION__ is
defined with value 199901L
to indicate that C99 support is available. As with the __STDC__
macro for C90, __STDC_VERSION__ can be used to write code that
will compile differently for C90 and C99 compilers, as in this example that
ensures that inline is available in either case.

#if __STDC_VERSION__ >= 199901L
  /* "inline" is a keyword */
#else
# define inline /* nothing */
#endif

Reference:
http://en.wikipedia.org/wiki/C_(programming_language)

(中文)Chinese C wiki page: http://zh.wikipedia.org/wiki/C%E8%AF%AD%E8%A8%80
 

 

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