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. It is widely
used on many different
software
platforms, and there are 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 (
procedural)
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 (for example, memory-mapped device control
registers), 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 (as a few distinct passes communicating via
intermediate files) 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 (not a delimiter).
C also exhibits the following more specific characteristics:
- lack of nested function
definitions
- 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 ( ...
)- the equal-sign is for assignment (copying), more like Fortran, than like ALGOL's ":=" assignment.
- two consecutive equal-signs are to test for equality (compare
to
.EQ. in Fortran or the
equal-sign in BASIC and ALGOL)
&& and || in place of ALGOL's
"∧" (AND) and "∨" (OR) (these are semantically distinct from the
bit-wise operators
& and | because they will never
evaluate the right operand if the result can be determined from the
left alone (short-circuit
evaluation)).
- 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:
A number of these features are available as extensions in some
compilers, or can be 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 [news:comp.std.c comp.std.c] and
[news:comp.lang.c 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 (in this example, presumably discarding input through the
next new-line) as an allowed
extension. Use of such
nonstandard extensions generally limits
software portability.
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.
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 Ritchie and
Thompson, incorporating several ideas from colleagues. Eventually
they decided to port the operating system to a
PDP-11. B's lack of functionality to take advantage
of some of the PDP-11's features, notably
byte
addressability, led to the development of an early version of the C
programming language.
The original PDP-11 version of the Unix system was developed in
assembly language. By 1973, with the addition of
struct types, the C language had become powerful
enough that most of the
Unix kernel 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.)
K&R C
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.
K&R introduced several language features:
- standard I/O library
long int data typeunsigned int data type- compound assignment operators of the form
=op (such as =-) were changed to
the form op= to remove the semantic ambiguity
created by such constructs as i=-10, which had been
interpreted as i =- 10 instead of the
possibly intended i = -10
Even after the publication of the 1989 C 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 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, if its value was used.
For example:
long int SomeFunction();/* int OtherFunction(); */
/* int */ CallingFunction(){
long int test1;
register /* int */ test2;
test1 = SomeFunction();
if (test1 > 0)
test2 = 0;
else
test2 = OtherFunction();
return test2;
}
All the above commented-out
int declarations could be
omitted in K&R C.
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
unofficial features were added to the language, supported by
compilers from AT&T and some other vendors. These
included:
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
computers,
minicomputers, 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. 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 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 ISO C standard.
National adoption of updates 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 unofficial features subsequently introduced. 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. The syntax
for parameter declarations was also augmented to include the style
used in C++, although 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 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 to take advantage of features
available only in Standard C.
C99
After the ANSI/ISO standardization process, the C language
specification remained relatively static for some time, whereas
C++ continued to evolve, largely during its own
standardization effort. In 1995 Normative Amendment 1 to the 1990 C
standard was published, to correct some details and to add more
extensive support for international character sets. 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. The international C standard is maintained by
the
working group ISO/IEC JTC1/SC22/WG14.
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, 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.
GCC,
Sun Studio and other C compilers now
support many or all of the new features of C99.
C1X
In 2007, work began in anticipation of another revision of the C
standard, informally called "C1X". The C standards committee has
adopted guidelines to limit the adoption of new features that have
not been tested by existing implementations.
Uses
C's primary use is for "
system
programming", including implementing
operating systems and
embedded system applications, due to a
combination of desirable characteristics such as code portability
and efficiency, ability to access specific hardware addresses,
ability to
"pun" types to match
externally imposed data access requirements, and low
runtime demand on system
resources.
One consequence of C's wide acceptance and efficiency is that
compilers, libraries, and interpreters of
other
programming languages are often implemented 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,
it is not necessary to develop machine-specific code generators.
Some compilers which use C this way are
BitC,
Gambit, the
Glasgow Haskell Compiler,
Squeak, and
Vala.
Unfortunately, C was designed as a programming language, not as a
compiler target language, and is thus less than ideal for use as an
intermediate language. This has led to development of C-based
intermediate languages such as
C--.
C has also been widely used to implement
end-user applications, but as
applications became larger, much of that development shifted to
other languages.
Syntax
Unlike languages such as
FORTRAN 77, C
source code is
free-form which
allows arbitrary use of whitespace to format code, rather than
column-based or text-line-based restrictions. Comments may appear
either between the delimiters
/* and
*/,
or (in C99) following
// until the end of the
line.
Each source file contains declarations and function definitions.
Function definitions, in turn, contain declarations and statements.
Declarations either define new types using keywords such as
struct,
union, 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
do-
while,
while, 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 (see below) 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.
Although mimicked by many languages because of its widespread
familiarity, C's syntax has often been criticized. For example,
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."
Some specific problems worth noting are:
- Not checking number and types of arguments when the function
declaration has an empty parameter list. (This provides backward compatibility with K&R C, which lacked
prototypes.)
- Some questionable choices of operator precedence, as mentioned
by Kernighan and Ritchie above, such as
== binding
more tightly than & and | in
expressions like x & 1 == 0.
- The use of the
= operator, used in mathematics for
equality, to indicate assignment, following the precedent of
Fortran, PL/I, and
BASIC, but unlike ALGOL
and its derivatives. Ritchie made this syntax design decision
consciously, based primarily on the argument that assignment occurs
more often than comparison.
- Similarity of the assignment and equality operators
(
= and ==), making it easy to
accidentally substitute one for the other. C's weak type system
permits each to be used in the context of the other without a
compilation error (although some compilers produce warnings). For
example, the conditional expression in if (a=b) is
only true if a is not zero after the assignment.
- A lack of infix operators for
complex objects, particularly for string operations, making
programs which rely heavily on these operations (implemented as
functions instead) somewhat difficult to read.
- A declaration syntax that some find unintuitive, particularly
for function pointers. (Ritchie's
idea was to declare identifiers in contexts resembling their use:
"declaration reflects
use".)
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:
- arithmetic (
+, -, *, /, %)
- equality testing (
==, !=)
- order relations
(
<, <=, >,
>=)
- boolean logic (
!,
&&, ||)
- bitwise logic (
~,
&, |, ^)
- bitwise shifts
(
<<, >>)
- assignment
(
=, +=, -=, *=,
/=, %=, &=,
|=, ^=, <<=,
>>=)
- increment and decrement (
++, --)
- reference and dereference
(
&, *, [ ])
- conditional evaluation (
? :)
- member selection
(
., ->)
- type conversion (
(
))
- object size (
sizeof)
- function argument collection
(
( ))
- sequencing (
,)
- subexpression
grouping (
( ))
C has a
formal grammar, specified by
the C standard.
"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
int main(void){
printf("hello, world\n");
return 0;
}
The first line of the program contains a
preprocessing directive, indicated by
#include. This causes the preprocessor — the first
tool to examine source code as it is compiled — to substitute the
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 standard headers to
other headers having the same name. Double quotes may also be used
to 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
return
value, the value that is returned to the invoker (in this case
the run-time environment) as a result of evaluating the
main function, is an integer. The keyword
void as a parameter list indicates that the
main function takes no arguments.The
main
function actually has two arguments,
int argc and
char *argv[], respectively, which can be used to
handle
command line
arguments. The C standard requires that both forms of
main be supported, which is special treatment not
afforded any other function.
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 was declared
in
stdio.h and 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
return statement terminates the execution of the
main function and causes it to return the integer
value 0, which is interpreted by the run-time system as an exit
code indicating successful execution.
The closing curly brace indicates the end of the code for the
main function.
Data structures
C has a static
weak typing type system that shares some similarities with
that of other
ALGOL descendants such as
Pascal. There are
built-in types for integers of various sizes, both signed and
unsigned,
floating-point
numbers, characters, and enumerated types (
enum).
C99 added a
boolean datatype. There
are also derived types including
array,
pointer,
records (
struct), and
untagged
unions
(
union).
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 value in some other way.
Pointers
C supports the use of pointers, a very simple type of
reference that records, in
effect, 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 and also
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. (See
Array-pointer
interchangeability below.) Pointers are used for many different
purposes in C.
Text
strings are commonly manipulated using pointers into arrays of
characters.
Dynamic memory
allocation, which is described below, is performed using
pointers. Many data types, such as
tree, are commonly implemented as
dynamically allocated
struct objects linked together
using pointers. Pointers to functions are useful for
callbacks from event
handlers.
A
null pointer is a pointer
value that points to no valid location (it is often represented by
address zero). Dereferencing a null pointer is therefore
meaningless, typically resulting in a run-time error. Null pointers
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.
Void pointers (
void *) point to objects of unknown
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.
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 be deallocated and reused (
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 (see below) means that true
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 the compiler
may provide bounds checking as an option. 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.
C does not have a special provision for declaring multidimensional
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 "multidimensional array"
can be thought of as increasing in
row-major order.
Although C supports static arrays, it is not required that array
indices be validated (
bounds
checking). For example, one can try to write to the sixth
element of an array with five elements, generally yielding
undesirable results. This type of bug, called a
buffer overflow or
buffer
overrun, is notorious for causing a number of security
problems. Since
bounds
checking elimination technology was largely nonexistent when C
was defined, bounds checking came with a severe performance
penalty, particularly in numerical computation. A few years
earlier, some
Fortran compilers had a switch
to toggle bounds checking on or off; however, this would have been
much less useful for C, where array arguments are passed as simple
pointers.
Multidimensional 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
A distinctive (but potentially confusing) feature of C is its
treatment of arrays and pointers. The array-subscript notation
x[i] can also be used when
x is a
pointer; the interpretation (using pointer arithmetic) is to access
the
(i + 1)th object of several adjacent data
objects pointed to by
x, counting the object that
x points to (which is
x[0]) as the first
element of the array.
Formally,
x[i] is equivalent to
*(x + i).
Since the type of the pointer involved is known to the compiler at
compile time, the address that
x + i points to is
not the address pointed to by
x incremented
by
i bytes, but rather incremented by
i
multiplied by the size of an element that
x points to.
The size of these elements can be determined with the operator
sizeof by applying it to any
dereferenced element of
x, as in
n = sizeof
*x or
n = sizeof x[0].
Furthermore, in most expression contexts (a notable exception is
sizeof x), 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 number of elements in a declared array
a can be
determined as
sizeof x / sizeof x[0].
An interesting demonstration of the interchangeability of pointers
and arrays is shown below. The four assignments are equivalent and
each is valid C code.
/* x designates an array */x[i] = 1; /* equivalent to *(x + i) */
- (x + i) = 1;
- (i + x) = 1;
i[x] = 1; /* uncommon usage, but correct: i[x] is equivalent to *(i
+ x) */
Note that the last line contains the uncommon, but semantically
correct, expression
i[x], which appears to interchange
the index variable
i with the array variable
x. This last line might be found in
obfuscated C code,
but its use is rare among C programmers.
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, unlike a pointer variable.
Consequently, what an array "points to" cannot be changed, and it
is impossible to assign a value to an array variable. (Array
values may be copied, however, e.g., by using the
memcpy function.)
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:
- 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
free
These three approaches are appropriate in different situations and
have various tradeoffs. For example, static memory allocation has
no allocation overhead, automatic allocation may involve a small
amount of overhead, and dynamic memory allocation can potentially
have a great deal of overhead for both allocation and deallocation.
On the other hand, stack space is typically much more limited and
transient than either static memory or heap space, and dynamic
memory allocation allows 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 preferred
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
grow in size at runtime, and since static allocations (and
automatic allocations in C89 and C90) must have a fixed size at
compile-time, there are many situations in which dynamic allocation
must be used. Prior to the C99 standard, variable-sized arrays were
a common example of this (see
malloc for an example of dynamically allocated
arrays).
Automatically and dynamically allocated objects are only
initialized 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 occur.
Another issue is that heap memory allocation has to be manually
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 has been
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 to release memory too soon and continue to access it;
however, since the allocation system can re-allocate or itself use
the freed memory, unpredictable behavior is likely to occur when
the multiple users corrupt each other's data. Typically, the
symptoms will appear in a portion of the program far removed from
the actual error. Such issues are ameliorated in languages with
automatic garbage
collection or
RAII.
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 "math library").
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 (“freestanding” [embedded] C
implementations may provide only a subset of the standard library).
This library supports stream input and output, memory allocation,
mathematics, character strings, and time values.
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
Java,
Perl, and
Python.
Language tools
Tools have been created to help C programmers avoid some of the
problems inherent in the language, such as statements with
undefined behavior or statements that are not a good practice
because they are more likely to result in unintended behavior or
run-time errors.
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.
There are also compilers, libraries and operating system level
mechanisms for performing array bounds checking,
buffer overflow detection,
serialization and
automatic garbage
collection, that are not a standard part of C.
Tools such as
Purify,
Valgrind, and linking with libraries containing
special versions of the
memory allocation
functions can help uncover runtime memory errors.
Related languages
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 (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.
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 (see
Compatibility of C and C++ for an
exhaustive list of differences).
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 (the designer of D) considered
undesirable, including the
C
preprocessor and
trigraph. 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.
See also
Footnotes
References
- Brian Kernighan, Dennis Ritchie: The C Programming
Language. Also known as K&R — The original book on C.
- 1st, Prentice Hall 1978; ISBN 0-13-110163-3. Pre-ANSI C.
- 2nd, Prentice Hall 1988; ISBN 0-13-110362-8. ANSI C.
- ISO/IEC 9899. Official C99 documents, including
technical corrigenda and a rationale. As of 2007 the latest version
of the standard is .
- 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 compilers. The book contains a BNF grammar for C.
- 5th, Prentice Hall 2002; ISBN 0-13-089592-X.
- Derek M. Jones: The New C Standard: A Cultural and Economic
Commentary, Addison-Wesley, ISBN 0-201-70917-1, online
material
- Robert
Sedgewick: Algorithms in C, Addison-Wesley, ISBN
0-201-31452-5 (Part 1–4) and ISBN 0-201-31663-3 (Part 5)
- William H. Press, Saul A. Teukolsky, William T. Vetterling,
Brian P. Flannery: Numerical
Recipes in C (The Art of Scientific Computing), ISBN
0-521-43108-5
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