C (programming language)

C

File:K&R C.jpg The C Programming Language, Brian Kernighan and Dennis Ritchie, the original edition that served for many years as an informal specification of the language.
Paradigm	imperative (procedural) systems implementation language
Designed by	Dennis Ritchie
Developer	Dennis Ritchie & Bell Labs
First appeared	1972
Stable release	C99 / March 2000
Typing discipline	static, weak
Website	www.iso.org/standard/74528.html,%20https://www.open-std.org/jtc1/sc22/wg14/
Major implementations
GCC, MSVC, Borland C, Watcom C
Dialects
ObjC, C++
Influenced by
B (BCPL,CPL), Algol68, Assembly, Pascal
Influenced
awk, csh, C++, ObjC, Concurrent C, Java, Javascript, etc.

The C programming language (often, just "C") is a general-purpose, procedural, imperative computer programming language developed in the early 1970s by Dennis Ritchie for use on the Unix operating system. It has since spread to many other operating systems, and is now one of the most widely used programming languages. C also has had a great influence on many other popular languages, especially C++ which was originally designed as an enhancement to C. It is the most commonly used programming language for writing system software, though it is also widely used for writing applications. Though not originally designed as a language for teaching, and despite its somewhat unforgiving character, C is commonly used in computer science education, in part because the language is so pervasive.

Philosophy

C is a minimalistic programming language. Among its design goals were that it could be compiled in a straightforward manner using a relatively simple compiler, provide low-level access to memory, generate only a few machine language instructions for each of its core language elements, and not require extensive run-time support. As a result, it is possible to write C code at a low level of abstraction analogous to assembly language; in fact C is sometimes referred to (and not always pejoratively) as "high-level assembly" or "portable assembly."

Because of simplicity, the language has therefore become available on a very wide range of platforms. Furthermore, despite its low-level nature, the language was designed to enable (and 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 minimal change to its source code.

C was originally developed (along with the Unix operating system with which it has long been associated) by programmers and for programmers, with few users other than its own designers in mind. Nevertheless, it has achieved very widespread popularity, finding application in contexts far removed from its roots as a language for systems-programming.

Characteristics

As an Algol-based language, C has the following characteristics:

• A procedural programming paradigm, with facilities for structured programming
• Lexical variable scope and recursion
• A static, weak type system which prevents many meaningless operations
• Function parameters are generally passed by value (pass-by-reference is achieved in C by explicitly passing pointer values)
• Heterogeneous aggregate data types (called "structs") which allow related data elements to be combined and manipulated as a unit
• A small set (around 30) of reserved keywords

C also has the following specific properties:

• Low-level access to computer memory via machine addresses and typed pointers
• Function pointers allow for a rudimentary form of closures and runtime polymorphism
• Array indexing as a secondary notion, defined in terms of pointer arithmetic
• A standardized C preprocessor for macro definition, source code file inclusion, conditional compilation, etc.
• A simple, small core language, with functionality such as mathematical functions and file handling provided by library routines
• C discarded the well established logical connectives and and or of most other algol derivatives and replaced them with && and ||, which
  • Were invented in order to make bit-wise operations (& and |) syntactically distinct — C's predecessor B used & and | for both meanings.
  • Never evaluate the right operand if the result can be determined from the left alone (as & and | of B used within an if statement).
• C popularized the controversial decision to free the equal-sign for assignment use by replacing = with == (inherited from B).

C lacks features found in some other systems implementation languages:

• No non-scalar operations such as copying of arrays or strings (old versions of C did not even copy structs automatically)
• No automatic garbage collection
• No bounds checking of arrays
• No semi-dynamic (i.e. stacked, runtime-sized) arrays until the C99 standard (despite not requiring garbage collection)
• No syntax for ranges, such as the A..B notation used in both newer and older languages
• No nested function definitions (although some compilers provide them, for example, GCC)
• 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
• Rudimentary support for modular programming
• No compile-time polymorphism in the form of function or operator overloading; only rudimentary support for generic programming
• No support for object-oriented programming; in particular, no support for polymorphism, inheritance and limited (inter-module only) support for encapsulation, even though there are libraries offering object systems for C, and many object-oriented languages are themselves written in C
• No native support for multithreading and networking, though these facilities are provided by popular libraries
• No standard libraries for graphics and several other application programming needs

Although the list of built-in features C lacks is long, this has contributed significantly to its acceptance, as new C compilers can be developed quickly for new platforms. The relatively low-level nature of the language affords the programmer close control over what the program is doing, while allowing solutions that can be specially tailored and aggressively optimized for a particular platform. This allows the code to run efficiently on very limited hardware, such as mass-produced consumer embedded systems, which today are as capable as the first machines used to implement C. Often, only hand-tuned assembly language code runs faster, although advances in compiler technology have narrowed this gap.

A number of the above missing features are available through the use of third party libraries. In some cases, a missing feature can be approximated within C. For example, the original implementation of C++ consisted of a preprocessor that translated the C++ syntax into C source code. Most object oriented functions include a special "this" pointer, which refers to the current object. By passing this pointer as a function argument in C, the same functionality can be performed in C. For example, in C++ one might write:

stack->push(val);

while in C, one would write:

push(stack,val);

where the stack argument of C is a pointer to a struct which is equivalent to the this pointer of C++, which is a pointer to an object.

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", but Ken Thompson credits it as being 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 video-game. 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, the entire 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-11/20 assembly language, was rewritten in C. This was one of the first operating system kernels implemented in a language other than assembly. (Earlier instances include the Multics system (written in PL/I), and MCP (Master Control Program) for Burroughs B5000 written in ALGOL in 1961.)

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

K&R C was often considered the most basic part of the language that a C compiler must support. For many years, even after the introduction of ANSI C, it was considered the "lowest common denominator" to which C programmers restricted themselves when maximum portability was desired, since not all compilers were updated to fully support ANSI C, and because with care, K&R C code can be written to be legal ANSI C as well.

In these early versions of C, only functions that returned a non-integer value needed to be declared if used before the function definition. If a function definition appears before it is referenced, no prototype is required, regardless of the return type. 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 with a prototype before use. In K&R, OtherFunction declaration could be omitted.

Since the K&R prototype 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.

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:

• void functions
• Functions returning struct or union types (rather than pointers)
• Assignment for struct data types
• const qualifier to make an object read-only
• Enumerated types

The large amount of extensions and lack of a standard library, together with the language popularity and the fact that not even the AT&T compilers precisely implemented the K&R specification, led to the necessity of standardization.

ANSI C and ISO C

File:Kr c prog lang.jpg

The C Programming Language, 2nd edition, is a widely used reference on ANSI 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. 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, 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 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++), void *, 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.

ANSI C is now supported by almost all the widely used C compilers. Most C code being written nowadays is based on ANSI C. 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 certain data types and byte endianness.

To mitigate the differences between K&R C and the ANSI C standard, the __STDC__ macro can be used to split code into ANSI and K&R sections.

#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 ANSI C) 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.

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, 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, or macros of variable arity
• restrict qualification to allow more aggressive code optimization

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 diagnose the omission but also assume int and continue translating the program.

GCC and several other C compilers now support most of the new features of C99. However, there has been less support from vendors such as Microsoft and Borland that have been mainly focused on C++, since C++ provides similar functionality in often incompatible ways (e.g., the complex template class). Microsoft's Brandon Bray said "In general, we have seen little demand for many C99 features. Some features have more demand than others, and we will consider them in future releases provided they are compatible with C++." [1]

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

Usage

One consequence of C's wide acceptance and efficiency is that the compilers, libraries, and interpreters of other higher-level languages are often implemented in C.

C is used as an intermediate language by some higher-level languages. This is implemented in one of two ways, as languages which:

• Can output object code, machine code, or another representation (e.g., bytecodes), and C source code. Examples: some Lisp dialects, Squeak's C-subset Slang.
• Do not output object code, machine code, or another representation, but output C source code only. Examples: Eiffel, Sather; Esterel.

C source code is then input to a C compiler, which then outputs finished object or machine code. This is done to gain portability and optimization. C compilers exist for nearly all processors and operating systems, and most C compilers output is well optimized. Thus, any language that outputs C source code becomes very portable, and able to yield optimized code.

Unfortunately, C is 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--.

Syntax

Main article: C 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, as well as the pointer-to symbol *, specify built-in types. Sections of code are enclosed in braces ({ and }) to indicate the extent to which declarations and control structures apply.

As an imperative language, C depends on statements to do most of the work. Most statements are expression statements which simply evaluate an expression—in the process, cause variables to receive new values or values to be printed. Control-flow statements are also available for conditional or iterative execution, constructed with reserved keywords such as if, else, switch, do, while, and for. Arbitrary jumps are possible with goto. A variety of built-in operators perform primitive arithmetic, Boolean logical, comparative, bitwise logical, and array indexing operations and assignment. Expressions can also invoke functions, including a large number of standard library functions, for performing many common tasks.

"hello, world" example

The following simple application appeared in the first edition of K&R, and has become the model for an introductory program in most programming textbooks, regardless of programming language. The program prints out "hello, world" to the standard output, which is usually a terminal or screen display. However, it might be a file or some other hardware device, depending on how standard output is mapped at the time the program is executed.

main()
{
    printf("hello, world\n");
}

The above program will compile correctly on most modern compilers that are not in compliance mode. However, it produces several warning messages when compiled with a compiler that conforms to the ANSI C standard, and won't compile at all if the compiler strictly conforms to the C99 standard. The current, ANSI C or C99, "hello world" program is written as follows:

#include <stdio.h>

int main(void)
{
    printf("hello, world\n");

    return 0;
}

What follows is a line-by-line analysis of the above program:

#include <stdio.h>

This first line of the program is a preprocessing directive, #include. This causes the preprocessor — the first tool to examine source code when it is compiled — to substitute for that line the entire text of the file to which it refers. In this case, the header stdio.h, which contains the definitions of standard input and output functions such as printf, will replace that line. The angle brackets surrounding stdio.h indicate that stdio.h can be found using an implementation-defined search strategy. Double quotes may also be used for headers, thus allowing the implementation to supply (up to) two strategies. Typically, angle brackets are reserved for headers supplied by the implementation, and double quotes for local or installation-specific headers.

int main(void)

This next line indicates that a function named main is being defined. The main function serves a special purpose in C programs: When the program is executed, main is the function called by the run-time environment—otherwise it acts like any other function in the program. The type specifier int indicates that the return value, the value of evaluating the main function that is returned to its invoker (in this case the run-time environment), is an integer. The keyword (void) in between the parentheses indicates that the main function takes no arguments. See also void.

{

This opening curly brace indicates the beginning of the definition of the main function.

    printf("hello, world\n");

This line calls (executes the code for) a function named printf, which is declared in the included header stdio.h. 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 the newline character, which on output signifies the beginning of the next line. The return value of the printf function is of type int, but no use was made of it so it will be quietly discarded. (A more careful program might test this value to determine whether the operation succeeded.)

    return 0;

This line terminates the execution of the main function and causes it to return the integral value 0, which is interpreted by the run-time system as an exit code indicating successful execution.

}

This closing curly brace indicates the end of the code for the main function.

If the above code were compiled and executed, it would do the following:

• Print the string "hello, world" onto the standard output device (typically but not always a terminal),
• Move the current position indicator to the beginning of the next line, then
• Return a "successful" exit status to the calling process (such as a command shell or script).

Data structures

C has a 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). There are also derived types including arrays, pointers, 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. (The use of type casts obviously sacrifices some of the safety normally provided by the type system.)

Pointers

C allows the use of pointers, a very simple type of reference that records, in effect, the address or location of an object in memory. Pointers can be dereferenced to access the data stored at the address pointed to. Pointers can be manipulated using normal assignments and also pointer arithmetic. The runtime representation of a pointer value is typically a raw memory address, but at compile time, a pointer's type includes the type of the data pointed to, which allows expressions including pointers to be type-checked. Pointer arithmetic increments the address of a pointer based on the size of that 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. It is also possible to use pointers to functions.

A null pointer is a pointer value that points to no valid location (its internal value is usually zero). (Dereferencing a null pointer is therefore meaningless, typically resulting in a runtime 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 *) also exist, and point to objects of unknown type, and can therefore be used as a "generic pointer". Since the size and type of the pointed-to object is not known, void pointers cannot be dereferenced, nor is pointer arithmetic on them possible, although they can easily be (and in fact implicitly are) converted to and from any other object pointer type.

Arrays

Array types in C are always one-dimensional and, 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 perfectly straightforward to allocate a block of memory (of arbitrary size) at run-time using the standard library 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. However, 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 some level of bounds checking as an option. Array bounds violations are therefore possible and rather common in carelessly written code (see also the "Criticism" section below), and can lead to various repercussions: illegal memory accesses, corruption of data, buffer overrun, run-time exceptions, etc.

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. There are provisions for accessing the array as a whole, or only particular elements of the array. However, because of the recursive nature of the type system, sub-array access is limited to row-by-row access.

Array–pointer interchangeability

A unique (and sometimes 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 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 contexts (sizeof array being a notable exception), the name of an array is automatically converted to a pointer to the array's first element; this implies that arrays are never copied as a whole when named as arguments to functions, but rather only the address of its first element is passed. Therefore, although C's function calls use pass-by-value semantics, arrays are effectively passed by reference.

The number of elements in an array a can be determined as sizeof a / sizeof a[0], provided that the name is "in scope" (visible).

An interesting demonstration of the remarkable interchangeability of pointers and arrays is shown below. These four lines are equivalent and completely correct. Note how the last line shows the strange code i[x] = 1;, which has the index variable i apparently interchanged with the array variable x. This last line might be found in obfuscated C code.

x[i] = 1;
*(x + i) = 1;
*(i + x) = 1;
i[x] = 1; /* strange, but correct */

There is, however, a distinction to be made between arrays and pointer variables. Even though the name of an array is in most contexts converted to a pointer (to its first element), this pointer does not itself occupy any storage. Consequently, you cannot change what an array "points to", and it is impossible to assign anything to an array.

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 can be 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 has a small amount of overhead during initialization, 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 than either static memory or heap space, and only 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 hassle of manually allocating and releasing storage. Unfortunately, many data structures can grow in size at runtime; since automatic and static allocations must have a fixed size at compile-time, there are many situations in which dynamic allocation must be used. Variable-sized arrays are a common example of this (see "malloc" for an example of dynamically allocated arrays).

Libraries

The C programming language uses libraries as its primary method of extension. In C, a library is a collection of functions contained within a single 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, the header file from that library must be declared at the top of a source file, and the library must be linked to the program, which in many cases requires compiler flags (e.g., -lmath).

The most common C library is the C standard library, which is specified by the ISO and ANSI C standard and comes standard with every modern C compiler. The ANSI C standard library provides functionality for 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 generates fast 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.

Criticism

Many beginning programmers have difficulty learning C's syntax and peculiarities, and even many expert programmers find C programs difficult to maintain and debug. A popular saying, repeated by such notable language designers as Bjarne Stroustrup, is that "C makes it easy to shoot yourself in the foot." [3] In other words, C permits many operations that are generally not desirable, and thus many simple programming errors are not detected by the compiler and may not even be readily apparent at runtime. This potentially leads to programs with unpredictable behavior and security holes.

The designers wanted to avoid compile- and run-time checks that were too expensive when C was first implemented. With time, external tools were developed to perform some of these checks. Nothing prevents an implementation from providing such checks, but nothing requires it to, either. The safe C dialect Cyclone addresses some of these concerns.

Even Kernigan and Ritchie made reference to the basic design philosophy of C in their response to criticism of C not being a strongly-typed language: "Nevertheless, C retains the basic philosophy that programmers know what they are doing; it only requires that they state their intentions explicitly."

Memory allocation

One issue to be aware of when using C is that automatically and dynamically allocated objects are not necessarily initialized (depending on what facility is used to allocate memory); they initially have an indeterminate value (typically whatever values are present in the memory space they occupy, which might not even be a legal bit pattern for that type). This value is highly unpredictable and can vary between two machines, two program runs, or even two calls to the same function. If the program attempts to use such 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 common problem is that heap memory has to be manually synchronized with its actual usage in any program for it to be correctly reused as much as possible. For example, if an automatic pointer variable goes out of scope or has its value overwritten while still referencing a particular allocation that is not freed via a call to free(), then that memory cannot be recovered for later reuse and is essentially lost to the program, a phenomenon known as memory leak. Conversely, it is possible to release memory too soon, and in some cases continue to be able to use it, but since the allocation system can re-allocate the memory at any time for unrelated reasons, this results in unpredictable behavior, typically manifested in portions of the program far removed from the erroneously written segment. Such issues are ameliorated in languages with automatic garbage collection or RAII.

Pointers

Pointers are a primary source of potential danger. Because they are typically unchecked, a pointer can be made to point to any arbitrary location (even within code), causing unpredictable effects. Although properly-used pointers point to safe places, they can be moved to unsafe places using pointer arithmetic; the memory they point to may be deallocated and reused (dangling pointers); they may be uninitialized (wild pointers); or they may be directly assigned a 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. Other languages attempt to address these problems by using more restrictive reference types.

Arrays

Although C has native support for static arrays, it is not required to verify that array indexes are valid (bounds checking). For example, one can write to the sixth element of an array with five elements, yielding generally undesirable results. This type of bug, called a buffer overflow, has been notorious as the source of a number of security problems. On the other hand, since bounds checking elimination technology was largely nonexistent when C was defined, bounds checking came with a severe performance penalty, particularly in numerical computation.

Multidimensional arrays are commonly used in numerical algorithms (mainly from applied linear algebra) to store matrices. The structure of the C array is particularly well suited to this particular task, provided one remembers to count indices starting from 0 instead of 1. This issue is discussed in the book Numerical Recipes in C, chapter 1.2, page 20ff (read online). In that book there is also a solution based on negative addressing which introduces other dangers. Starting indices at 0 has been assimilated into the computing culture, and is no longer as alien a notion as it seemed when C was first introduced.

Variadic functions

Another source of bugs is variadic functions, which take a variable number of arguments. Unlike other prototyped C functions, checking the types of arguments to variadic functions at compile-time is, in general, impossible without additional information. If the wrong type of data is passed, the effect is unpredictable, and often fatal. Variadic functions also handle null pointer constants in a way which is often surprising to those unfamiliar with the language semantics. For example, NULL must be cast to the desired pointer type when passed to a variadic function. The printf family of functions supplied by the standard library, used to generate formatted text output, has been noted for its error-prone variadic interface, which relies on a format string to specify the number and type of trailing arguments.

However, type-checking of variadic functions from the standard library is a quality-of-implementation issue; many modern compilers do type-check printf calls, producing warnings if the argument list is inconsistent with the format string. Even so, not all printf calls can be checked statically since the format string can be built at runtime, and other variadic functions typically remain unchecked.

Syntax

Although mimicked by many languages because of its widespread familiarity, C's syntax has been often targeted as one of its weakest points. For example, Kernighan and Ritchie say in the second edition 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." Bjarne Stroustrup said of C++ (which is superficially similar to C): "Within C++, there is a much smaller and cleaner language struggling to get out. […] the C++ semantics is much cleaner than its syntax." [4] Some specific problems worth noting are:

• A function prototype with an empty parameter list allows any set of parameters, a syntax problem introduced for 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. Ritchie made this syntax design decision consciously, based primarily on the argument that assignment occurs more often than comparison. However, as explained by computer scientist Damian Conway in his "Seven Deadly Sins of Introductory Programming Language Design": "Many students, when confronted with this operator, become confused as to the nature of assignment and its relationship to equality. […] [A different syntax] seems to evoke less confusion, [because it] reinforces the notion of procedural transfer of value, rather than transitive equality of value.".[5]
• Similarly, the similarity of the assignment and equality operators (= and ==) makes it easy to substitute one for the other, and C's weak type system permits each to be used in the context of the other without a compiler error (although some produce warnings). [6]
• A lack of infix operators for complex objects, particularly for string operations, making programs which rely heavily on these operations difficult to read. The Lisp language, with no infix operators whatsoever, exhibits this problem to an even greater extent.
• Heavy reliance on punctuation-based symbols even where this is arguably less clear, such as "&&" and "||" instead of "and" and "or," respectively. Some are also confused about the difference between bit-wise operators ("&" and "|") and logical operators ("&&" and "||").
• Unintuitive declaration syntax, particularly for function pointers. In the words of language researcher Damian Conway speaking about the very similar C++ declaration syntax:

Specifying a type in C++ is made difficult by the fact that some of the components of a declaration (such as the pointer specifier) are prefix operators while others (such as the array specifier) are postfix. These declaration operators are also of varying precedence, necessitating careful bracketing to achieve the desired declaration. Furthermore, if the type ID is to apply to an identifier, this identifier ends up at somewhere between these operators, and is therefore obscured in even moderately complicated examples (see Appendix A for instance). The result is that the clarity of such declarations is greatly diminished. Ben Werther & Damian Conway. A Modest Proposal: C++ Resyntaxed. Section 3.1.1. 1996.

Economy of expression

One occasional criticism of C is that it can be concise to the point of being cryptic. A classic example that appears in K&R is the following function to copy the contents of string t to string s:

void strcpy(char *s, char *t)
{
    while (*s++ = *t++);
}

In this example, t is a pointer to a null-terminated array of characters, s is a pointer to an array of characters. Every loop of the single while statement does the following:

• Copies the character pointed to by t (initially set to point to the first character of the string to be copied) to the corresponding character position in s (initially set to point to the first character of the character array to be copied to)
• Advances the pointers s and t to point to the next character. Note that the values of s and t can safely be changed, because they are local copies of the pointers to the corresponding arrays
• Tests whether the character copied (the result of the assignment statement) is a null character signifying the end of the string. Note that the test could have been written "((*s++ = *t++) != '\0')" (where '\0' is the null character); however, in C, both Boolean values and characters are represented as small integers and are therefore interchangeable, and consequently the test is true as long as the character has any non-zero value (i.e., is any character other than a string-terminating null)
• As long as the character is not a null, the condition is true, causing the while loop to repeat. (In particular, because the character copy occurs before the condition is evaluated, the final terminating null is guaranteed to be copied as well)
• The body of the while loop is an empty statement, signified by the terminating semi-colon. (It is not uncommon for the body of while or for loops to be empty.)

The above code is functionally equivalent to:

void strcpy(char *s, char *t)
{
    char aux;
    do {
        *s = *t;
        aux = *s;
        s++;
        t++;
    } while (aux != '\0');
}

In a modern optimising compiler, these two pieces of code produce identical assembly code, so the smaller code does not produce smaller output. In more verbose languages such as Pascal, the above single statement would require several statements to implement. For C programmers, the economy of style is idiomatic and leads to shorter expressions; for critics, being able to do too much with a single line of C code can lead to problems in comprehension.

Maintenance

There are other problems in C that don't directly result in bugs or errors, but make it harder for programmers to build a robust, maintainable, large-scale system. Examples of these include:

• A fragile system for importing definitions (#include) that relies on literal text inclusion and redundantly keeping prototypes and function definitions in sync, and drastically increases build times.
• A cumbersome compilation model that forces manual dependency tracking and inhibits compiler optimizations between modules (except by link-time optimization).
• A weak type system that lets many clearly erroneous programs compile without errors.

Tools for mitigating issues with C

Tools have been created to help C programmers avoid these problems in many cases.

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.

There are also compilers, libraries and operating system level mechanisms for performing array bounds checking, buffer overflow detection, and automatic garbage collection, that are not a standard part of C.

Many compilers, most notably Visual C++, deal with the long compilation times inflicted by header file inclusion using precompiled headers, a system where declarations are stored in an intermediate format that is quick to parse. Building the precompiled header files in the first place is expensive, but this is generally done only for system header files, which are larger and more numerous than most application header files and also change much less often.

Cproto is a program that will read a C source file and output prototypes of all the functions within the source file. This program can be used in conjunction with the "make" command to create new files containing prototypes each time the source file has been changed. These prototype files can be included by the original source file (e.g., as "filename.p"), which reduces the problems of keeping function definitions and source files in agreement.

It should be recognized that these tools are not a panacea. Because of C's flexibility, some types of errors involving misuse of variadic functions, out-of-bounds array indexing, and incorrect memory management cannot be detected on some architectures without incurring a significant performance penalty. However, some common cases can be recognized and accounted for.

Related languages

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 preprocessors -- source code was translated into C, and then compiled with a C compiler.

C++

The C++ programming language was derived from C and is Bjarne Stroustrup's answer to adding 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. Considered a principal superset of C, C++ supports most of C, with some few but relevant exceptions (mostly of stronger typing restriction; see Compatibility of C and C++ for an exhaustive list of differences).

Objective-C

Objective-C is a very "thin" layer on top of and 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 is taken from Smalltalk. Objective-C and C++ differ in philosophies -- see the Objective-C article for details.

See also

• C preprocessor
• C standard library
• C string
• C syntax
• C variable types and declarations
• Comparison of programming languages
• C++
• C#
• List of articles with C programs
• Objective-C
• Operators in C and C++
• Programming tools: Cygwin, Dev-C/C++, DJGPP, GNU Compiler Collection, LCC, Linker, make, lint, Small-C, C--
• Pascal and C
• C to Java Virtual Machine compilers

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.
• ISO/IEC 9899. The official C:1999 standard, along with defect reports and a rationale. As of 2005 the latest version is ISO/IEC 9899:TC2.
• 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.
  • 4th, Prentice Hall 1994; ISBN 0-133-26224-3.
  • 5th, Prentice Hall 2002; ISBN 0-130-89592-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

External links

Tutorials

• C Programming at Wikibooks
• Everything You Ever Wanted to Know about C Types
• C Programming (course at University of Strathclyde Computer Centre)
• The C Book by M.Banahan-D.Brady-M.Doran (Addison-Wesley, 2nd ed.) — A very interesting and complete book for beginners/intermediate, now off-print and free.
• C Unleashed by Richard Heathfield and others contributors in comp.lang.c (Sams) — An interesting book about advanced ISO-C.
• Essential C — Short C guide, great for programmers new to C.
• Everything you need to know about pointers in C

Resources

• Basic concepts in the C language
• ISO C Working Group (official Web site)
• comp.lang.c Frequently Asked Questions
• comp.lang.c Wiki
• GNU C Library documentary (can also download source files)
• Programming in C (document collection at Lysator)
• The New C Standard: An economic and cultural commentary — An unpublished book about "detailed analysis of the International Standard for the C language. (PDF)"

Optimization techniques

• Programming Optimization
• C optimization tutorial

C99

• The current Standard (C99 with Technical corrigenda TC1 and TC2 included) - in PDF
• Open source development using C99 — Is your C code up to standard? by Peter Seebach
• Are you Ready For C99?

Support

• C Forum at Cprogramming.com
• C and C++ at Daniweb

History

• The Development of the C Language by Dennis M. Ritchie

Miscellaneous

• International Obfuscated C Code Contest