C (programming language)

Early developments

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 Dennis Ritchie and Ken Thompson, incorporating several ideas from colleagues. Eventually, they decided to port the operating system to a PDP-11. The original PDP-11 version of Unix was also developed in assembly language.^[12]

K&R C

In 1978, Brian Kernighan and Dennis Ritchie published the first edition of The C Programming Language.^[23] Known as K&R from the initials of its authors, the book 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". As this was released in 1978, it is now also referred to as C78.^[24] The second edition of the book^[25] covers the later ANSI C standard, described below.

K&R introduced several language features:

• Standard I/O library
• long int data type
• unsigned int data type
• Compound assignment operators of the form =op (such as =-) were changed to the form op= (that is, -=) to remove the semantic ambiguity created by constructs such as i=-10, which had been interpreted as i =- 10 (decrement i by 10) instead of the possibly intended i = -10 (let i be −10).

Even after the publication of the 1989 ANSI 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.

Although later versions of C require functions to have an explicit type declaration, K&R C only requires functions that return a type other than int to be declared before use. Functions used without prior declaration were presumed to return int.

For example:

long long_function();

calling_function()
{
    long longvar;
    register intvar;
    longvar = long_function();
    if (longvar > 1)
          intvar = 0;
    else
          intvar = int_function();
    return intvar;
}

The declaration of long_function() (on line 1) is required since it returns long; not int. Function int_function can be called (line 11) even though it is not declared since it returns int. Also, variable intvar does not need to be declared as type int since that is the default type for register keyword.

Since function declarations did not include information about arguments, type checks were not performed, although some compilers would issue a warning if different calls to a function used different numbers or types of arguments. Tools such as Unix's lint utility were developed that (among other things) checked for consistency of function use across multiple source files.

In the years following the publication of K&R C, several features were added to the language, supported by compilers from AT&T (in particular PCC^[26]) and other vendors. These included:

• void functions; functions returning no value
• Functions returning struct or union types
• Assignment for struct variables
• Enumerated types

The popularity of the language, lack of agreement on standard library interfaces, and lack of compliance to the K&R specification, led to standardization efforts.^[27]

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 increased significantly.

In 1983 the American National Standards Institute (ANSI) formed a committee, X3J11, to establish a standard specification of C. X3J11 based the C standard on the Unix implementation; however, the non-portable portion of the Unix C library was handed off to the IEEE working group 1003 to become the basis for the 1988 POSIX standard. In 1989, the C 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 international C standard, maintained by the working group ISO/IEC JTC1/SC22/WG14. National adoption of an update 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 subsequently introduced unofficial features. 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. Although the syntax for parameter declarations was augmented to include the style used in C++, the K&R interface continued to be permitted, for compatibility with existing source code.

C89 is supported by current C compilers, and most modern C code 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 prevent the use on a K&R C-based compiler of features available only in Standard C.

After the ANSI/ISO standardization process, the C language specification remained relatively static for several years. In 1995, Normative Amendment 1 to the 1990 C standard (ISO/IEC 9899/AMD1:1995, known informally as C95) was published, to correct some details and to add more extensive support for international character sets.^[28]

C99

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.^[29]

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 and flexible array members, improved support for IEEE 754 floating point, 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, Solaris Studio, and other C compilers now^[when?] support many or all of the new features of C99. The C compiler in Microsoft Visual C++, however, implements the C89 standard and those parts of C99 that are required for compatibility with C++11.^{[30][needs update]}

In addition, the C99 standard requires support for identifiers using Unicode in the form of escaped characters (e.g. \u0040 or \U0001f431) and suggests support for raw Unicode names.

C11

Work began in 2007 on another revision of the C standard, informally called "C1X" until its official publication of ISO/IEC 9899:2011 on December 8, 2011. The C standards committee adopted guidelines to limit the adoption of new features that had not been tested by existing implementations.

The C11 standard adds numerous new features to C and the library, including type generic macros, anonymous structures, improved Unicode support, atomic operations, multi-threading, and bounds-checked functions. It also makes some portions of the existing C99 library optional, and improves compatibility with C++. The standard macro __STDC_VERSION__ is defined as 201112L to indicate that C11 support is available.

C17

C17 is an informal name for ISO/IEC 9899:2018, a standard for the C programming language published in June 2018. It introduces no new language features, only technical corrections, and clarifications to defects in C11. The standard macro __STDC_VERSION__ is defined as 201710L to indicate that C17 support is available.

C23

C23 is an informal name for the current major C language standard revision. It was known as "C2X" through most of its development. It builds on past releases, introducing features like new keywords, additional meaning for auto to provide type inference when declaring variables, new types including nullptr_t and _BitInt(N), and expansions to the standard library.^[31]

C23 was published in October 2024 as ISO/IEC 9899:2024.^[32] The standard macro __STDC_VERSION__ is defined as 202311L to indicate that C23 support is available.

C29

C29 (C2Y) are informal names for the next major C language standard revision, after C23 (C2X), that is hoped to be released no later in the 2020s, hence the '2' in "C2Y". An early working draft of C2Y was released in February 2024 as N3220 by the working group ISO/IEC JTC1/SC22/WG14.^[33]

Embedded C

Historically, embedded C programming requires non-standard extensions to the C language to support exotic features such as fixed-point arithmetic, multiple distinct memory banks, and basic I/O operations.

In 2008, the C Standards Committee published a technical report extending the C language^[34] to address these issues by providing a common standard for all implementations to adhere to. It includes a number of features not available in normal C, such as fixed-point arithmetic, named address spaces, and basic I/O hardware addressing.

Character set

The basic C source character set includes the following characters:^[38]

• Lowercase and uppercase letters of the ISO basic Latin alphabet: a–z, A–Z
• Decimal digits: 0–9
• Graphic characters: ! " # % & ' ( ) * + , - . / : ; < = > ? [ \ ] ^ _ { | } ~
• Whitespace characters: space, horizontal tab, vertical tab, form feed, newline

The newline character indicates the end of a text line; it need not correspond to an actual single character, although for convenience C treats it as such.

The POSIX standard mandates a portable character set which adds a few characters (notably "@") to the basic C source character set. Both standards do not prescribe any particular value encoding—ASCII and EBCDIC both comply with these standards, since they include at least those basic characters, even though they use different encoded values for those characters.

Additional multi-byte encoded characters may be used in string literals, but they are not entirely portable. Since C99 multi-national Unicode characters can be embedded portably within C source text by using \uXXXX or \UXXXXXXXX encoding (where X denotes a hexadecimal character).

The basic C execution character set contains the same characters, along with representations for the null character, alert, backspace, and carriage return.^[38]

Run-time support for extended character sets has increased with each revision of the C standard.

Reserved words

All versions of C have reserved words that are case sensitive. As reserved words, they cannot be used for variable names.

C89 has 32 reserved words:

C99 added five more reserved words: (‡ indicates an alternative spelling alias for a C23 keyword)

C11 added seven more reserved words:^[39] (‡ indicates an alternative spelling alias for a C23 keyword)

C23 reserved fifteen more words:

Most of the recently reserved words begin with an underscore followed by a capital letter, because identifiers of that form were previously reserved by the C standard for use only by implementations. Since existing program source code should not have been using these identifiers, it would not be affected when C implementations started supporting these extensions to the programming language. Some standard headers do define more convenient synonyms for underscored identifiers. Some of those words were added as keywords with their conventional spelling in C23 and the corresponding macros were removed.

Prior to C89, entry was reserved as a keyword. In the second edition of their book The C Programming Language, which describes what became known as C89, Kernighan and Ritchie wrote, "The ... [keyword] entry, formerly reserved but never used, is no longer reserved." and "The stillborn entry keyword is withdrawn."^[40]

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: +, -, *, /, %
• assignment: =
• augmented assignment: +=, -=, *=, /=, %=, &=, |=, ^=, <<=, >>=
• bitwise logic: ~, &, |, ^
• bitwise shifts: <<, >>
• Boolean logic: !, &&, ||
• conditional evaluation: ? :
• equality testing: ==, !=
• calling functions: ( )
• increment and decrement: ++, --
• member selection: ., ->
• object size: sizeof
• type: typeof, typeof_unqual since C23
• order relations: <, <=, >, >=
• reference and dereference: &, *, [ ]
• sequencing: ,
• subexpression grouping: ( )
• type conversion: (typename)

C uses the operator = (used in mathematics to express equality) to indicate assignment, following the precedent of Fortran and PL/I, but unlike ALGOL and its derivatives. C uses the operator == to test for equality. The similarity between the operators for assignment and equality may result in the accidental use of one in place of the other, and in many cases the mistake does not produce an error message (although some compilers produce warnings). For example, the conditional expression if (a == b + 1) might mistakenly be written as if (a = b + 1), which will be evaluated as true unless the value of a is 0 after the assignment.^[41]

The C operator precedence is not always intuitive. For example, the operator == binds more tightly than (is executed prior to) the operators & (bitwise AND) and | (bitwise OR) in expressions such as x & 1 == 0, which must be written as (x & 1) == 0 if that is the coder's intent.^[42]

Data types

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The type system in C is static and weakly typed, which makes it similar to the type system of ALGOL descendants such as Pascal.^[43] There are built-in types for integers of various sizes, both signed and unsigned, floating-point numbers, and enumerated types (enum). Integer type char is often used for single-byte characters. C99 added a Boolean data type. There are also derived types including arrays, pointers, records (struct), and 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 data object in some other way.

Some find C's declaration syntax unintuitive, particularly for function pointers. (Ritchie's idea was to declare identifiers in contexts resembling their use: "declaration reflects use".)^[44]

C's usual arithmetic conversions allow for efficient code to be generated, but can sometimes produce unexpected results. For example, a comparison of signed and unsigned integers of equal width requires a conversion of the signed value to unsigned. This can generate unexpected results if the signed value is negative.

int func(int n, int m) {
    float (*p)[n][m] = malloc(sizeof *p);
    if (p == NULL) {
         return -1;
    }
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            (*p)[i][j] = i + j;
        }
    }
    print_array(n, m, p);
    free(p);
    return 1;
}

int func(int n, int m) {
    // Caution: checks should be made to ensure n * m * sizeof(float) does NOT exceed limitations for auto VLAs and is within available size of stack.
    float p[n][m]; // auto VLA is held on the stack, and sized when the function is invoked
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < m; j++) {
            p[i][j] = i + j;
        }
    }
    print_array(n, m, p);
    // no need to free(p) since it will disappear when the function exits, along with the rest of the stack frame
    return 1;
}

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 principal ways to allocate memory for objects:^[18]

• 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 realloc or free.

These three approaches are appropriate in different situations and have various trade-offs. For example, static memory allocation has little allocation overhead, automatic allocation may involve slightly more overhead, and dynamic memory allocation can potentially have a great deal of overhead for both allocation and deallocation. The persistent nature of static objects is useful for maintaining state information across function calls, automatic allocation is easy to use but stack space is typically much more limited and transient than either static memory or heap space, and dynamic memory allocation allows convenient 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 simplest 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 change in size at run time, and since static allocations (and automatic allocations before C99) must have a fixed size at compile time, there are many situations in which dynamic allocation is necessary.^[18] Prior to the C99 standard, variable-sized arrays were a common example of this. (See the article on C dynamic memory allocation for an example of dynamically allocated arrays.) Unlike automatic allocation, which can fail at run time with uncontrolled consequences, the dynamic allocation functions return an indication (in the form of a null pointer value) when the required storage cannot be allocated. (Static allocation that is too large is usually detected by the linker or loader, before the program can even begin execution.)

Unless otherwise specified, static objects contain zero or null pointer values upon program startup. Automatically and dynamically allocated objects are initialized only 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 can occur.

Heap memory allocation has to be synchronized with its actual usage in any program 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 it is deallocated explicitly, 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 for memory to be freed but referenced subsequently, leading to unpredictable results. Typically, the failure symptoms appear in a portion of the program unrelated to the code that causes the error, making it difficult to diagnose the failure. Such issues are ameliorated in languages with automatic garbage collection.

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. 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 "link the math library").^[18]

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 (implementations which target limited environments such as embedded systems may provide only a subset of the standard library). This library supports stream input and output, memory allocation, mathematics, character strings, and time values. Several separate standard headers (for example, stdio.h) specify the interfaces for these and other standard library facilities.

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.^[18]

Rationale for use in systems programming

C is widely used for systems programming in implementing operating systems and embedded system applications.^[52] This is for several reasons:

• The C language permits platform hardware and memory to be accessed with pointers and type punning, so system-specific features (e.g. Control/Status Registers, I/O registers) can be configured and used with code written in C – it allows fullest control of the platform it is running on.
• The code generated by compilation does not demand many system features, and can be invoked from some boot code in a straightforward manner – it is simple to execute.
• The C language statements and expressions typically map well to sequences of instructions for the target processor, and consequently there is a low run-time demand on system resources – it is fast to execute.
• With its rich set of operators, the C language can use many of the features of target CPUs. Where a particular CPU has more esoteric instructions, a language variant can be constructed with perhaps intrinsic functions to exploit those instructions – it can use practically all the target CPU's features.
• The language makes it easy to overlay structures onto blocks of binary data, allowing the data to be comprehended, navigated and modified – it can write data structures, even file systems.
• The language supports a rich set of operators, including bit manipulation, for integer arithmetic and logic, and perhaps different sizes of floating point numbers – it can process appropriately structured data effectively.
• C is a fairly small language, with only a handful of statements, and without too many features that generate extensive target code – it is comprehensible.
• C has direct control over memory allocation and deallocation, which gives reasonable efficiency and predictable timing to memory-handling operations, without any concerns for sporadic stop-the-world garbage collection events – it has predictable performance.
• C permits the use and implementation of different memory allocation schemes, including a typical malloc and free; a more sophisticated mechanism with arenas; or a version for an OS kernel that may suit DMA, use within interrupt handlers, or integrated with the virtual memory system.
• Depending on the linker and environment, C code can also call libraries written in assembly language, and may be called from assembly language – it interoperates well with other lower-level code.
• C and its calling conventions and linker structures are commonly used in conjunction with other high-level languages, with calls both to C and from C supported – it interoperates well with other high-level code.
• C has a mature and broad ecosystem, including libraries, frameworks, open source compilers, debuggers and utilities, and is the de facto standard. It is likely the drivers already exist in C, or that there is a similar CPU architecture as a back-end of a C compiler, so there is reduced incentive to choose another language.

Games

Computer games are often built from a combination of languages. C has featured significantly, especially for those games attempting to obtain best performance from computer platforms. Examples include Doom from 1993.^[53]

World Wide Web

Historically, C was sometimes used for web development using the Common Gateway Interface (CGI) as a "gateway" for information between the web application, the server, and the browser.^[54] C may have been chosen over interpreted languages because of its speed, stability, and near-universal availability.^[55] It is no longer common practice for web development to be done in C,^[56] and many other web development languages are popular. Applications where C-based web development continues include the HTTP configuration pages on routers, IoT devices and similar, although even here some projects have parts in higher-level languages e.g. the use of Lua within OpenWRT.

Two popular web servers, Apache HTTP Server and nginx, are written in C.^{[57][58][better source needed]} C's close-to-the-metal approach allows for the construction of these high-performance software systems.^{[59][better source needed]}

C as an intermediate language

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, additional machine-specific code generators are not necessary. C has some features, such as line-number preprocessor directives and optional superfluous commas at the end of initializer lists, that support compilation of generated code. However, some of C's shortcomings have prompted the development of other C-based languages specifically designed for use as intermediate languages, such as C--. Also, contemporary major compilers GCC and LLVM both feature an intermediate representation that is not C, and those compilers support front ends for many languages including C.

Computationally intensive libraries

C enables programmers to create efficient implementations of algorithms and data structures, because the layer of abstraction from hardware is thin, and its overhead is low, an important criterion for computationally intensive programs. For example, the GNU Multiple Precision Arithmetic Library, the GNU Scientific Library, Mathematica, and MATLAB are completely or partially written in C. Many languages support calling library functions in C; for example, the Python-based framework NumPy uses C for the high-performance and hardware-interacting aspects.

Other languages are written in C

A consequence of C's wide availability and efficiency is that compilers, libraries and interpreters of other programming languages are often implemented in C.^[60] For example, the reference implementations of Python,^[61] Perl,^[62] Ruby,^[63] and PHP^[64] are written in C.

Mitigations for C's problems

For some purposes, restricted styles of C have been adopted, e.g. MISRA C or CERT C, in an attempt to reduce the opportunity for unwanted behaviour.^[73] Databases such as CWE attempt to count the ways that systems in general, especially those coded in C, have potential vulnerabilities, along with recommendations for mitigation.

There are tools that can mitigate some of the drawbacks. Contemporary C compilers include checks which may generate warnings to help identify many potential bugs.

C's use of pointers can be made less risky by use of instruction set architecture extensions such as CHERI or Permission Overlay Extensions. These techniques change the fundamental nature of pointers at a hardware level to include bounds checks and purposes, which can help prevent buffer over-runs and inappropriate heap accesses.

Since the early 2020s the Linux kernel has sections written in Rust, a language which has specific measures to improve safety.^[74]