C (programming language): Difference between revisions

{{Redirect|C programming language|the book|The C Programming Language}}

| logo = The C Programming Language logo.svg

| logo caption = Logotype used on the cover of the first edition of ''[[The C Programming Language]]''<ref name="C in a Nutshell">{{Cite book|url={{GBurl|id=4Mfe4sAMFUYC}}|title=C in a Nutshell|last1=Prinz|first1=Peter|last2=Crawford|first2=Tony|date=December 16, 2005|publisher=O'Reilly Media, Inc.|isbn=9780596550714|page=3|language=en}}</ref>

| latest preview version = C2y (N3220)

| influenced = [[:Category:C programming language family|Numerous]]: [[AMPL]], [[AWK]], [[C shell|csh]], [[C++]], [[C--]], [[C Sharp (programming language)|C#]], [[Objective-C]], [[D (programming language)|D]], [[Go (programming language)|Go]], [[Java (programming language)|Java]], [[JavaScript]], [[JS++]], [[Julia (programming language)|Julia]], [[Limbo (programming language)|Limbo]], [[LPC (programming language)|LPC]], [[Perl]], [[PHP]], [[Pike (programming language)|Pike]], [[Processing (programming language)|Processing]], [[Python (programming language)|Python]], [[Rust (programming language)|Rust]], [[Seed7]], [[V (programming language)|V (Vlang)]], [[Vala (programming language)|Vala]], [[Verilog]] (HDL),<ref name="vinsp">{{cite web|title=Verilog HDL (and C)|url=http://cs.anu.edu.au/courses/ENGN3213/lectures/lecture6_VERILOG_2010.pdf|date=June 3, 2010|access-date=August 19, 2013|publisher=The Research School of Computer Science at the Australian National University|quote=1980s: Verilog first introduced; Verilog inspired by the C programming language|url-status=dead|archive-url=https://web.archive.org/web/20131106064022/http://cs.anu.edu.au/courses/ENGN3213/lectures/lecture6_VERILOG_2010.pdf|archive-date=November 6, 2013}}</ref> [[Nim (programming language)|Nim]], [[Zig (programming language)|Zig]]

| influenced by = [[B (programming language)|B]] ([[BCPL]], [[CPL (programming language)|CPL]]), [[ALGOL 68]],{{efn |"The scheme of type composition adopted by C owes considerable debt to Algol 68, although it did not, perhaps, emerge in a form that Algol's adherents would approve of."{{sfnp|Ritchie|1993a|p=8}}{{sfnp|Ritchie|1993b|p=8}}{{sfnp|Ritchie|2003}} }} [[PL/I]], [[Fortran]]

| website = {{ubl|{{URL|https://www.c-language.org/|c-language.org}}|{{URL|https://www.iso.org/standard/82075.html|iso.org}}|{{URL|www.open-std.org/jtc1/sc22/wg14/|open-std.org}}}}

'''C'''{{efn|Pronounced {{IPAc-en|'|s|iː}}, like the letter {{gloss |[[c]]}}.<ref>{{cite web |title=The name is based on, and pronounced like the letter C in the English alphabet |url=https://eng.ichacha.net/pronounce/the%20c%20programming%20language.html |url-status=live |archive-url=https://web.archive.org/web/20221117151137/https://eng.ichacha.net/pronounce/the%20c%20programming%20language.html |archive-date=November 17, 2022 |access-date=November 17, 2022 |website=the c programming language sound |publisher=English Chinese Dictionary |language=en-US}}</ref>}} is a [[general-purpose programming language]]. It was created in the 1970s by [[Dennis Ritchie]] and remains widely used and influential. By design, C gives the [[programmer]] relatively direct access to the features of the typical [[Central processing unit|CPU]] architecture; customized for the target [[Instruction set architecture|instruction set]]. It has been and continues to be used to implement [[operating system]]s (especially [[Kernel (operating system)|kernels]]<ref>{{Cite web |last=Munoz |first=Daniel |title=After All These Years, the World is Still Powered by C Programming {{!}} Toptal |url=https://www.toptal.com/c/after-all-these-years-the-world-is-still-powered-by-c-programming |access-date=June 15, 2024 |website=Toptal Engineering Blog}}</ref>), [[device driver]]s, and [[protocol stack]]s, but its use in [[application software]] has been decreasing.<ref>{{cite web |date=August 9, 2016 |title=C Language Drops to Lowest Popularity Rating |url=https://www.developer.com/news/c-language-drops-to-lowest-popularity-rating/ |url-status=dead |archive-url=https://web.archive.org/web/20220822225609/https://www.developer.com/news/c-language-drops-to-lowest-popularity-rating/ |archive-date=August 22, 2022 |access-date=August 1, 2022 |website=Developer.com |language=en-US}}</ref> C is used on computers that range from the largest [[supercomputer]]s to the smallest [[microcontroller]]s and [[embedded system]]s.

A successor to the programming language [[B (programming language)|B]], C was originally developed at [[Bell Labs]] by Ritchie between 1972 and 1973 to construct utilities running on [[Unix]]. It was applied to re-implementing the kernel of the Unix operating system.{{sfnp|Ritchie|1993a}} During the 1980s, C gradually gained popularity. It has become one of the most widely used [[programming language]]s,<ref name="langpop">{{cite web |url=http://www.langpop.com/ |title=Programming Language Popularity |year=2009 |access-date=January 16, 2009 |url-status=dead |archive-url= https://web.archive.org/web/20090116080326/http://www.langpop.com/ |archive-date=January 16, 2009}}</ref><ref name="TIOBE-2009">{{cite web |url=http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html | title=TIOBE Programming Community Index |year=2009 |access-date=May 6, 2009 |url-status=dead |archive-url= https://web.archive.org/web/20090504181627/http://www.tiobe.com/index.php/content/paperinfo/tpci/index.html |archive-date=May 4, 2009}}</ref> with C [[compiler]]s available for practically all modern [[computer architecture]]s and [[operating system]]s. The book ''[[The C Programming Language]]'', co-authored by the original language designer, served for many years as the ''de facto'' standard for the language.<ref name="ward198308">{{cite news |last=Ward |first=Terry A.  |url=https://archive.org/stream/byte-magazine-1983-08/1983_08_BYTE_08-08_The_C_Language#page/n267/mode/2up |title=Annotated C / A Bibliography of the C Language |work=Byte |date=August 1983 |access-date=January 31, 2015 |pages=268}}</ref><ref name="C in a Nutshell"/> C has been standardized since 1989 by the [[American National Standards Institute]] (ANSI) and, subsequently, jointly by the [[International Organization for Standardization]] (ISO) and the [[International Electrotechnical Commission]] (IEC).

Since 2000, C has consistently ranked among the top four languages in the [[TIOBE index]], a measure of the popularity of programming languages.<ref>{{cite web|title=TIOBE Index for September 2024|url=https://www.tiobe.com/tiobe-index/|access-date=September 20, 2024|archive-date=September 18, 2024|archive-url=https://web.archive.org/web/20240918165843/https://www.tiobe.com/tiobe-index/|url-status=live}}</ref> Originally, C was popular mostly due to being easier to use than other programming languages.{{cn |date=July 2025}} Currently, C is popular mostly due to speed, efficiency, low memory usage, and simplicity.{{cn |date=July 2025}} C uses approximately 80 times less energy than Python, Perl, and PHP. <ref>{{cite web|title=Original work in SLE'17|url=https://sites.google.com/view/energy-efficiency-languages/results}}</ref> On average, C uses less energy than [[Fortran]], despite Fortran being faster on average.{{fact |date=July 2025}}

[[File:Ken n dennis.jpg|thumb|[[Dennis Ritchie]] (right), the inventor of the C programming language, with [[Ken Thompson]]]]

 

* [[Executable code]] is contained in [[function]]s; no script-like syntax

* [[Arithmetic]], [[bitwise]], and logic operators, including {{codes|+|+{{=}}|++|&amp;|{{!!}}|d=,}}, etc.

* User-defined identifiers are not distinguished from keywords; i.e. by a [[Sigil (computer programming)|sigil]]

* A function cannot be nested inside a function; but some translators support this

* Function pointers support [[run-time polymorphism]]

* Supports [[Recursion (computer science)|recursion]]; a function may call itself

* User-defined [[Union type|union]] types support overlapping members; allowing multiple data types to share the same [[memory location]]

* Lacks a [[String (computer science)|string type]] but has syntax for [[null-terminated string|null-terminated strings]] with associated [[C string handling|handling]] in its standard library

* Resulting compiled code has relatively straightforward needs on the underlying platform; making it desirable for operating and [[embedded system|embedded]] systems

[[File:Hello World Brian Kernighan 1974.jpg|thumb|"Hello, World!" program by [[Brian Kernighan]] (1978)]]

The next code line declares the [[entry point]] function <code>main()</code>. The [[run-time environment]] calls this function to begin program execution. The type specifier <code>int</code> indicates that the function returns an integer value. The <code>void</code> parameter list indicates that the function consumes no arguments. The run-time environment actually passes two arguments (typed <code>int</code> and <code>char *[]</code>), but this implementation ignores them. The ISO C standard (section 5.1.2.2.1) requires syntax that either is void or these two arguments {{endash}} a special treatment not afforded to other functions.

| [[#C2Y|C2Y]]

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.{{sfnp|Ritchie|1993a}}

Thompson wanted a programming language for developing utilities for the new platform. He first tried writing a [[Fortran]] compiler, but he soon gave up the idea and instead created a cut-down version of the recently developed [[systems programming language]] called [[BCPL]]. The official description of BCPL was not available at the time,<ref name="NFDsZ">{{cite web |url=https://www.lysator.liu.se/c/dmr-on-histories.html |first=Dennis |last=Ritchie |title=BCPL to B to C |website=lysator.liu.se |access-date=September 10, 2019 |archive-date=December 12, 2019 |archive-url=https://web.archive.org/web/20191212221532/http://www.lysator.liu.se/c/dmr-on-histories.html |url-status=live }}</ref> and Thompson modified the syntax to be less 'wordy' and similar to a simplified [[ALGOL]] known as SMALGOL.<ref name="Ars">{{Cite web |last=Jensen |first=Richard |date=December 9, 2020 |title="A damn stupid thing to do"—the origins of C |url=https://arstechnica.com/features/2020/12/a-damn-stupid-thing-to-do-the-origins-of-c/ |access-date=March 28, 2022 |website=Ars Technica |language=en-us |archive-date=March 28, 2022 |archive-url=https://web.archive.org/web/20220328143845/https://arstechnica.com/features/2020/12/a-damn-stupid-thing-to-do-the-origins-of-c/ |url-status=live }}</ref> He called the result [[B (programming language)|''B'']],{{sfnp|Ritchie|1993a}} describing it as "BCPL semantics with a lot of SMALGOL syntax".<ref name=Ars /> Like BCPL, B had a [[bootstrapping]] compiler to facilitate porting to new machines.<ref name=Ars /> Ultimately, few utilities were written in B because it was too slow and could not take advantage of PDP-11 features such as [[byte]] addressability.

Unlike BCPL's <code>// comment</code> marking comments up to the end of the line, B adopted <code>/* comment */</code> as the comment delimiter, more akin to PL/1, and allowing comments to appear in the middle of lines. (BCPL's comment style would be reintroduced in C++.){{sfnp|Ritchie|1993a}}

In 1971 Ritchie started to improve B, to use the features of the more-powerful PDP-11. A significant addition was a character data type. He called this ''New B'' (NB).<ref name=Ars /> Thompson started to use NB to write the [[Research Unix|Unix]] kernel, and his requirements shaped the direction of the language development.<ref name="Ars" /><ref name="unixport" />

The C compiler and some utilities made with it were included in [[Version 2 Unix]], which is also known as [[Research Unix]].<ref name="QtqTh">{{cite tech report |first=M. D. |last=McIlroy |author-link=Doug McIlroy |year=1987 |url=http://www.cs.dartmouth.edu/~doug/reader.pdf |title=A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986 |series=CSTR |number=139 |institution=Bell Labs |format=PDF |page=10 |access-date=February 1, 2015 |archive-date=November 11, 2017 |archive-url=https://web.archive.org/web/20171111151817/http://www.cs.dartmouth.edu/~doug/reader.pdf |url-status=live }}</ref>

Unix was one of the first operating system kernels implemented in a language other than [[assembly language|assembly]]. Earlier instances include the [[Multics]] system (which was written in [[PL/I]]) and [[Master Control Program]] (MCP) for the [[Burroughs large systems|Burroughs B5000]] (which was written in [[ALGOL]]) in 1961. In around <!--Better?: {{Circa|1977}}--> 1977, Ritchie and [[Stephen C. Johnson]] made further changes to the language to facilitate [[Software portability|portability]] of the Unix operating system. Johnson's [[Portable C Compiler]] served as the basis for several implementations of C on new platforms.<ref name="unixport">{{cite journal |last1=Johnson |first1=S. C. |author-link1=Stephen C. Johnson |last2=Ritchie |first2=D. M. |author-link2=Dennis Ritchie |title=Portability of C Programs and the UNIX System |journal=Bell System Tech. J. |year=1978 |volume=57 |issue=6 |pages=2021–2048 |doi=10.1002/j.1538-7305.1978.tb02141.x |citeseerx=10.1.1.138.35 |s2cid=17510065 |issn = 0005-8580 }} (Note: The PDF is an OCR scan of the original, and contains a rendering of "IBM 370" as "IBM 310".)</ref>

In 1978 [[Brian Kernighan]] and [[Dennis Ritchie]] published the first edition of ''[[The C Programming Language]]''.{{sfnp|Kernighan|Ritchie|1978}} Known as ''K&R'' from the initials of its authors, the book served for many years as an informal [[Specification (technical standard)|specification]] of the language. The version of C that it describes is commonly referred to as "'''K&R C'''<!--boldface per WP:R#PLA-->". As this was released in 1978, it is now also referred to as ''C78''.<ref name="qOvzA">{{cite book |url=https://nxmnpg.lemoda.net/7/c78 |title=FreeBSD Miscellaneous Information Manual |date=May 30, 2011 |edition=FreeBSD 13.0 |chapter=C manual pages |access-date=January 15, 2021 |archive-url=https://web.archive.org/web/20210121024455/https://nxmnpg.lemoda.net/7/c78 |archive-date=January 21, 2021 |url-status=live}} [https://www.freebsd.org/cgi/man.cgi?query=c78&apropos=0&sektion=0&manpath=FreeBSD+9-current&arch=default&format=html] {{Webarchive|url=https://web.archive.org/web/20210121033654/https://www.freebsd.org/cgi/man.cgi?query=c78&apropos=0&sektion=0&manpath=FreeBSD+9-current&arch=default&format=html|date=January 21, 2021}}</ref> The second edition of the book{{sfnp|Kernighan|Ritchie|1988}} covers the later [[ANSI C]] standard, described below.

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 [[Portable C Compiler|PCC]]<ref name="SkKfZ">{{cite report |first1=Bjarne |last1=Stroustrup |author-link=Bjarne Stroustrup |title=Sibling rivalry: C and C++ |publisher=AT&T Labs |number=TD-54MQZY |year=2002 |url=http://stroustrup.com/sibling_rivalry.pdf |access-date=April 14, 2014 |archive-date=August 24, 2014 |archive-url=https://web.archive.org/web/20140824072719/http://www.stroustrup.com/sibling_rivalry.pdf |url-status=live }}</ref>) and other vendors. These included:

The popularity of the language, lack of agreement on [[C standard library|standard library]] interfaces, and lack of compliance to the K&R specification, lead to standardization efforts.<ref>{{Cite web |url=https://www.cs.man.ac.uk/~pjj/cs211/c_rationale/node2.html |title=Rationale for American National Standard for Information Systems – Programming Language – C |access-date=July 17, 2024 |archive-url=https://web.archive.org/web/20240717164722/https://www.cs.man.ac.uk/~pjj/cs211/c_rationale/node2.html |archive-date=July 17, 2024}}</ref>

<!-- [[WP:NFCC]] violation: [[File:The C Programming Language cover.svg|thumb|240x240px|The cover of the book, ''[[The C Programming Language]]'', second edition by [[Brian Kernighan]] and [[Dennis Ritchie]] covering ANSI C]] -->

During the late 1970s and 1980s, versions of C were implemented for a wide variety of [[mainframe computer]]s, [[minicomputer]]s, and [[microcomputer]]s, 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. 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'''.

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.

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 [[Computing platform|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]].

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.<ref name="NWUon">{{cite book |author=<!--Staff writer(s); no by-line.--> |title=C Integrity |url=https://www.iso.org/standard/23909.html |publisher=International Organization for Standardization |date=March 30, 1995 |access-date=July 24, 2018 |archive-date=July 25, 2018 |archive-url=https://web.archive.org/web/20180725033429/https://www.iso.org/standard/23909.html |url-status=live }}</ref>

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.<ref name="WG14">{{cite web |title=JTC1/SC22/WG14&nbsp;– C |url=http://www.open-std.org/jtc1/sc22/wg14/ |work=Home page |publisher=ISO/IEC |access-date=June 2, 2011 |archive-date=February 12, 2018 |archive-url=https://web.archive.org/web/20180212100115/http://www.open-std.org/JTC1/SC22/WG14/ |url-status=live }}</ref>

C99 introduced several new features, including [[inline function]]s, several new [[data type]]s (including <code>long long int</code> and a <code>complex</code> type to represent [[complex number]]s), [[variable-length array]]s and [[flexible array member]]s, improved support for [[IEEE 754]] floating point, support for [[variadic macro]]s (macros of variable [[arity]]), and support for one-line comments beginning with <code>//</code>, 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 <code>int</code> implicitly assumed. A standard macro <code>__STDC_VERSION__</code> is defined with value <code>199901L</code> to indicate that C99 support is available. [[GNU Compiler Collection|GCC]], [[Solaris Studio]], and other C compilers now{{when|date=August 2022}} 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]].<ref name="YTKIv">{{cite web |url=http://www.drdobbs.com/cpp/interview-with-herb-sutter/231900562 |title=Interview with Herb Sutter |website=[[Dr. Dobbs]] |author=Andrew Binstock |date=October 12, 2011 |access-date=September 7, 2013 |archive-date=August 2, 2013 |archive-url=https://web.archive.org/web/20130802070446/http://www.drdobbs.com/cpp/interview-with-herb-sutter/231900562 |url-status=live }}</ref>{{update inline|date=February 2021}}

{{Main|C11 (C standard revision)}}

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 <code>__STDC_VERSION__</code> is defined as <code>201112L</code> to indicate that C11 support is available.

{{Main|C17 (C standard revision)}}

{{Main|C23 (C standard revision)}}

C23 is an informal name for the current major C language standard revision and was known as "C2X" through most of its development. It builds on past releases, introducing features like new keywords, types including <code>nullptr_t</code> and <code>_BitInt(N)</code>, and expansions to the standard library.<ref>{{cite web |title=ISO/IEC 9899:2024 (en) — N3220 working draft |url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf |access-date=11 July 2025}}</ref>

C23 was published in October 2024 as ISO/IEC 9899:2024.<ref name="N3132">{{cite web |title=WG14-N3132 : Revised C23 Schedule |url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3132.pdf |website=open-std.org |archive-url=https://web.archive.org/web/20230609204739/https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3132.pdf |archive-date=June 9, 2023 |date=June 4, 2023 |url-status=live}}</ref> The standard macro <code>__STDC_VERSION__</code> is defined as <code>202311L</code> to indicate that C23 support is available.

C2Y is an informal name for the next major C language standard revision, after C23 (C2X), that is hoped to be released 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.<ref name="N3220">{{cite web |title=WG14-N3220 : Working Draft, C2y |url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf |website=open-std.org |archive-url=https://web.archive.org/web/20240226053735/https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf |archive-date=February 26, 2024 |date=February 21, 2024 |url-status=live}}</ref>

In 2008, the C Standards Committee published a [[technical report]] extending the C language<ref name="TR18037">{{cite web |title=TR 18037: Embedded C |url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1169.pdf |website=open-std.org |id=ISO/IEC JTC1 SC22 WG14 N1169 |date=April 4, 2006 |access-date=July 26, 2011 |archive-date=February 25, 2021 |archive-url=https://web.archive.org/web/20210225224616/https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1169.pdf |url-status=live }}</ref> 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.

C has a [[formal grammar]] specified by the C standard.<ref name="h&s5e">{{cite book |last1=Harbison |first1=Samuel P. |last2=Steele |first2=Guy L. |author-link2=Guy L. Steele, Jr. |title=C: A Reference Manual |edition=5th |publisher=[[Prentice Hall]] |year=2002 |location=[[Englewood Cliffs, NJ]] |isbn=978-0-13-089592-9}} Contains a [[Backus-Naur form|BNF]] grammar for C.</ref> Line endings are generally not significant in C; however, line boundaries do have significance during the preprocessing phase. Comments may appear either between the delimiters <code>/*</code> and <code>*/</code>, or (since C99) following <code>//</code> until the end of the line. Comments delimited by <code>/*</code> and <code>*/</code> do not nest, and these sequences of characters are not interpreted as comment delimiters if they appear inside [[String literal|string]] or character literals.{{sfnp|Kernighan|Ritchie|1988|p=192}}

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 (computer science)|side effect]] of the evaluation, [[Function (computer programming)|functions may be called]] and [[Assignment (computer science)|variables 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 <code>if</code> ... [<code>else</code>] conditional execution and by <code>do</code> ... <code>while</code>, <code>while</code>, and <code>for</code> iterative execution (looping). The <code>for</code> statement has separate initialization, testing, and reinitialization expressions, any or all of which can be omitted. <code>break</code> and <code>continue</code> can be used within the loop. Break is used to leave the innermost enclosing loop statement and continue is used to skip to its reinitialisation. There is also a non-structured <code>[[goto]]</code> statement which branches directly to the designated [[Label (computer science)|label]] within the function. <code>[[Switch statement|switch]]</code> selects a <code>case</code> to be executed based on the value of an integer expression. Different from many other languages, control-flow will [[Switch statement#Fallthrough|fall through]] to the next <code>case</code> unless terminated by a <code>break</code>.

Expressions can use a variety of built-in operators 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 (<code>&&</code>, <code>||</code>, <code>[[?:]]</code> 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.

The basic C source character set includes the following characters:

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.

The basic C execution character set contains the same characters, along with representations for [[Bell character|alert]], [[backspace]], and [[carriage return]]. [[Run time (program lifecycle phase)|Run-time]] support for extended character sets has increased with each revision of the C standard.

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

C11 added seven more reserved words:<ref name="ISOIEC 9899">{{Cite web|url=http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf|title=ISO/IEC 9899:201x (ISO C11) Committee Draft|website=open-std.org|date=December 2, 2010|access-date=September 16, 2011|archive-date=December 22, 2017|archive-url=https://web.archive.org/web/20171222215122/http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf|url-status=live}}</ref> (‡ indicates an alternative spelling alias for a C23 keyword)

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.

C uses the operator <code>=</code> (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 <code>==</code> 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 <code>if (a == b + 1)</code> might mistakenly be written as <code>if (a = b + 1)</code>, which will be evaluated as <code>true</code> unless the value of <code>a</code> is <code>0</code> after the assignment.<ref name="AutoTX-8">{{cite web |url=http://www.cs.ucr.edu/~nxiao/cs10/errors.htm |title=10 Common Programming Mistakes in C++ |website=Cs.ucr.edu |access-date=June 26, 2009 |archive-date=October 21, 2008 |archive-url=https://web.archive.org/web/20081021080953/http://www.cs.ucr.edu/~nxiao/cs10/errors.htm |url-status=live }}</ref>

The [[type system]] in C is [[static typing|static]] and [[Strong and weak typing|weakly typed]], which makes it similar to the type system of [[ALGOL]] descendants such as [[Pascal (programming language)|Pascal]].<ref name="Nmlwr">{{cite journal |last1=Feuer |first1=Alan R. |last2=Gehani |first2=Narain H. |date=March 1982 |title=Comparison of the Programming Languages C and Pascal |journal=ACM Computing Surveys |volume=14 |issue=1 |pages=73–92 |doi=10.1145/356869.356872 |s2cid=3136859}}</ref> There are built-in types for integers of various sizes, both signed and unsigned, [[floating-point number]]s, and enumerated types (<code>enum</code>). Integer type <code>char</code> is often used for single-byte characters. C99 added a [[Boolean data type]]. There are also derived types including [[Array (data type)|arrays]], [[Pointer (computer programming)|pointers]], [[record (computer science)|records]] (<code>[[Struct (C programming language)|struct]]</code>), and [[union (computer science)|unions]] (<code>union</code>).

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 conversion|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.

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.

C supports the use of [[Pointer (computer programming)|pointers]], a type of [[Reference (computer science)|reference]] that records 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 or [[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.

Pointers are used for many purposes in C. [[Text string]]s are commonly manipulated using pointers into arrays of characters. [[Dynamic memory allocation]] is performed using pointers; the result of a <code>malloc</code> is usually [[Type conversion|cast]] to the data type of the data to be stored. Many data types, such as [[Tree (data structure)|trees]], are commonly implemented as dynamically allocated <code>struct</code> objects linked together using pointers. Pointers to other pointers are often used in multi-dimensional arrays and arrays of <code>struct</code> objects. Pointers to functions (''[[function pointer]]s'') are useful for passing functions as arguments to [[higher-order function]]s (such as [[qsort]] or [[bsearch]]), in [[dispatch table]]s, or as [[callbacks]] to [[event handler]]s.<ref name="bk21st" />

A ''[[null pointer]] value'' explicitly points to no valid location. Dereferencing a null pointer value is undefined, often resulting in a [[segmentation fault]]. Null pointer values 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. In appropriate contexts in source code, such as for assigning to a pointer variable, a ''null pointer constant'' can be written as <code>0</code>, with or without explicit casting to a pointer type, as the <code>NULL</code> macro defined by several standard headers or, since C23 with the constant <code>nullptr</code>. In conditional contexts, null pointer values evaluate to <code>false</code>, while all other pointer values evaluate to <code>true</code>.

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 continue to be used after deallocation ([[dangling pointer]]s); they may be used without having been initialized ([[wild pointer]]s); 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 (computer science)|reference]] types.

[[Array (data type)|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 <code>malloc</code> function, and treat it as an array.

Since arrays are always accessed (in effect) via pointers, array accesses are typically ''not'' checked against the underlying array size, although some compilers may provide [[bounds checking]] as an option.<ref name="fedoraproject">For example, gcc provides _FORTIFY_SOURCE. {{cite web |url=http://fedoraproject.org/wiki/Security/Features |title=Security Features: Compile Time Buffer Checks (FORTIFY_SOURCE) |publisher=fedoraproject.org |access-date=August 5, 2012 |archive-date=January 7, 2007 |archive-url=https://web.archive.org/web/20070107153447/http://fedoraproject.org/wiki/Security/Features |url-status=live }}</ref><ref name="Programming with C">{{Cite book|title=Programming with C|last1=เอี่ยมสิริวงศ์|first1=โอภาศ|publisher=SE-EDUCATION PUBLIC COMPANY LIMITED|year=2016|isbn=978-616-08-2740-4|location=Bangkok, Thailand|pages=225–230}}</ref> Array bounds violations are therefore possible 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 [[multi-dimensional array]]s, but rather relies on [[Recursion (computer science)|recursion]] within the type system to declare arrays of arrays, which effectively accomplishes the same thing. The index values of the resulting "multi-dimensional array" can be thought of as increasing in [[row-major order]]. Multi-dimensional 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, in early versions of C 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 was to allocate the array with an additional "row vector" of pointers to the columns.) C99 introduced "variable-length arrays" which address this issue.

<syntaxhighlight lang="c">

int func(int N, int M)

{

  float (*p)[N] [M] = malloc(sizeof *p);

  if (p == 0)

    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;

And here is a similar implementation using C99's ''Auto [[Variable length array|VLA]]'' feature:{{efn|Code of <code>print_array</code> (not shown) slightly differs,{{why|date=November 2023}} too.}}

<syntaxhighlight lang="c">

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;

The subscript notation <code>x[i]</code> (where <code>x</code> designates a pointer) is [[syntactic sugar]] for <code>*(x+i)</code>.<ref name="Raymond1996">{{cite book |last1=Raymond |first1=Eric S. |author-link=Eric S. Raymond |title=The New Hacker's Dictionary |edition=3rd |url={{GBurl|id=g80P_4v4QbIC|p=432}} |access-date=August 5, 2012 |date=October 11, 1996 |publisher=MIT Press |isbn=978-0-262-68092-9 |page=432 }}</ref> Taking advantage of the compiler's knowledge of the pointer type, the address that <code>x + i</code> points to is not the base address (pointed to by <code>x</code>) incremented by <code>i</code> bytes, but rather is defined to be the base address incremented by <code>i</code> multiplied by the size of an element that <code>x</code> points to. Thus, <code>x[i]</code> designates the <code>i+1</code>th element of the array.

* [[Static memory allocation]]: space for the object is provided in the binary at compile-time; these objects have an [[Variable (programming)#Scope and extent|extent]] (or lifetime) as long as the binary which contains them is loaded into memory.

* [[C dynamic memory allocation|Dynamic memory allocation]]: blocks of memory of arbitrary size can be requested at run-time using library functions such as <code>malloc</code> from a region of memory called the [[Memory management|heap]]; these blocks persist until subsequently freed for reuse by calling the library function <code>realloc</code> or <code>free</code>.

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 runtime, 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.<ref name="bk21st" /> 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 (computing)|linker]] or [[Loader (computing)|loader]], before the program can even begin execution.)

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 is 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]].

The C programming language uses [[Library (computing)|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 flag]]s (e.g., <code>-lm</code>, shorthand for "link the math library").<ref name="bk21st" />

The most common C library is the [[C standard library]], which is specified by the [[ISO standard|ISO]] and [[ANSI C]] standards and comes with every C implementation (implementations which target limited environments such as [[embedded system]]s 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, <code>stdio.h</code>) specify the interfaces for these and other standard library facilities.

{{More citations needed section|date=July 2014}}

 

Memory management checking tools like [[IBM Rational Purify|Purify]] or [[Valgrind]] and linking with libraries containing special versions of the [[malloc|memory allocation functions]] can help uncover runtime errors in memory usage.<ref>{{Cite web |title=CS107 Valgrind Memcheck |url=https://web.stanford.edu/class/archive/cs/cs107/cs107.1236/resources/valgrind.html |access-date=June 23, 2023 |website=web.stanford.edu}}</ref><ref>{{Cite journal |last1=Hastings |first1=Reed |last2=Joyce |first2=Bob |title=Purify: Fast Detection of Memory Leaks and Access Errors |url=https://web.stanford.edu/class/cs343/resources/purify.pdf |journal=Pure Software Inc. |pages=9}}</ref>

C is widely used for [[systems programming]] in implementing [[operating system]]s and [[embedded system]] applications.<ref name="Zh3CW">{{Cite book|title=Programming and problem solving with C++ |edition=6th |last1=Dale |first1=Nell B. |last2=Weems |first2=Chip |year=2014 |location=Burlington, Massachusetts |publisher=Jones & Bartlett Learning |isbn=978-1449694289 |oclc=894992484}}</ref> This is for several reasons:

* The code generated after compilation does not demand many [[runtime system|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 on to sequences of instructions for the target processor, and consequently there is a low [[Run time (program lifecycle phase)|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 function]]s to exploit those instructions – it can use practically all the target CPU's features.

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.<ref>{{cite web |title=Development of Doom |url=https://doomwiki.org/wiki/Development_of_Doom |website=DoomWiki.org |access-date=2025-03-02 |language=en |date=2 March 2025}}</ref>

===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 [[GNU Compiler Collection|GCC]] and [[LLVM]] both feature an [[intermediate representation]] that is not C, and those compilers support front ends for many languages including C.

A consequence of C's wide availability and efficiency is that [[compiler]]s, libraries and [[Interpreter (computing)|interpreters]] of other programming languages are often implemented in C.<ref>{{Cite web |date=November 13, 2018 |title=C – the mother of all languages |url=https://ict.iitk.ac.in/c-the-mother-of-all-languages/ |access-date=October 11, 2022 |website=ICT Academy at IITK |language=en-US |archive-date=May 31, 2021 |archive-url=https://web.archive.org/web/20210531161841/https://ict.iitk.ac.in/c-the-mother-of-all-languages/ |url-status=dead }}</ref> For example, the [[reference implementation]]s of [[Python (programming language)|Python]],<ref>{{Cite web |title=1. Extending Python with C or C++ |website=Python 3.10.7 documentation |url=https://docs.python.org/3/extending/extending.html |access-date=October 11, 2022 |archive-date=November 5, 2012 |archive-url=https://web.archive.org/web/20121105232707/https://docs.python.org/3/extending/extending.html |url-status=live }}</ref> [[Perl]],<ref>{{Cite web |title=An overview of the Perl 5 engine |url=https://opensource.com/article/18/1/perl-5-engine |access-date=October 11, 2022 |website=Opensource.com |first1=Michael |last1=Conrad |date=January 22, 2018 |language=en |archive-date=May 26, 2022 |archive-url=https://web.archive.org/web/20220526105419/https://opensource.com/article/18/1/perl-5-engine |url-status=live }}</ref> [[Ruby (programming language)|Ruby]],<ref>{{Cite web |title=To Ruby From C and C++ |url=https://www.ruby-lang.org/en/documentation/ruby-from-other-languages/to-ruby-from-c-and-cpp/ |access-date=October 11, 2022 |website=Ruby Programming Language |archive-date=August 12, 2013 |archive-url=https://web.archive.org/web/20130812003928/https://www.ruby-lang.org/en/documentation/ruby-from-other-languages/to-ruby-from-c-and-cpp/ |url-status=live }}</ref> and [[PHP]]<ref>{{Cite web |date=August 3, 2022 |title=What is PHP? How to Write Your First PHP Program |url=https://www.freecodecamp.org/news/what-is-php-write-your-first-php-program/ |access-date=October 11, 2022 |website=freeCodeCamp |first1=Michael |last1=Para |language=en |archive-date=August 4, 2022 |archive-url=https://web.archive.org/web/20220804050401/https://www.freecodecamp.org/news/what-is-php-write-your-first-php-program/ |url-status=live }}</ref> are written in C.

===Once used for web development===

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.<ref name="Dobbs 1995">{{cite book |title=Dr. Dobb's Sourcebook |publisher=Miller Freeman, Inc. |date=November–December 1995 |location=U.S.}}</ref> C may have been chosen over [[interpreted language]]s because of its speed, stability, and near-universal availability.<ref name="linuxjournal 2005">{{cite web |url=http://www.linuxjournal.com/article/6863 |publisher=linuxjournal.com |title=Using C for CGI Programming |access-date=January 4, 2010 |date=March 1, 2005 |archive-date=February 13, 2010 |archive-url=https://web.archive.org/web/20100213075858/http://www.linuxjournal.com/article/6863 |url-status=live }}</ref>  It is no longer common practice for web development to be done in C,<ref>{{cite web |last1=Perkins |first1=Luc |title=Web development in C: crazy? Or crazy like a fox? |url=https://medium.com/@lucperkins/web-development-in-c-crazy-or-crazy-like-a-fox-ff723209f8f5 |website=Medium |language=en |date=September 17, 2013 |access-date=April 8, 2022 |archive-date=October 4, 2014 |archive-url=https://web.archive.org/web/20141004135317/https://medium.com/@lucperkins/web-development-in-c-crazy-or-crazy-like-a-fox-ff723209f8f5 |url-status=live }}</ref> and many other [[web development#Server-side languages|web development languages]] are popular.  Applications where C-based web development continues include the [[HTTP]] configuration pages on [[Router (computing)|routers]], [[IoT]] devices and similar, although even here some projects have parts in higher-level languages e.g. the use of [[Lua (programming language)|Lua]] within [[OpenWRT]].

===Web servers===

The two most popular [[web server]]s, [[Apache HTTP Server]] and [[Nginx]], are both written in C. These web servers interact with the operating system, listen on TCP ports for HTTP requests, and then serve up static web content, or cause the execution of other languages handling to 'render' content such as [[PHP]], which is itself primarily written in C.  C's close-to-the-metal approach allows for the construction of these high-performance software systems.

===End-user applications===

C has also been widely used to implement [[End-user (computer science)|end-user]] applications.<ref>{{Cite web |title=After All These Years, the World is Still Powered by C Programming |first1=Daniel |last1=Munoz |url=https://www.toptal.com/c/after-all-these-years-the-world-is-still-powered-by-c-programming |access-date=November 17, 2023 |website=Toptal Engineering Blog |language=en}}</ref> However, such applications can also be written in newer, higher-level languages.

Ritchie himself joked about the deficiencies of the language he created:<ref>{{cite magazine |last=Metz |first=Cade |date=2011-10-13 |title=Dennis Ritchie: The Shoulders Steve Jobs Stood On |url=https://www.wired.com/2011/10/thedennisritchieeffect/ |url-access=subscription |magazine=Wired |access-date=April 19, 2022 |archive-date=April 12, 2022 |archive-url=https://web.archive.org/web/20220412005125/http://www.wired.com/2011/10/thedennisritchieeffect/ |url-status=live }}</ref>

While C has been popular, influential and hugely successful, it has drawbacks, including:

* The standard [[dynamic memory]] handling with <code>malloc</code> and <code>free</code> is error prone. Improper use can lead to [[memory leaks]] and [[dangling pointers]].<ref>{{cite web |author=Internet Security Research Group |title=What is memory safety and why does it matter? |url=https://www.memorysafety.org/docs/memory-safety/ |website=Prossimo |access-date=March 3, 2025}}</ref>

* The use of pointers and the direct manipulation of memory means corruption of memory is possible, perhaps due to programmer error, or insufficient checking of bad data.

* There is some [[type checking]], but it does not apply to areas like [[variadic functions]], and the type checking can be trivially or inadvertently circumvented. It is [[Strong and weak typing|weakly typed]].

* Since the code generated by the compiler contains few checks itself, there is a burden on the programmer to consider all possible outcomes, to protect against buffer overruns, array bounds checking, [[stack overflow]]s, memory exhaustion, and consider [[Race condition#In software|race conditions]], thread isolation, etc.

* The use of pointers and the run-time manipulation of these means there may be two ways to access the same data (aliasing), which is not determinable at compile time. This means that some optimisations that may be available to other languages are not possible in C. Fortran is considered faster.

* There is limited standardisation in support for low-level variants in generated code, for example: different function [[calling conventions]] and [[Application binary interface|ABI]]; different [[Data structure alignment|structure packing]] conventions; different byte ordering within larger integers (including endianness). In many language implementations, some of these options may be handled with the preprocessor directive <code>[[C preprocessor#Compiler-specific preprocessor features|#pragma]]</code>,<ref>{{Cite web |last=corob-msft |title=Pragma directives and the __pragma and _Pragma keywords |url=https://learn.microsoft.com/en-us/cpp/preprocessor/pragma-directives-and-the-pragma-keyword |access-date=September 24, 2022 |website=Microsoft Learn |date=March 31, 2022 |language=en-us |archive-date=September 24, 2022 |archive-url=https://web.archive.org/web/20220924075131/https://learn.microsoft.com/en-us/cpp/preprocessor/pragma-directives-and-the-pragma-keyword |url-status=live }}</ref><ref>{{Cite web |title=Pragmas (The C Preprocessor) |url=https://gcc.gnu.org/onlinedocs/cpp/Pragmas.html |access-date=September 24, 2022 |website=GCC, the GNU Compiler Collection |archive-date=June 17, 2002 |archive-url=https://web.archive.org/web/20020617041757/https://gcc.gnu.org/onlinedocs/cpp/Pragmas.html |url-status=live }}</ref> and some with additional keywords e.g. use <code>[[__cdecl]]</code> calling convention. The directive and options are not consistently supported.<ref>{{cite web |title=Pragmas |url=https://www.intel.com/content/www/us/en/develop/documentation/cpp-compiler-developer-guide-and-reference/top/compiler-reference/pragmas.html |publisher=Intel |work=Intel C++ Compiler Classic Developer Guide and Reference |access-date=April 10, 2022 |language=en |archive-date=April 10, 2022 |archive-url=https://web.archive.org/web/20220410113529/https://www.intel.com/content/www/us/en/develop/documentation/cpp-compiler-developer-guide-and-reference/top/compiler-reference/pragmas.html |url-status=live }}</ref>

* The language does not directly support object orientation, [[type introspection|introspection]], run-time expression evaluation, generics, etc.

* There are few guards against inappropriate use of language features, which may lead to [[Software maintenance|unmaintainable]] code. In particular, the [[C preprocessor]] can hide troubling effects such as double evaluation and worse.<ref>{{cite web |title=In praise of the C preprocessor |url=https://apenwarr.ca/log/20070813 |website=apenwarr |date=August 13, 2007 |access-date=July 9, 2023}}</ref> This facility for tricky code has been celebrated with competitions such as the ''[[International Obfuscated C Code Contest]]'' and the ''[[Underhanded C Contest]]''.

* C lacks standard support for [[exception handling]] and only offers [[return code]]s for error checking. The [[Setjmp.h|<code>setjmp</code> and <code>longjmp</code>]] standard library functions have been used<ref>{{cite web|last1=Roberts |first1=Eric S. |title=Implementing Exceptions in C |date=March 21, 1989 |url=http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/tech_reports/SRC-RR-40.pdf |archive-url=https://web.archive.org/web/20170115152453/http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/tech_reports/SRC-RR-40.pdf |archive-date=January 15, 2017 |url-status=live |access-date=January 4, 2022 |publisher=[[DEC Systems Research Center]] |id=SRC-RR-40}}</ref> to implement a try-catch mechanism via macros.

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 bugs.  Databases such as [[Common Weakness Enumeration|CWE]] attempt to count the ways C etc. has vulnerabilities, along with recommendations for mitigation.

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

[[File:Tiobe index 2020 may.png|alt=|thumb|The [[TIOBE index]] graph, showing a comparison of the popularity of various programming languages<ref name="MmjNC">{{cite magazine |url=https://www.wired.com/2013/01/java-no-longer-a-favorite/ |title=Is Java Losing Its Mojo? |first1=Robert |last1=McMillan |date=August 1, 2013 |magazine=[[Wired (magazine)|Wired]] |access-date=March 5, 2017 |archive-date=February 15, 2017 |archive-url=https://web.archive.org/web/20170215115409/https://www.wired.com/2013/01/java-no-longer-a-favorite/ |url-status=live }}</ref>]]

Many languages developed after C, were influenced by and borrowed aspects of C, including [[C++]], [[C Sharp (programming language)|C#]], [[C shell]], [[D (programming language)|D]], [[Go (programming language)|Go]], [[Java (programming language)|Java]], [[JavaScript]], [[Julia (programming language)|Julia]], [[Limbo (programming language)|Limbo]], [[LPC (programming language)|LPC]], [[Objective-C]], [[Perl]], [[PHP]], [[Python (programming language)|Python]], [[Ruby (programming language)|Ruby]], [[Rust (programming language)|Rust]], [[Swift (programming language)|Swift]], [[Verilog]] and [[SystemVerilog]].<ref name="vinsp" /><ref name="kafmy">{{Cite book|title=Pillars of computing : a compendium of select, pivotal technology firms |last1=O'Regan |first1=Gerard |isbn=978-3319214641 |oclc=922324121 |date=September 24, 2015|publisher=Springer }}</ref> Some claim that the most pervasive influence has been syntactical {{endash}} that these languages combine the statement and expression syntax of C with type systems, data models and large-scale program structures that differ from those of C, sometimes radically.

When [[object-oriented programming]] 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 compiler]]s; source code was translated into C, and then compiled with a C compiler.<ref name="dSI6f">{{Cite book |title=Languages and compilers for parallel computing : 16th international workshop, LCPC 2003, College Station, TX, USA, October 2–4, 2003 : revised papers|last1=Rauchwerger |first1=Lawrence |year=2004 |publisher=Springer |isbn=978-3540246442 |oclc=57965544}}</ref>

The [[C++]] programming language (originally named "C with [[Class (programming)|Classes]]") was devised by [[Bjarne Stroustrup]] as an approach to providing [[Object-oriented programming|object-oriented]] functionality with a C-like syntax.<ref name="stroustrup 1993">{{cite web |url=http://www.stroustrup.com/hopl2.pdf |title=A History of C++: 1979–1991 |first1=Bjarne |last1=Stroustrup |author-link=Bjarne Stroustrup |year=1993 |access-date=June 9, 2011 |archive-date=February 2, 2019 |archive-url=https://web.archive.org/web/20190202050609/http://www.stroustrup.com/hopl2.pdf |url-status=live }}</ref> 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{{when|date=August 2022}} supports most of C, with [[Compatibility of C and C++|a few exceptions]].

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|isbn=978-1449399917}}

* {{cite book |last1=Perry |first1=Greg |last2=Miller |first2=Dean |title=C Programming: Absolute Beginner's Guide |edition=3 |year=2013 |publisher=Que |isbn=978-0789751980}}

* {{cite book |last1=Deitel |first1=Paul |last2=Deitel |first2=Harvey |title=C: How to Program |edition=8 |year=2015 |publisher=Pearson |isbn=978-0133976892}}

* {{cite book |last1=Gustedt |first1=Jens |title=Modern C |edition=2 |year=2019 |publisher=Manning |isbn=978-1617295812}} <small>''[https://gustedt.gitlabpages.inria.fr/modern-c/ (free)]''</small>

** {{cite web |url=https://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf |archive-url=https://web.archive.org/web/20071025205438/http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf |archive-date=October 25, 2007 |url-status=live |title=C99 with Technical corrigenda TC1, TC2, and TC3 included}}&nbsp;{{small|(3.61&nbsp;MB)}}