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| {{C Standard Library}}
| | Hi there. Let me start by introducing the author, her title is Sophia. Since I was eighteen I've been working as a bookkeeper but quickly my spouse and I will begin our own company. What me and my family love is bungee jumping but I've been taking on new things recently. Ohio is exactly where her home is.<br><br>Have a look at my web-site; [http://www.clubwww1music.com/profile.php?u=MaSvf tarot readings] |
| {{Cleanup|reason=misleading information|date=August 2013}}
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| The '''C standard library''' is the [[standard library]] for the [[C (programming language)|C programming language]], as specified in the [[ANSI C]] standard.<ref name="c99io">[[International Organization for Standardization|ISO]]/[[International Electrotechnical Commission|IEC]] (1999). ''[[C99|ISO/IEC 9899:1999(E): Programming Languages - C]] §7.19.1 para 1''</ref> It was developed at the same time as the [[C POSIX library]], which is a superset of it.<ref>{{cite web
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| | url = http://ftp.gnu.org/old-gnu/Manuals/glibc-2.2.3/html_chapter/libc_1.html
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| | title = The GNU C Library - Introduction
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| | accessdate = 2013-12-05
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| | publisher = gnu.org
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| }}</ref><ref>{{cite web
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| | url = http://stackoverflow.com/questions/9376837/difference-bewteen-c-standard-library-and-c-posix-library
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| | title = Difference bewteen C standard library and C POSIX library
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| | year = 2012 | accessdate = 2013-12-05
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| | publisher = stackoverflow.com
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| }}</ref> Since ANSI C was adopted by the [[International Organization for Standardization]],<ref name=ISOAdoptedAnsiC>{{cite web|title=C Standards|url=http://www.keil.com/support/docs/1893.htm|publisher=Keil|accessdate=24 November 2011}}</ref> the C standard library is also called the '''ISO C library'''.
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| The C standard library provides macros, type definitions, and functions for tasks like [[character string (computer science)|string]] handling, mathematical computations, input/output processing, memory allocation and several other [[operating system]] services.
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| ==Application programming interface==
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| ==={{anchor|headers}} Header files ===
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| The [[application programming interface]] (API) of the C standard library is declared in a number of [[header file]]s. Each header file contains one or more function declarations, data type definitions, and macros.
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| After a long period of stability, three new header files (<code>iso646.h</code>, <code>wchar.h</code>, and <code>wctype.h</code>) were added with ''Normative Addendum 1'' (NA1), an addition to the C Standard ratified in 1995. Six more header files (<code>complex.h</code>, <code>fenv.h</code>, <code>inttypes.h</code>, <code>stdbool.h</code>, <code>stdint.h</code>, and <code>tgmath.h</code>) were added with [[C99]], a revision to the C Standard published in 1999, and five more files (<code>stdalign.h</code>, <code>stdatomic.h</code>, <code>stdnoreturn.h</code>, <code>threads.h</code>, and <code>uchar.h</code>) with [[C11 (C standard revision)|C11]] in 2011. In total, there are now 29 header files:
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| {| class="wikitable sortable"
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| |-
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| ! Name !! From !! Description
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| |-
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| | <code>[[assert.h|<assert.h>]]</code> || || Contains the [[assertion (computing)|assert]] macro, used to assist with detecting logical errors and other types of bug in debugging versions of a program.
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| | <code>[[complex.h|<complex.h>]]</code> || C99 || A [[C mathematical functions#complex.h|set of functions]] for manipulating [[complex number]]s.
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| | <code>[[ctype.h|<ctype.h>]]</code> || || Defines [[C character classification|set of functions]] used to classify characters by their types or to convert between upper and lower case in a way that is independent of the used [[character set]] (typically [[ASCII]] or one of its extensions, although implementations utilizing [[EBCDIC]] are also known).
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| |-
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| | <code>[[errno.h|<errno.h>]]</code> || || For testing error codes reported by library functions.
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| | <code>[[fenv.h|<fenv.h>]]</code> || C99 || Defines a [[C mathematical functions#fenv.h|set of functions]] for controlling [[floating-point]] environment.
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| | <code>[[float.h|<float.h>]]</code> || || Defines [[C data types#float.h|macro constants]] specifying the implementation-specific properties of the [[floating-point]] library.
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| | <code>[[inttypes.h|<inttypes.h>]]</code> || C99 || Defines [[C data types#inttypes.h|exact width integer types]].
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| | <code>[[iso646.h|<iso646.h>]]</code> || NA1 || Defines [[C alternative tokens|several macros]] that implement alternative ways to express several standard tokens. For programming in [[ISO 646]] variant character sets.
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| | <code>[[limits.h|<limits.h>]]</code> || || Defines [[C data types#limits.h|macro constants]] specifying the implementation-specific properties of the integer types.
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| | <code><locale.h></code> || || Defines [[C localization functions|localization functions]].
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| | <code>[[math.h|<math.h>]]</code> || || Defines [[C mathematical functions|common mathematical functions]].
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| | <code>[[setjmp.h|<setjmp.h>]]</code> || || Declares the macros <code>setjmp</code> and <code>longjmp</code>, which are used for non-local exits.
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| | <code>[[signal.h|<signal.h>]]</code> || || Defines [[C signal handling|signal handling functions]].
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| | <code><stdalign.h></code> || C11 || For querying and specifying the [[Data structure alignment|alignment]] of objects.
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| | <code>[[stdarg.h|<stdarg.h>]]</code> || || For accessing a varying number of arguments passed to functions.
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| | <code><stdatomic.h></code> || C11 || For [[Linearizability|atomic operations]] on data shared between threads.
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| | <code>[[stdbool.h|<stdbool.h>]]</code> || C99 || Defines [[C data types#stdbool.h|a boolean data type]].
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| | <code>[[stddef.h|<stddef.h>]]</code> || || Defines [[C data types#stddef.h|several useful types and macros]].
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| | <code>[[stdint.h|<stdint.h>]]</code> || C99 || Defines [[C data types#stdint.h|exact width integer types]].
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| | <code>[[stdio.h|<stdio.h>]]</code> || || Defines [[C file input/output|core input and output functions]]
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| | <code><stdlib.h></code> || || Defines [[C string handling#stdlib.h|numeric conversion functions]], [[C mathematical functions#stdlib.h|pseudo-random numbers generation functions]], [[C dynamic memory allocation|memory allocation]], [[C process control|process control functions]]
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| | <code><stdnoreturn.h></code> || C11 || For specifying non-returning functions.
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| | <code>[[string.h|<string.h>]]</code> || || Defines [[C string handling|string handling functions]].
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| | <code>[[tgmath.h|<tgmath.h>]]</code> || C99 || Defines [[C mathematical functions#tgmath.h|type-generic mathematical functions]].
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| | <code><threads.h></code> || C11 || Defines functions for managing multiple [[Thread (computer science)|Thread]]s as well as [[Mutual exclusion|mutexes]] and [[Monitor (synchronization)|condition variables]].
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| | <code>[[time.h|<time.h>]]</code> || || Defines [[C date and time functions|date and time handling functions]]
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| | <code><uchar.h></code> || C11 || Types and functions for manipulating [[Unicode]] characters.
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| | <code>[[wchar.h|<wchar.h>]]</code> || NA1 || Defines [[C string handling|wide string handling functions]].
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| | <code>[[wctype.h|<wctype.h>]]</code> || NA1 || Defines [[C character classification|set of functions]] used to classify wide characters by their types or to convert between upper and lower case
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| |}
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| Three of the header files (<code>complex.h</code>, <code>stdatomic.h</code>, <code>threads.h</code>) are conditional features that implementations need not support.
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| The [[POSIX]] standard added several nonstandard C headers for Unix-specific functionality. Many have found their way to other architectures. Examples include <code>unistd.h</code> and <code>signal.h</code>. A number of other groups are using other nonstandard headers - most flavors of Linux have <code>alloca.h</code> and HP OpenVMS has the <code>va_count()</code> function.
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| === Documentation ===
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| On Unix-like systems, the authoritative documentation of the actually implemented API is provided in form of [[man page]]s. On most systems, man pages on standard library functions are in section 3; section 7 may contain some more generic pages on underlying concepts (e.g. <code>man 7 math_error</code> in [[Linux]]).
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| == Implementations ==
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| [[Unix-like]] systems typically have a C library in [[shared library]] form, but the header files (and compiler toolchain) may be absent from an installation so C development may not be possible. The C library is considered part of the operating system on Unix-like systems.{{Citation needed|date=October 2010}} The C functions, including the ISO C standard ones, are widely used by programs, and are regarded as if they were not only an implementation of something in the C language, but also de facto part of the operating system interface. Unix-like operating systems generally cannot function if the C library is erased.
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| By contrast, on Microsoft Windows, the core system dynamic libraries ([[Dynamic-link library|DLLs]]) do not provide an implementation of the C standard library; this is provided by each compiler individually. Compiled applications written in C are either statically linked with a C library, or linked to a dynamic version of the library that is shipped with these applications, rather than relied upon to be present on the targeted systems. Functions in a compiler's C library are not regarded as interfaces to Microsoft Windows.
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| Many other implementations exist, provided with both various operating systems and C compilers.
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| Although there exist too many implementations to list, some popular implementations follow:
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| *[[BSD libc]], implementations distributed under [[Berkeley Software Distribution|BSD]] operating systems.
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| *[[GNU C Library]], used in [[GNU/Linux]] and [[GNU]]/[[HURD]].
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| *[[Windows library files#Msvcrt.dll and Msvcpp.dll|Microsoft C Run-time Library]], part of [[Microsoft Visual C++]]
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| *[[dietlibc]], an alternative small implementation of the C standard library (MMU-less)
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| *[[uClibc]], a C standard library for embedded [[Linux]] systems (MMU-less)
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| *[[Newlib]], a C standard library for embedded systems (MMU-less)<ref>{{cite web|url=http://www.cygwin.com/ml/newlib/2006/msg00224.html |title=Re: Does Newlib support mmu-less CPUs? |publisher=Cygwin.com |date=23 March 2006 |accessdate=28 October 2011}}</ref>
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| *[[klibc]], primarily for booting Linux systems.
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| *[[EGLIBC]], variant of glibc for embedded systems.
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| *[[musl]], another lightweight C standard library implementation for Linux systems<ref>{{cite web|url=http://www.etalabs.net/musl/ |title=musl libc |publisher=Etalabs.net |date= |accessdate=28 October 2011}}</ref>
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| *[[Bionic (software)|Bionic]], originally developed by Google for the Android embedded system operating system, derived from BSD libc.
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| === Compiler built-in functions ===
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| Some compilers (for example, [[GNU Compiler Collection|GCC]]<ref>[http://gcc.gnu.org/onlinedocs/gcc-4.1.1/gcc/Other-Builtins.html#Other-Builtins Other built-in functions provided by GCC], GCC Manual</ref>) provide built-in versions of many of the functions in the C standard library; that is, the implementations of the functions are written into the compiled [[object file]], and the program calls the built-in versions instead of the functions in the C library [[shared object]] file. This reduces function call overhead, especially if function calls are replaced with [[inline function|inline]] variants, and allows other forms of [[compiler optimization|optimization]] (as the compiler knows the [[control flow|control-flow]] characteristics of the built-in variants), but may cause confusion when debugging (for example, the built-in versions cannot be replaced with [[instrumentation (computer programming)|instrumented]] variants).
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| However, the built-in functions must behave like ordinary functions in accordance with ISO C. The main implication is that the program must be able to create a pointer to these functions by taking their address, and invoke the function by means of that pointer. If two pointers to the same function are derived in two different translation unit in the program, these two pointers must compare equal; that is, the address comes by resolving the name of the function, which has external (program-wide) linkage.
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| ===Linking, libm===
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| Under Linux and FreeBSD,<ref>{{cite web|title=Compiling with cc|url=http://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/tools-compiling.html|accessdate=03/02/2013}}</ref> the mathematical functions (as declared in <code>math.h</code>) are bundled separately in the mathematical library [[libm]]. If any of them are used, the linker must be given the directive <code>-lm</code>.
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| ===Detection===
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| According to the C standard the macro <code>__STDC_HOSTED__</code> shall be defined to '''1''' if the implementation is hosted. A hosted implementation has all the headers specified by the C standard. An implementation can also be ''freestanding'' which means that these headers will not be present. If an implementation is ''freestanding'', it shall define <code>__STDC_HOSTED__</code> to '''0'''.
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| == Concepts, problems and workarounds ==
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| === Buffer overflow vulnerabilities ===
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| Some functions in the C standard library have been notorious for having [[buffer overflow]] vulnerabilities and generally encouraging buggy programming ever since their adoption.<ref>[[Morris worm]] that takes advantage of the well-known vulnerability in <code>gets()</code> have been created as early as in 1988.</ref> The most criticized items are:
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| * [[C string handling|string-manipulation routines]], including <code>strcpy()</code> and <code>strcat()</code>, for lack of bounds checking and possible buffer overflows if the bounds aren't checked manually;
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| * string routines in general, for [[side effect (computer science)|side-effects]], encouraging irresponsible buffer usage, not always guaranteeing valid [[null-terminated string|null-terminated]] output, linear length calculation;<ref>in C standard library, string length calculation and looking for a string's end have [[Linear time|linear time complexities]] and are inefficient when used on the same or related strings repeatedly</ref>
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| * <code>[[printf]]()</code> family routines, for spoiling the [[call stack|execution stack]] when the format string doesn't match the arguments given. This fundamental flaw created an entire class of attacks: [[format string attack]]s;
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| * <code>[[gets()|gets]]()</code> and <code>[[scanf]]()</code> family I/O routines, for lack of (either any or easy) input length checking.
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| Except the extreme case with <code>gets()</code>, all the security vulnerabilities can be avoided by introducing auxiliary code to perform memory management, bounds checking, input checking, etc. This is often done in form of wrappers that make standard library functions safer and easier to use. This dates back to as early as ''[[The Practice of Programming]]'' book by B. Kernighan and R. Pike where the authors commonly use wrappers that print error messages and quit the program if an error occurs.
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| The ISO C committee published Technical reports [http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1225.pdf TR 24731-1] and is working on [http://www.open-std.org/JTC1/SC22/WG14/www/docs/n1337.pdf TR 24731-2] to propose adoption of some functions with bounds checking and automatic buffer allocation, correspondingly. The former has met severe criticism with some praise,<ref>[http://stackoverflow.com/questions/372980/do-you-use-the-tr-24731-safe-functions-in-your-c-code Do you use the TR 24731 ‘safe’ functions in your C code?] - Stack overflow</ref><ref>{{cite web|url=http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1106.txt |title=Austin Group Review of ISO/IEC WDTR 24731 |date= |accessdate=28 October 2011}}</ref> the latter received mixed responses. Despite this, TR 24731-1 has been implemented into Microsoft's C standard library and its compiler issues warnings when using old 'insecure' functions.
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| ===Threading problems, vulnerability to race conditions===
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| The <code>[[mktemp]]()</code> and <code>[[strerror]]()</code> routines are criticized for being [[thread safety|thread unsafe]] and otherwise vulnerable to [[race condition]]s.
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| === Error handling ===
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| The error handling of the functions in the C standard library is not consistent and sometimes confusing. This can be fairly well summarized by the Linux manual page <code>math_error</code> which says:
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| <blockquote>
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| The current (version 2.8) situation under glibc is messy. Most (but not all)
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| functions raise exceptions on errors. Some also set errno. A few functions set
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| errno, but don't raise an exception. Very few functions do neither.
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| </blockquote>
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| == Standardization ==
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| {{main|C (programming language)#History}}
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| The original [[C (programming language)|C language]] provided no built-in functions such as I/O operations, unlike traditional languages such as [[COBOL]] and [[Fortran]].{{Citation needed|date=November 2010}} Over time, user communities of C shared ideas and implementations of what is now called C standard libraries. Many of these ideas were incorporated eventually into the definition of the standardized C language.
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| Both [[Unix]] and C were created at [[Bell Labs|AT&T's Bell Laboratories]] in the late 1960s and early 1970s. During the 1970s the C language became increasingly popular. Many universities and organizations began creating their own variants of the language for their own projects. By the beginning of the 1980s compatibility problems between the various C implementations became apparent. In 1983 the [[American National Standards Institute]] (ANSI) formed a committee to establish a standard specification of C known as "[[ANSI C]]". This work culminated in the creation of the so-called C89 standard in 1989. Part of the resulting standard was a set of [[software libraries]] called the ANSI C standard library.
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| === POSIX standard library ===
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| {{Main|C POSIX library}}
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| [[POSIX]] (and [[Single Unix Specification|SUS]]) specifies a number of routines that should be available over and above those in the C standard library proper; these are often implemented alongside the C standard library functionality, with varying degrees of closeness. For example, [[glibc]] implements functions such as [[fork (operating system)|fork]] within <code>libc.so</code>, but before [[NPTL]] was merged into glibc it constituted a separate library with its own linker flag argument. Often, this POSIX-specified functionality will be regarded as part of the library; the C library proper may be identified as the ANSI or [[International Organization for Standardization|ISO]] C library.
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| ===BSD libc===
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| '''BSD libc''' is an implementation of C standard library used by [[BSD]] [[Operating System]]s such as [[FreeBSD]], [[NetBSD]] and [[OpenBSD]]. It first appeared in [[BSD#4.4BSD and descendants|4.4BSD]], which was released in 1994. BSD libc has some extensions that are not defined in the original standard. Some of the extensions of BSD libc are:
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| * <code>sys/tree.h</code> - contains an implementation of [[Red-black tree]] and [[Splay tree]].<ref>{{cite web|url=http://man.freebsd.org/tree|title=tree |publisher=Man.freebsd.org |date=2007-12-27 |accessdate=2013-08-25}}</ref>
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| * <code>sys/queue.h</code> -Implementations of [[Linked list]], [[queue (data structure)|queues]], [[tail queue]], etc.<ref>{{cite web|url=http://man.freebsd.org/queue |title=queue |publisher=Man.freebsd.org |date=2011-05-13 |accessdate=2013-08-25}}</ref>
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| * <code>fgetln()</code> - defined in <code>stdio.h</code>. This can be used to read a file line by line.<ref>{{cite web|url=http://man.freebsd.org/fgetln |title=fgetln |publisher=Man.freebsd.org |date=1994-04-19 |accessdate=2013-08-25}}</ref>
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| * <code>fts.h</code> - contains some functions to traverse a file hierarchy.<ref>{{cite web|url=http://man.freebsd.org/fts |title=fts|publisher=Man.freebsd.org |date=2012-03-18 |accessdate=2013-08-25}}</ref>
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| * <code>db.h</code> - some functions to connect to the [[Berkeley DB]].<ref>{{cite web|url=http://man.freebsd.org/db |title=db|publisher=Man.freebsd.org |date=2010-09-10 |accessdate=2013-08-25}}</ref>
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| * <code>strlcat()</code> and <code>strlcpy()</code> - secure alternates for <code>strncat()</code> and <code>strncpy()</code>.<ref>Miller, Todd C. and Theo de Raadt. [http://www.usenix.org/events/usenix99/millert.html strlcpy and strlcat - consistent, safe, string copy and concatenation]. Proceedings of the 1999 USENIX Annual Technical Conference, June 6–11, 1999, pp. 175–178.</ref>
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| * <code>err.h</code> - contains some functions to print formatted error messages.<ref>{{cite web|url=http://man.freebsd.org/err |title=err|publisher=Man.freebsd.org |date=2012-03-29 |accessdate=2013-08-25}}</ref>
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| * <code>vis.h</code> - contains the <code>vis()</code> function. This function is used to display non-printable characters in a visual format.<ref>{{cite web|title=vis(3)|url=http://www.freebsd.org/cgi/man.cgi?query=vis&sektion=3&apropos=0&manpath=FreeBSD+9.1-RELEASE|publisher=Man.FreeBSD.org|accessdate=14 September 2013}}</ref>
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| == The C standard library in other languages ==
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| Some languages include the functionality of the standard C library in their own libraries. The library may be adapted to better suit the language's structure, but the operation semantics are kept similar. The [[C++]] language, for example, includes the functionality of the C standard library in the [[namespace (programming)|namespace]] <code>std</code> (e.g., <code>std::printf</code>, <code>std::atoi</code>, <code>std::feof</code>), in header files with similar names to the C ones (<code>cstdio</code>, <code>cmath</code>, <code>cstdlib</code>, etc.). Other languages that take similar approaches are [[D (programming language)|D]] and the main implementation of [[Python (programming language)|Python]] known as [[CPython]]. In the latter, for example, the built-in file objects are defined as "implemented using C's <code>stdio</code> package",<ref>{{cite web|url=http://docs.python.org/library/stdtypes.html#bltin-file-objects |title=The Python Standard Library: 6.9. File Objects |publisher=Docs.python.org |date= |accessdate=28 October 2011}}</ref> so that the available operations (open, read, write, etc.) are expected to have the same behavior as the corresponding C functions.
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| ==Comparison to standard libraries of other languages==
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| The C standard library is small compared to the standard libraries of some other languages. The C library provides a basic set of mathematical functions, string manipulation, type conversions, and file and console-based I/O. It does not include a standard set of "[[container (data structure)|container types]]" like the [[C++]] [[Standard Template Library]], let alone the complete [[graphical user interface]] (GUI) toolkits, networking tools, and profusion of other functionality that [[Java (software platform)|Java]] and the [[.NET Framework]] provides as standard. The main advantage of the small standard library is that providing a working ISO C environment is much easier than it is with other languages, and consequently porting C to a new platform is comparatively easy.
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| == See also ==
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| *[[C++ standard library]]
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| == References ==
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| {{Reflist|30em}}
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| ==Further reading==
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| * {{cite book|last=Plauger|first=P. J.|title=The Standard C library|year=1992|publisher=Prentice Hall|location=Englewood Cliffs, N.J|isbn=0-13-131509-9}}
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| == External links ==
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| *[http://www.acm.uiuc.edu/webmonkeys/book/c_guide/index.html The C Library Reference Guide]
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| *[http://www.schweikhardt.net/identifiers.html Handy list of which headers are in which standard]
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| * Microsoft [http://msdn.microsoft.com/en-us/library/abx4dbyh.aspx C Run-Time Libraries] on MSDN
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| * NetBSD [http://netbsd.gw.com/cgi-bin/man-cgi?intro+3+NetBSD-current C libraries manual] and [http://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libc/ full C library source]
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| * [http://man.cat-v.org/unix-1st/3/ Manual pages for the original C standard libraries in Unix]
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| {{CProLang}}
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| {{use dmy dates|date=January 2012}}
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| {{DEFAULTSORT:C Standard Library}}
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| [[Category:C standard library| ]]
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| [[Category:C programming language]]
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