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C11 (C standard revision)
open-in-new
<h2 id="changes-from-c99">Changes from C99</h2> <p>The standard includes several changes to the <a href="/facts/C99/o5uMWjTQ">C99</a> language and library specifications, such as<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p> <ul><li><a href="/facts/Data_structure_alignment/Bqk6zMNS">Alignment</a> specification (_Alignas specifier, _Alignof operator, aligned_alloc function, <stdalign.h> header)</li> <li>The _Noreturn function specifier and the <stdnoreturn.h> header</li> <li>Type-generic expressions using the _Generic keyword. For example, the following macro <a href="/facts/Cube_root/c8klHLFb">cbrt(x)</a> translates to cbrtl(x), cbrt(x) or cbrtf(x) depending on the type of x:</li></ul> #define cbrt(x) _Generic((x), long double: cbrtl, \ default: cbrt, \ float: cbrtf)(x) <ul><li><a href="/facts/Thread_(computer_science)/hh8WHyPd">Multi-threading</a> support (_Thread_local storage-class specifier, <threads.h> header including thread creation/management functions, <a href="/facts/Mutual_exclusion/1dwvCxDW">mutex</a>, <a href="/facts/Condition_variable/g2zfRatV">condition variable</a> and <a href="/facts/Thread-local_storage/BPDI8VDU">thread-specific storage</a> functionality, as well as <stdatomic.h><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> for atomic operations supporting the C11 memory model).</li> <li>Improved <a href="/facts/Unicode/DDu3MfPV">Unicode</a> support based on the C Unicode Technical Report ISO/IEC TR 19769:2004 (char16_t and char32_t types for storing <a href="/facts/UTF-16/ofSmbCem">UTF-16</a>/<a href="/facts/UTF-32/YF6Me5na">UTF-32</a> encoded data, including conversion functions in <uchar.h> and the corresponding u and U string literal prefixes, as well as the u8 prefix for <a href="/facts/UTF-8/xlaVHSyf">UTF-8</a> encoded literals).<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a></li> <li>Removal of the <a href="/facts/Gets()/m5n82Fqz">gets</a> function (in favor of safer fgets), which was deprecated in the previous C language standard revision, ISO/IEC 9899:1999/Cor.3:2007(E).</li> <li><a href="/facts/Bounds_checking/vnAjD4Lp">Bounds-checking</a> interfaces (Annex K).<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a></li> <li>Analyzability features (Annex L).</li> <li>More macros for querying the characteristics of floating-point types, concerning <a href="/facts/Denormal_number/6qB24xrS">subnormal floating-point numbers</a> and the number of decimal digits the type is able to store.</li> <li>Anonymous <i>structures</i> and <i>unions</i>, useful when unions and structures are nested, e.g. in struct T { int tag; union { float x; int n; }; };.</li> <li><a href="/facts/Assertion_(computing)/lxSfLdLY">Static assertions</a>, which are evaluated during translation at a later phase than #if and #error, when types are understood by the translator.</li> <li>An exclusive create-and-open mode ("…x" suffix) for <a href="/facts/C_file_input/output/m5n82Fqz">fopen</a>. This behaves like O_CREAT|O_EXCL in <a href="/facts/Open_(system_call)/TP39FgXh">POSIX</a>, which is commonly used for <a href="/facts/File_locking/QektVCQs">lock files</a>.</li> <li>The <a href="/facts/C_process_control/jyvQdka8">quick_exit</a> function as a third way to terminate a program, intended to do at least minimal deinitialization.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a></li> <li>A new timespec_get function and corresponding structure in <time.h> with a degree of <a href="/facts/POSIX/mQkSt234">POSIX</a> compatibility.</li> <li>Macros for the construction of <a href="/facts/Complex_number/2w7mqI7e">complex values</a> (partly because real + imaginary*I might not yield the expected value if imaginary is infinite or <a href="/facts/NaN/DNnYJ4FU">NaN</a>).<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a></li></ul> <h2 id="optional-features">Optional features</h2> <p>The new revision allows implementations to not support certain parts of the standard — including some that had been mandatory to support in the 1999 revision.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> Programs can use predefined macros to determine whether an implementation supports a certain feature or not. </p> Optional features in C11<table><tbody><tr><th>Feature</th><th>Feature test macro</th><th>Availability in C99<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a></th></tr><tr><td>Analyzability (Annex L)</td><td>__STDC_ANALYZABLE__</td><td>Not available</td></tr><tr><td>Bounds-checking interfaces (Annex K)</td><td>__STDC_LIB_EXT1__</td><td>Not available</td></tr><tr><td>Multithreading (<threads.h>)</td><td>__STDC_NO_THREADS__</td><td>Not available</td></tr><tr><td>Atomic primitives and types (<stdatomic.h> and the _Atomic type qualifier)<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a></td><td>__STDC_NO_ATOMICS__</td><td>Not available</td></tr><tr><td><a href="/facts/IEEE_754-2008/gqqJPoYC">IEC 60559</a> floating-point arithmetic (Annex F)</td><td>__STDC_IEC_559__</td><td>Optional</td></tr><tr><td><a href="/facts/IEEE_754-2008/gqqJPoYC">IEC 60559</a> compatible complex arithmetic (Annex G)</td><td>__STDC_IEC_559_COMPLEX__</td><td>Optional</td></tr><tr><td>Complex types (<complex.h>)</td><td>__STDC_NO_COMPLEX__</td><td>Mandatory for hosted implementations</td></tr><tr><td>Variable-length arrays<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a></td><td>__STDC_NO_VLA__</td><td>Mandatory</td></tr></tbody></table> <h2 id="compiler-support">Compiler support</h2> <p>Some features of C11 are supported by the <a href="/facts/GNU_Compiler_Collection/t07jh01k">GCC</a> starting with version 4.6,<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> <a href="/facts/Clang/MH7cjC67">Clang</a> starting with version 3.1,<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> <a href="/facts/IBM_XL_C/zfnTqxGW">IBM XL C</a> starting with version 12.1,<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> and <a href="/facts/Microsoft_Visual_C%2B%2B/RM3sbYw5">Microsoft Visual C++</a> starting with VS 2019 (16.8)<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> in September 2020. </p> <h2 id="criticism">Criticism</h2> <p class="note">See also: <a href="/facts/C_string_handling/V0AYCYR2">C string handling § Replacements</a></p> <p>The optional bounds-checking interfaces (Annex K) remain controversial and have not been widely implemented, and their deprecation or removal from the next standard revision has been proposed.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> Even Microsoft, a main proponent of this interface, does not conform to the definition.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> In addition, Annex K does not include the more useful TR24731-2 (dynamic allocation functions), such as vasprintf and open_memstream.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> The few open-source implementations include <a href="/facts/Open_Watcom/GFxPBuGb">Open Watcom</a> C/C++'s "Safer C" library<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> and safeclib.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> </p> <h2 id="see-also">See also</h2> <ul> <li>Computer programming portal</li></ul> <ul><li><a href="/facts/C%2B%2B23/8sBupF9c">C++23</a>, <a href="/facts/C%2B%2B20/hwpB1Vzp">C++20</a>, <a href="/facts/C%2B%2B17/B3gKNeER">C++17</a>, <a href="/facts/C%2B%2B14/CXukL6pA">C++14</a>, <a href="/facts/C%2B%2B11/fI3YCoKQ">C++11</a>, <a href="/facts/C%2B%2B03/oKxXn8nY">C++03</a>, <a href="/facts/C%2B%2B98/Et0F2qn9">C++98</a>, versions of the C++ programming language standard</li> <li><a href="/facts/Compatibility_of_C_and_C%2B%2B/xXuvKjlQ">Compatibility of C and C++</a></li></ul> <h2 id="further-reading">Further reading</h2> <ul><li><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf">N1570</a> (working draft of C11 standard); WG14; 2011. (<a href="https://port70.net/~nsz/c/c11">HTML and ASCII versions</a>)</li> <li><a href="https://www.iso.org/standard/57853.html">ISO/IEC 9899:2011</a> (official C11 standard); <a href="/facts/International_Organization_for_Standardization/IacYR7Yg">ISO</a>; 2011.</li> <li>Plum, Thomas (April 6, 2012). <a href="http://drdobbs.com/cpp/232800444">"C Finally Gets A New Standard"</a>. <i><a href="/facts/Dr._Dobb%27s_Journal/6Qov1hUK">Dr. Dobb's Journal</a></i>.</li> <li><a href="https://web.archive.org/web/20131203031224/http://www.owasp.org.cn/OWASP_Conference/2011/17_.pdf">Safe C API—Concise solution of buffer overflow, The OWASP Foundation, OWASP AppSec, Beijing 2011 </a></li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://www.open-std.org/jtc1/sc22/wg14/www/wg14_document_log.htm">C Language Working Group 14 (WG14) Documents</a></li> <li><a href="http://www.open-std.org/JTC1/SC22/wg14/www/docs/n1250.pdf">C1X Charter</a> - WG14</li> <li>Safe C Library of Bounded APIs</li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"ISO/IEC 9899:2011 - Information technology -- Programming languages -- C". www.iso.org. <a href="https://www.iso.org/standard/57853.html" target="_blank">https://www.iso.org/standard/57853.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>"WG14 N1250 The C1X Charter" (PDF). <a href="http://www.open-std.org/JTC1/SC22/wg14/www/docs/n1250.pdf" target="_blank">http://www.open-std.org/JTC1/SC22/wg14/www/docs/n1250.pdf</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>"Subsetting the C Standard". www.open-std.org. <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1460.htm" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1460.htm</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>WG14 N1570 Committee Draft — April 12, 2011 <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>"Defect report #411". ISO/IEC JTC1/SC22/WG14 - C. February 2012. Retrieved 2012-05-04. <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_411.htm" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_411.htm</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>WG14 N1516 Committee Draft — October 4, 2010 <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1516.pdf" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1516.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"Atomic operations library - cppreference.com". en.cppreference.com. <a href="http://en.cppreference.com/w/c/atomic" target="_blank">http://en.cppreference.com/w/c/atomic</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>"WG14 N1286 — "On Support for TR-19769 and New Character Types", Nick Stoughton, Larry Dwyer" (PDF). <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1286.pdf" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1286.pdf</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Berin Babcock-McConnell. "API02-C. Functions that read or write to or from an array should take an argument to specify the source or target size". <a href="https://www.securecoding.cert.org/confluence/display/c/API02-C.+Functions+that+read+or+write+to+or+from+an+array+should+take+an+argument+to+specify+the+source+or+target+size" target="_blank">https://www.securecoding.cert.org/confluence/display/c/API02-C.+Functions+that+read+or+write+to+or+from+an+array+should+take+an+argument+to+specify+the+source+or+target+size</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>"Abandoning a Process". www.open-std.org. <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1327.htm" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1327.htm</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>"Creation of complex value". www.open-std.org. <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1464.htm" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1464.htm</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>WG14 N1548 Committee Draft — December 2, 2010 6.10.8.3 Conditional feature macros <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1548.pdf</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>ISO 9899:1999 6.10.8 Predefined macro names <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>"WG14 N1558 Mar 14-18 meeting minutes (draft)" (PDF). <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1558.pdf" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1558.pdf</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>ISO 9899:2011 Programming Languages - C 6.7.6.2 4 <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>"GCC 4.6 Release Series — Changes, New Features, and Fixes - GNU Project - Free Software Foundation (FSF)". gcc.gnu.org. <a href="https://gcc.gnu.org/gcc-4.6/changes.html#c" target="_blank">https://gcc.gnu.org/gcc-4.6/changes.html#c</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>"Clang 3.1 Release Notes". llvm.org. <a href="http://llvm.org/releases/3.1/docs/ClangReleaseNotes.html#cchanges" target="_blank">http://llvm.org/releases/3.1/docs/ClangReleaseNotes.html#cchanges</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>"Support for ISO C11 added to IBM XL C/C++ compilers". www.ibm.com. 17 April 2014. <a href="http://www.ibm.com/developerworks/rational/library/support-iso-c11/index.html" target="_blank">http://www.ibm.com/developerworks/rational/library/support-iso-c11/index.html</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>"C11 and C17 Standard Support Arriving in MSVC". devblogs.microsoft.com. 14 September 2020. <a href="https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support-arriving-in-msvc/" target="_blank">https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-support-arriving-in-msvc/</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>"N1969 — Updated Field Experience With Annex K — Bounds Checking Interfaces". www.open-std.org. <a href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1969.htm" target="_blank">http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1969.htm</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Leffler, Jonathan. "c - Do you use the TR 24731 'safe' functions?". Stack Overflow. <a href="https://stackoverflow.com/a/373911" target="_blank">https://stackoverflow.com/a/373911</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>"c - Do you use the TR 24731 'safe' functions?". Stack Overflow. <a href="https://stackoverflow.com/a/17353714" target="_blank">https://stackoverflow.com/a/17353714</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>"Safer C Library - Open Watcom". 3 May 2015. Archived from the original on 2015-05-03. <a href="https://web.archive.org/web/20150503192244/http://openwatcom.org/index.php/Safer_C_Library" target="_blank">https://web.archive.org/web/20150503192244/http://openwatcom.org/index.php/Safer_C_Library</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>"safec: Safe C Library - README". rurban.github.io. <a href="https://rurban.github.io/safeclib/doc/safec-3.5.2/index.html" target="_blank">https://rurban.github.io/safeclib/doc/safec-3.5.2/index.html</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> </ol>