Editing c/c11

{{c title|C11}}{{C standards}}
'''C11''' is the latest major revision of the [[C]] standard which was ratified by ISO on December 8, 2011 by the the international standardization working group for the [[C]] programming language, ''ISO/IEC JTC1/SC22/WG14''<ref>[http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=57853  Information technology -- Programming languages -- C]</ref>. C11 obsoletes the previous standard, C99, published in 1999. In April of 2011 the final draft ([http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf N1570]) was published. The draft passed its final review on October 10, 2011 which was consequently ratified and published as '''ISO/IEC 9899:2011'''. C11 was also later adopted by INCITS as an ANSI standard on May 23, 2012 as '''INCITS/ISO/IEC 9899-2012'''<ref>[http://webstore.ansi.org/RecordDetail.aspx?sku=INCITS%2FISO%2FIEC+9899-2012 INCITS/ISO/IEC 9899-2012]</ref>.

The C11 standard adds concurrency support, provides a mechanism for {{C|generic selection|type-generic expressions}}, adds alignment-related facilities, {{C|static assertion}}, Unicode support, floating-point characteristic macros, no-return functions, anonymous structures and unions
, and various bound-checking and reentrancy functions.

== Version detection ==
C11 can be detected via the '''__STDC_VERSION__''' mandatory macro and the '''__STDC_LIB_EXT1__''' conditional feature macro which must equal ''201112L''. Note that the official standard has a defect where the macro's definition is set to '201ymmL'. The defect has been corrected in Defect Report [http://www.open-std.org/jtc1/sc22/wg14/www/docs/dr_411.htm #411].

<source lang="C">
#if __STDC_VERSION__ >= 201112L
    /* C11 support */
#endif
</source>

== New headers ==
C11 introduces 5 new [[C standard library|standard headers]]: <[[stdalign.h]]>, <[[stdatomic.h]]>, <[[stdnoreturn.h]]>, <[[threads.h]]>, and <[[uchar.h]]>. The two concerned with concurrency (stdatomic.h and threads.h) are optional.

== No-return functions ==
Some functions do not ever return for various reasons. The new '''_Noreturn''' keyword can be used to mark functions that do not return to their caller such as ''exit()'', ''longjmp()'', and ''thrd_exit()''.  Using _Noreturn can assist compiler's optimizers as well as eliminating possible diagnostic messages.

Note that the use of _Noreturn must be limited to functions that cannot return under any circumstance. No-return functions that may return under certain conditions will exhibit undefined behavior.

== Alignment ==
{{main|c/alignment of objects|l1=Alignment of objects}}
C11 provides a new mechanism for working with objects alignments. The '''_Alignof''' operator was added which yields the alignment requirement (an  integer constant) of its operand type (which remains unevaluted). A set of convenient macros such as ''alignof'' and ''alignas'' are defined in [[stdalign.h|<stdalign.h>]].

For example:

<source lang="C">printf("The alignment of char is %zd.\n", _Alignof (char));</source>

Which must print 1 for the alignment of char. Additionally, the alignment can be specified as an integer using the '''_Alignas''' alignment specifier.

On each platform, a some type has the largest alignment requirement. C11 added the '''max_align_t''' object type whose alignment is as great as is supported by the implementation.

=== Extended alignment ===
Extended alignment is the ability for an implementation to support alignment requirement greater than _Alignof (max_align_t). A type having extended alignment requirement is called an ''over-aligned type''. That feature is optional and need not be supported.

== Aligned memory allocations ==
C11 added the {{C|stdlib.h/aligned_alloc|aligned_alloc}} function which allocates space for an object whose alignment is provided by you.

== Unicode support ==
C11 adds support for Unicode characters and string literals. A set of prefixes have been added to support [[UTF-8]], [[UTF-16]], and [[UTF-32]] character strings. The ''u8'' prefix for strings creates a UTF-8 encoded string. Likewise the ''u'' and ''U''' prefixes create UTF-16 and UTF-32 strings respectively. For example:

<source lang="C">
#include <uchar.h>

/* UTF-8 */
char u8str[] = u8"ππ This is a UTF-8 string ππ.";
char u8chr = u8'π';

/* UTF-16 */
#ifdef __STD_UTF_16__
    char16_t u16str[] = u"ΩΩ This is a UTF-16 string ΩΩ.";
    char16_t u16char = u'Ω';
#endif

/* UTF-32 */
#ifdef __STD_UTF_32__
    char32_t u32str[] = U"δδ This is a UTF-32 string δδ.";
    char32_t u32char = U'δ';
#endif
</source>

== Type-generic expressions ==
{{main|c/generic selection|l1=Generic selection}}
C11 now provides a mechanism for type-generic expressions, a mechanism needed to perform type-generic math without compiler magic. The feature is achieved via the addition of the new '''_Generic''' keyword.

== Static assertion ==
{{main|c/static assertion|l1=Static assertion}}
C11 added support for compile-time assertions. Static assertions are evaluated after the preprocessor compilation phase is over, thereby allowing the use of C operators such as ''sizeof''.

== Bounds-checking interfaces annex (Annex K) ==
The Bounds-checking interfaces annex provides a set of functions with more restricted behaviors in order to allow for more secure coding practices.

=== Reentrant functions ===
The functions ''strtok_s'', ''wcstok_s'', ''bsearch_s'', and ''qsort_s'', were added which keep track of their state via an argument that is passed instead of a static buffer which would prevent them from being reentrant.

=== %n format specifier ===
In various format printing functions, all %specifiers with the exception of %n are used for output purpose. On the other hand, the %n specifier is user for input. The ''_s'' version of the functions was added which gets rid of the %n specifier entirely.

=== memset_s ===
The memset_s function is a special version of memset which must execute even if the compiler's optimizer thinks superfluous. The best case for this is when a function attempts to zero-out some confidential information such as a password right before returning thus making the optimizer thinks the memset is not doing anything.

== Analyzability annex (Annex L) ==
The Analyzability Annex identifies a small subset of all undefined behaviors and sets some implementation constraints on the resulting behavior in an attempt to mitigate potential issues. An implementation that conforms to Annex L must define '''__STDC_ANALYZABLE__'''.

== Concurrency ==
{{main|c/concurrency|l1=Concurrency}}
C11's largest addition to the C programming language, is the addition of semantics of multi‐threaded programs. The standard also added a facility for performing inter‐thread communication via atomic variables as well as thread-specific storage. Thread support is optional and must be tested for via the  '''__STDC_NO_THREADS__''' macro.

== Compiler support ==

{| class="wikitable"
|-
| style="background:#CCCCFF;text-align:center;" colspan="4" | C11 Compiler Support
|-
! Compiler Name !! Support Level !! style="text-align:center;" | Enforcement Switch !! Note
|-
| ACK || style="background:#FF6666;text-align:center;" | No Support || ||
|-
| Clang || style="background:#99FF66;text-align:center;" | Supported || style="text-align:center;" | -std=c11 || No support for (optional) <threads.h>
|-
| GCC || style="background:#99FF66;text-align:center;" | Supported || style="text-align:center;" | -std=c11<ref>[http://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html Options Controlling C Dialect]</ref> || No support for (optional) <threads.h><ref>[http://gcc.gnu.org/wiki/C11Status C11 Status]</ref>
|-
| TCC || style="background:#FFCC66;text-align:center;" | Partial || || 
|-
| ICC || style="background:#FFCC66;text-align:center;" | Partial || style="text-align:center;" | ||
|-
| Visual C++ || style="background:#FFCC66;text-align:center;" | Partial ||   || Missing most headers
|}

== References ==
{{reflist}}

== External links ==
* [http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1570.pdf C11 final draft]

[[Category:C standards]]