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C11 (previously C1X, formally ISO/IEC 9899:2011) is a past standard for the C programming language. It replaced C99 (standard ISO/IEC 9899:1999) and has been superseded by C17 (standard ISO/IEC 9899:2018). C11 mainly standardizes features already supported by common contemporary compilers, and includes a detailed memory model to better support multiple threads of execution. Due to delayed availability of conforming C99 implementations, C11 makes certain features optional, to make it easier to comply with the core language standard. The final draft, N1570, was published in April 2011. The new standard passed its final draft review on October 10, 2011 and was officially ratified by ISO and published as ISO/IEC 9899:2011 on December 8, 2011, with no comments requiring resolution by participating national bodies. A standard macro __STDC_VERSION__ is defined with value 201112L to indicate that C11 support is available.


Changes from C99

The standard includes several changes to the C99 language and library specifications, such as * Alignment specification (_Alignas specifier, _Alignof operator, aligned_alloc function, <stdalign.h> header) * The _Noreturn function specifier and the <stdnoreturn.h> header * Type-generic expressions using the _Generic keyword. For example, the following macro cbrt(x) translates to cbrtl(x), cbrt(x) or cbrtf(x) depending on the type of x:
#define cbrt(x) _Generic((x), long double: cbrtl, \ default: cbrt, \ float: cbrtf)(x)
* Multi-threading support (_Thread_local storage-class specifier, <threads.h> header including thread creation/management functions, mutex, condition variable and thread-specific storage functionality, as well as <stdatomic.h> for atomic operations supporting the C11 memory model). * Improved
Unicode Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...
support based on the C Unicode Technical Report ISO/IEC TR 19769:2004 (char16_t and char32_t types for storing UTF-16/ UTF-32 encoded data, including conversion functions in <uchar.h> and the corresponding u and U string literal prefixes, as well as the u8 prefix for
UTF-8 UTF-8 is a character encoding standard used for electronic communication. Defined by the Unicode Standard, the name is derived from ''Unicode Transformation Format 8-bit''. Almost every webpage is transmitted as UTF-8. UTF-8 supports all 1,112,0 ...
encoded literals). * Removal of the gets function (in favor of safer fgets), which was deprecated in the previous C language standard revision, ISO/IEC 9899:1999/Cor.3:2007(E). * Bounds-checking interfaces (Annex K). * Analyzability features (Annex L). * More macros for querying the characteristics of floating-point types, concerning subnormal floating-point numbers and the number of decimal digits the type is able to store. * Anonymous ''structures'' and ''unions'', useful when unions and structures are nested, e.g. in . * Static assertions, which are evaluated during translation at a later phase than #if and #error, when types are understood by the translator. * An exclusive create-and-open mode ("…x" suffix) for fopen. This behaves like O_CREAT, O_EXCL in
POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...
, which is commonly used for lock files. * The quick_exit function as a third way to terminate a program, intended to do at least minimal deinitialization. * A new timespec_get function and corresponding structure in <time.h> with a degree of
POSIX The Portable Operating System Interface (POSIX; ) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines application programming interfaces (APIs), along with comm ...
compatibility. * Macros for the construction of complex values (partly because real + imaginary*I might not yield the expected value if imaginary is infinite or NaN).


Optional features

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. Programs can use predefined macros to determine whether an implementation supports a certain feature or not.


Compiler support

Some features of C11 are supported by the GCC starting with version 4.6,
Clang Clang () is a compiler front end for the programming languages C, C++, Objective-C, Objective-C++, and the software frameworks OpenMP, OpenCL, RenderScript, CUDA, SYCL, and HIP. It acts as a drop-in replacement for the GNU Compiler ...
starting with version 3.1, IBM XL C starting with version 12.1, and Microsoft Visual C++ starting with VS 2019 (16.8) in September 2020.


Criticism

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. Even Microsoft, a main proponent of this interface, does not conform to the definition. In addition, Annex K does not include the more useful TR24731-2 (dynamic allocation functions), such as and . The few open-source implementations include Open Watcom C/C++'s "Safer C" library and safeclib.


See also

* C++23, C++20, C++17, C++14,
C++11 C++11 is a version of a joint technical standard, ISO/IEC 14882, by the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC), for the C++ programming language. C++11 replaced the prior vers ...
,
C++03 C++03 is a version of the ISO/ IEC 14882 standard for the C++ programming language. It is defined by two standards organizations, the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC), in ...
, C++98, versions of the C++ programming language standard * Compatibility of C and C++


References


Further reading


N1570
(working draft of C11 standard); WG14; 2011.
HTML and ASCII versions


(official C11 standard);
ISO The International Organization for Standardization (ISO ; ; ) is an independent, non-governmental, international standard development organization composed of representatives from the national standards organizations of member countries. Me ...
; 2011. *
Safe C API—Concise solution of buffer overflow, The OWASP Foundation, OWASP AppSec, Beijing 2011


External links




C1X Charter
- WG14 * Safe C Library of Bounded APIs {{ISO standards C (programming language) Programming language standards IEC standards ISO standards