C11 (C standard revision)

Changes from C99

The standard includes several changes to the C99 language and library specifications, such as:^[10]

• 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>^[11] for atomic operations supporting the C11 memory model).
• Improved Unicode 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 encoded literals).^[12]
• 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).^[13]
• 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 struct T { int tag; union { float x; int n; }; };.
• 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, 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 if termination with exit fails.^[14]
• A new timespec_get function and corresponding structure in <time.h> with a degree of POSIX 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).^[15]

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.^[16] Programs can use predefined macros to determine whether an implementation supports a certain feature or not.

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.^[20] Even Microsoft, a main proponent of this interface, does not conform to the definition.^[21] In addition, Annex K does not include the more useful TR24731-2 (dynamic allocation functions), such as vasprintf and open_memstream.^[22] The few open-source implementations include Open Watcom C/C++'s "Safer C" library^[23] and safeclib.^[24]

See also

• C++20, C++17, C++14, C++11, C++03, C++98, versions of the C++ programming language standard
• Compatibility of C and C++

References

External links

• The C1X Charter
• N1570, the final draft of C1X, dated 12 April 2011
• ISO C Working Group's official website
• The standard draft in simple ASCII and linked HTML forms, in addition to the PDF.
• Safe C Library of Bounded APIs
• Plum, Thomas (April 6, 2012). "C Finally Gets A New Standard". Dr. Dobb's Journal.
• Safe C API—Concise solution of buffer overflow, The OWASP Foundation, OWASP AppSec, Beijing 2011