# C11

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< c

ISO/IEC 9899:2011, a.k.a. C11, is a previous revision of the C standard.

## Obsolete

### Removed

- gets()

## New language features

- Multithreaded environments
  - Atomic objects (`_Atomic`)
  - Thread local storage (`_Thread_local`)
- Enhanced alignment support
  - Alignment query (`_Alignof`)
  - Alignment strengthening (`_Alignas`)
  - Over-aligned types
- Unicode support
  - `u`/`U` character constants
  - `u8`/`u`/`U` string literals
- Generic selection expressions (`_Generic`)
- Non-returning functions (`_Noreturn`)
- Anonymous struct and union members
- Fine-grained evaluation order
- Extending the lifetime of temporary objects
- `_Static_assert`
- Analyzability

### Feature test macros for optional features

- `__STDC_ANALYZABLE__`
  - Indicates analyzability is supported.
- `__STDC_LIB_EXT1__`
  - Indicates bounds checking functions are supported.
- `__STDC_NO_ATOMICS__`
  - Indicates atomic objects and the atomic operation library are not supported.
- `__STDC_NO_COMPLEX__`
  - Indicates complex types and the complex math functions are not supported.
    These features were mandatory in C99.
- `__STDC_NO_THREADS__`
  - Indicates thread local storage and the thread support library are not supported.
- `__STDC_NO_VLA__`
  - Indicates variable length arrays and variably modified types are not supported.
    These features were mandatory in C99.

## New library features

### New headers

- `<stdalign.h>`
- `<stdatomic.h>`
- `<stdnoreturn.h>`
- `<threads.h>`
- `<uchar.h>`

### Library features

- Concurrency support library
- `aligned_alloc()`
- UTF-16/32 type aliases
  - `char16_t`
  - `char32_t`
- UTF-16/32 conversion functions
  - mbrtoc16()
  - mbrtoc32()
  - c16rtomb()
  - c32rtomb()
- quick_exit
- at_quick_exit
- Exclusive modes of fopen() and freopen() ("x")
- Bounds checking functions
- `timespec`
- `timespec_get()`
- `CMPLX(F|L)?`
- New numeric limit macros
  - `(FLT|DBL|LDBL)_DECIMAL_DIG`
  - `(FLT|DBL|LDBL)_TRUE_MIN`
  - `(FLT|DBL|LDBL)_HAS_SUBNORM`
- Thread local errno

## Defect reports

Template:c/language/history/DR11

## Compiler support

Template:c/compiler support/11

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