The C standard published in 2011 is called C11.
Its formal designation is ISO/IEC 9899:2011, following the naming convention that combines the language name with the publication year. C11 replaced C99 and was later superseded by C17, the 2018 revision of the C standard.
C11 introduced or standardized several important features, including a language-level memory model, atomic types and operations, and support for multithreading through `<threads.h>` where implemented. It also added features such as the `_Generic` selection construct and the `_Static_assert` compile-time assertion.
The label can be confusing because the working draft was previously called C1X, reflecting the expectation that publication might occur before 2010. The final draft, N1570, appeared in April 2011; the official standard was published in December 2011. C11 support is commonly indicated by the predefined macro `__STDC_VERSION__` having the value `201112L`.