Which C standard was published in 1999?

The C standard published in 1999 was C99.

C99 is the common name for ISO/IEC 9899:1999, the international revision of the C programming-language standard. It followed the earlier C90 standard and was developed during the 1990s under the ISO/IEC working group responsible for C. ANSI adopted the international standard in May 2000, but the revision itself is identified by its 1999 publication year.

The update added several influential language features, including inline functions, variable-length arrays, designated initializers, compound literals, `long long` integers, `_Bool`, complex types, and the `restrict` qualifier. It also standardized features such as `//` comments and improved support for IEEE 754 floating-point arithmetic.

C99 is often confused with C89, C11, or C17. C89 refers to the 1989 ANSI standard, while C11 was published in 2011 and C17 in 2018. C99 remained largely compatible with earlier C, but removed or tightened some legacy behavior, including implicit `int` declarations. A compiler can commonly advertise C99 support through the predefined macro `__STDC_VERSION__` with the value `199901L`.