C99

C99 is the ISO/IEC 9899:1999 standard for the C programming language. It superseded the earlier C89/90 standard and was published in 1999 by ISO/IEC. The revision added a number of features and changes to the language and its standard library, with the goal of improving portability, robustness, and usability.

Key language changes include the introduction of the // single-line comment form, and the long long int type for at least 64-bit integers; a _Bool type with the stdbool.h header providing the bool, true, and false abstractions; and support for complex numbers via the _Complex and _Imaginary types, together with the complex.h header. The standard also added facilities for portable integer types (stdint.h), the portable formatting macros in inttypes.h, and the boolean type in stdbool.h. The concept of designated initializers allows initialization of struct and array members by name or index (for example, .x = 1). Compound literals and flexible array members were introduced, as well as variable-length arrays (VLAs). Variadic macros allow macros to accept a variable number of arguments, while the _Pragma operator enables pragma control within macros.

Other notable changes include the inline keyword for functions and the restrict qualifier for pointer aliases. The C99 standard also enhanced the standard library with new headers such as tgmath.h and complex.h, and refined support for mathematical and numeric types.

Adoption varied by compiler, and some features were implemented incompletely in practice. C99’s influence shaped subsequent standards, notably C11 and later revisions, and remains a reference point for modern C programming.