C90

C90, short for the 1990 version of the C programming language standard, is the ISO/IEC 9899:1990 standard, commonly known as ANSI C. It was the first international standard for C and followed the earlier ANSI C standard (C89) published in 1989. The standard was issued by ISO with ANSI as a coordinating body and laid out the rules for the C language syntax, semantics, and the standard library as used by mainstream compilers.

C90 defines data types including char, short, int, long, float, and double, along with type qualifiers such as const and volatile. It specifies pointers, arrays, structures, and unions, as well as storage classes including auto, extern, static, and register. The standard introduced function prototypes, enabling better type checking of function calls, while preserving backward compatibility with K&R-style declarations to accommodate legacy code. It also specifies the header files and library functions available to portable C programs, such as stdio.h, stdlib.h, string.h, math.h, time.h, limits.h, and float.h, along with macros and type limits.

Impact and legacy: C90 created a portable baseline that allowed code to be compiled across many architectures, and it guided compiler implementations for years. It was later complemented and largely superseded by subsequent standards, most notably ISO/IEC 1999 (C99) and beyond, with corrigenda issued in subsequent years. Today, C90 remains a historical reference point for the evolution of the C language and for maintaining older C codebases, particularly in embedded and systems programming.