ANSI C (C89/C90) was the first official contract for the C language — function prototypes, `const`, and a standard library (`stdio.h`, `stdlib.h`, etc.) that works the same on UnixAn influential multitasking OS from the 1970s — files, pipes, and C shaped Linux, macOS, and BSD., Windows, and embedded chips. Every modern C compiler still builds on this baseline.

C had been defined by a book (K&R), not a formal agreement. ANSI C was the first time compiler vendors signed off on one shared definition of the language.

A developer could write code on a SunSun Microsystems — maker of Java, Solaris, and SPARC; acquired by Oracle in 2010. workstationA powerful desktop machine for technical work — between personal computers and minicomputers in price and capability., ship it to a PCPersonal computer — a computer designed for individual use, especially desktop or laptop machines., or flash it onto a microcontroller — and expect `printf`, `malloc`, and function calls to work the same way. That reliability is why C became the default for operating systems and device drivers.