Fiwix is an operating system kernel written from scratch, based on the UNIX architecture and fully focused on being POSIX compatible. It is designed and developed mainly as a hobby OS and, since it serves also for educational purposes, the kernel code is kept as simple as possible for the benefit of students and OS enthusiasts. It is small in size with only 35,000 lines of code, runs on the 32-bit x86 hardware platform and is compatible with a good base of existing GNU applications.
It is based on the monolithic kernel design and it is written entirely in C language, leaving only for the Assembler the small needed parts. It follows the System V Application Binary Interface (or ABI) and is also Linux 2.0 System Call ABI mostly compatible.
Do not think of Fiwix as a new UNIX flavor with its own system utilities, libraries and compilers. Instead, think about it as a new UNIX-like kernel that takes advantage of the existing GNU applications.
The Fiwix 1.0 version comes with the Newlib v2.2.0 as its system C Library and with the Minix v2 as its primary file system.