The Tinyserial library is a space-saving alternative to the Arduino software distribution's libraries for reading and writing characters and strings to the USART0 serial port on the Atmel ATmega168 and ATmega328p MCUs found on Arduino Diecimilla and Duemilanove boards. While the Arduino software distribution's libraries provide interrupt-driven serial I/O with far more features and support more MCUs, the Tinyserial library provides only the most basic polling-based serial I/O. However, the Tinyserial library uses far less Flash and SRAM, thereby giving you room to implement larger and more complicated applications on your boards. The Tinyserial library respects the GNU libc ABI, so you can call into it from C and C++ programs.
libposix is an impementation of the core functionality of all Unix systems. It is a full, cross-platform implementation of the POSIX 2008 standard. It is meant to replace existing implementations of a Unix system's core libraries. It is an exact implementation of POSIX 2008 and nothing else (no extensions, no previous POSIX versions). However, it works well with possible extensions to the core system functionallity (for example, GNU or BSD).