Avr-libc

From Wiki

avr-libc is a very small library for AVR programming. It includes a couple of the most important functions from a standard GNU C Library, but most things are not appropriate and have been left out, but I don't think you'll miss much.

avr-libc also provides simple access to PINs and PORTSs, interrupts, timers, and other memory mapped registers in the AVR.