This is avr-libc. It is a C library implementation for use with GNU GCC and GNU binutils for development of programs for Atmel's AVR microcontrollers.

The current branch (1.3), requires the following versions of gcc and binutils:

  Tool            Minimal Version       Preferred Version
  ===========     ===============       =================
  binutils            2.13                   2.16
  gcc                 3.3                 Latest 3.4.x

For details on installing and using AVR Libc, see the users manual in the
doc/api directory or online at:

  http://savannah.nongnu.org/projects/avr-libc/