avr-libc can be freely used and redistributed, provided the following license conditions are met.

Portions of avr-libc are Copyright (c) 1999-2010 Werner Boellmann, Dean Camera, Pieter Conradie, Brian Dean, Keith Gudger, Wouter van Gulik, Bjoern Haase, Steinar Haugen, Peter Jansen, Reinhard Jessich, Magnus Johansson, Harald Kipp, Carlos Lamas, Cliff Lawson, Artur Lipowski, Marek Michalkiewicz, Todd C. Miller, Rich Neswold, Colin O'Flynn, Bob Paddock, Andrey Pashchenko, Reiner Patommel, Florin-Viorel Petrov, Alexander Popov, Michael Rickman, Theodore A. Roth, Juergen Schilling, Philip Soeberg, Anatoly Sokolov, Nils Kristian Strom, Michael Stumpf, Stefan Swanepoel, Helmut Wallner, Eric B. Weddington, Joerg Wunsch, Dmitry Xmelkov, Atmel Corporation, egnite Software GmbH, The Regents of the University of California. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: * Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. * Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. * Neither the name of the copyright holders nor the names of contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.