在gcc手册中找到了有关的解释：
unused：This
attribute, attached to a function, means that the function is meant to
be
possibly unused. GCC will not produce a
warning for this function.
===============================================================================
used： This
attribute, attached to a function, means that code must be emitted for
the
function even if it appears that the function is not referenced. This is
useful,
for example, when the function is referenced only in
inline assembly.
表示该函数或变量可能不使用，这个属性可以避免编译器产生警告信息。
__attribute__((unused)),布布扣,bubuko.com
原文：http://www.cnblogs.com/pengdonglin137/p/3615376.html