来源： https://blog.csdn.net/tabactivity/article/details/78558457
attribute是GNU C特色之一,在iOS用的比较广泛.系统中有许多地方使用到. attribute可以设置函数属性（Function Attribute ）、变量属性（Variable Attribute ）和类型属性（Type Attribute)等.
函数属性(Function Attribute)
- noreturn
- noinline
- always_inline
- pure
- const
- nothrow
- sentinel
- format
- format_arg
- no_instrument_function
- section
- constructor
- destructor
- used
- unused
- deprecated
- weak
- malloc
- alias
- warn_unused_result
- nonnull
类型属性(Type Attributes)
- aligned
- packed
- transparent_union,
- unused,
- deprecated
- may_alias
变量属性(Variable Attribute)
- aligned
- packed
Clang特有的
- availability
- overloadable