alignas说明符vs __attribute __(aligned),c 11

另外,从alignment support proposal(3.11)：

A fundamental alignment is represented by an alignment less than or equal to the greatest alignment supported by the implementation in all contexts,which is equal to alignof(std::max_align_t) (18.1).

An extended alignment is represented by an alignment greater than
alignof(std::max_align_t). It is implementation-defined whether any extended
alignments are supported and the contexts in which they are supported (7.1.6). A type
having an extended alignment requirement is an over-aligned type.

同样的文件(7.1.6)：

if the constant expression evaluates to an extended alignment and the implementation
does not support that alignment in the context of the declaration,the program is illformed

这可能也是答案的一部分.我目前无法访问完整的标准,有人应该可以确认.

对于__attribute __(aligned)和alignas之间的区别,我不认为它们在语义上是不同的,但是一个只是一个编译器扩展,另一个是由标准完全定义的.

为了回答你的最后一个问题,alignas只定义为：

alignas ( constant-expression ) 
alignas ( type-id )