C（C99+）标准需要（尽管隐式）定义 __ stdc __ 至1至1的符合实现。
但是，C标准不需要IEC 60559以定义 __ stdc_iec_iec_559 __ __ __ __ __ __ __ code> （至1）。
后果：
#if __STDC__ != 1
/* non-conforming implementation */
#endif
#if __STDC_IEC_559__ != 1
/* may be IEC 60559 conformant implementation */
#endif
在这里，我们看到这些宏的语义没有一致性。有什么想法吗？是可能的缺陷吗？
为什么C标准不需要IEC 60559符合实现来定义 __ stdc_iec_559 __ （至1）？
The C (C99+) standard requires (though implicitly) a conforming implementation to define __STDC__ to 1.
However, the C standard does not require an IEC 60559 conformant implementation to define __STDC_IEC_559__ (to 1).
Consequences:
#if __STDC__ != 1
/* non-conforming implementation */
#endif
#if __STDC_IEC_559__ != 1
/* may be IEC 60559 conformant implementation */
#endif
Here we see that there is no consistency in the semantics of these macros. Any ideas why? Is it a possible defect?
Why the C standard does not require an IEC 60559 conformant implementation to define __STDC_IEC_559__ (to 1)?
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评论（1）
定义它符合实施自身的最大利益。 C17 6.10.8.3：
从技术上讲，编译器可以不定义它，但没有任何意义，如果它确实符合IEC 60559。或
__ stdc_version __。值得注意的是，该常数自C99以来才存在，因此C90实现可能在不定义宏常数的情况下使用IEC 60559。
It's in the implementation's own best interest to define it. C17 6.10.8.3:
While it technically would be possible for the compiler not to define it, it doesn't make any sense not to, in case it does conform to IEC 60559. It's a quality of implementation stamp, just like
__STDC__or__STDC_VERSION__.Notably, this constant has only been around since C99, so it would be possible that C90 implementations use IEC 60559 without defining the macro constant.