错误：使用未声明的标识符“errno_t”的

下面是我死了简单的虚拟code：

Here is my dead simple dummy code:

#include <errno.h>

int main(void)
{
    errno_t e;
    return 0;
}

这令人惊讶引发此错误：

Which surprisingly raises this error:

main.c:5:5: error: use of undeclared identifier 'errno_t'
    errno_t x;
    ^

我开始的遵循痕迹的：当编译器看到＆LT; ...＆GT; 夹杂它会先看看 / usr / include目录其中，当然，我发现 errno.h中文件。其实它在一个单一的线，除了许可证评论，这是：

I started to follow the traces: when the compiler sees the <...> inclusions it will first look at /usr/include where of course I found errno.h file. Actually it has a single line in it, besides the license comment, which is:

#include <sys/errno.h>

现在，在 / usr / include目录中的 errno.h中 / SYS 我发现下面几行：

Now, at /usr/include/sys in errno.h I found the following lines:

#include <sys/cdefs.h>

#if defined(__STDC_WANT_LIB_EXT1__) && __STDC_WANT_LIB_EXT1__ >= 1
#include <sys/_types/_errno_t.h>
#endif

而在 / usr / include目录/ _types 在 _errno_t.h 我发现这一点：

typedef int errno_t;

因此​​，看起来，它的存在，它是整数类型的别名，的 errno.h中部分 - 就像它应该是。

So it looks like, it is there, and it is an alias of the integer type, and part of the errno.h -- just as it should be.

那么，为什么是不是包括在内？为什么编译器引发未声明的标识符错误？

Then why isn't it included? Why the compiler raises the undeclared identifier error?

在此先感谢！

相关信息：的

RELEVANT INFO:

Compiler:
    Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)`

Compiler flags:
    -std=c11 -I/usr/include/sys -I/usr/local/include

宏变量 __ STDC_WANT_LIB_EXT1 __ 将在定义 / usr / include目录中的 / SYS cdefs.h在以下行：

The macro variable __STDC_WANT_LIB_EXT1__ will be defined at /usr/include/sys in cdefs.h in the following lines:

/* If the developer has neither requested a strict language mode nor a version
 * of POSIX, turn on functionality provided by __STDC_WANT_LIB_EXT1__ as part
 * of __DARWIN_C_FULL.
 */
#if !defined(__STDC_WANT_LIB_EXT1__) && !defined(__STRICT_ANSI__) && __DARWIN_C_LEVEL >= __DARWIN_C_FULL
#define __STDC_WANT_LIB_EXT1__ 1
#endif

更新：的

UPDATE:

由于@PaulR在评论区说：如果我删除 -std = C11 标志，它编译。这是一样令人惊讶，如果被列入该标志作为引发的错误。所以我用一个子问题，延长了这个问题：

As @PaulR said in the comment section: if I remove the -std=c11 flag, it compiles. Which is just as surprising as the error raised if the flag was included. So I extend this question with a sub-question:

的不 errno_t 的C11标准的一部分，或者为什么没有收录，当编译器规定的标准？的

Is not errno_t part of the C11 standard, or why isn't it included, when the standard is specified for the compiler?