为啥"$$"也能当标识符

[MoRanjiang]

昨天做作业碰到的。
看各种资料都只说大小写字母下划线和数字，但是 GCC 编译了确实能跑。

[Shatyuka]

gcc 方言

[FrankHB]

登记在册的已知扩展。

ISO/IEC 9899 （反正好几个版本没改了）

J.5 Common extensions
1 The following extensions are widely used in many systems, but are not portable to all implementations. The inclusion of any extension that may cause a strictly conforming program to become invalid renders an implementation nonconforming. Examples of such extensions are new keywords, extra library functions declared in standard headers, or predefined macros with names that do not begin with an underscore.

J.5.2 Specialized identifiers
1 Characters other than the underscore _, letters, and digits, that are not part of the basic source character set (such as the dollar sign $, or characters in national character sets) may appear in an identifier (6.4.2).

原则上这个是“常见”的，比如并列的有：
J.5.3 Lengths and cases of identifiers
1 All characters in identifiers (with or without external linkage) are significant (6.4.2).
类似 C89 用 6 个字符以上的标识符都算。（虽然 C99 是 31 个。）

[weiweiwitch]

你这是对钱有意见？