是双引号（“）是预处理式的或未终止的字符串字面的？
双引号 (") 是预处理标记还是未终止的字符串文字？
C11, 6.4 词法元素，语法，1:
preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
punctuator
each non-white-space character that cannot be one of the above
C11, 6.4.5 字符串文字，语法，1:
string-literal:
encoding-prefix(opt) " s-char-sequence(opt) "
注意：GCC 认为它成为未终止的字符串文字：
#if 0
"
#endif
产生：
warning: missing terminating " character
Is double quote (") a preprocessing-token or an unterminated string literal?
C11, 6.4 Lexical elements, Syntax, 1:
preprocessing-token:
header-name
identifier
pp-number
character-constant
string-literal
punctuator
each non-white-space character that cannot be one of the above
C11, 6.4.5 String literals, Syntax, 1:
string-literal:
encoding-prefix(opt) " s-char-sequence(opt) "
Note: GCC considers it to be an unterminated string literal:
#if 0
"
#endif
produces:
warning: missing terminating " character
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评论（1）
C 2018 6.4.1 3说，预处理令牌的一个类别是“单个非白色空间字符，这些字符与其他预处理代币类别不符合其他情况”，下一个句子说“如果
'或“字符匹配最后一个类别，行为不确定。”因此，如果出现一个孤立的“出现（一个与其他未与另一个配对的与 s-char-sequence 之间的配对），则失败匹配字符串的词汇形式，并将其解析为与其他类别相匹配的单个非白色空间字符，并且该行为未通过标准来定义。 .2 3描述
＃include 带有“”q-char-sequence的指令> new-line ”。 i> new-line ”。表格显示为“”Q-char- sequence“”，然后是＃＃inclage段落正在讨论的类型的指令。）C 2018 6.4.1 3 says one category of preprocessing tokens is “single non-white-space characters that do not lexically match the other preprocessing token categories”, and the next sentence says “If a
’or a"character matches the last category, the behavior is undefined.” Thus, if an isolated"appears (one that is not paired with another"with an s-char-sequence between them), it fails to match the lexical form of a string literal and is parsed as a single non-white-space character that does not match the other categories, and the behavior is not defined by the standard. This explains the GCC message.(I note that 6.10.2 3 describes
#includedirectives with “"q-char-sequence"new-line”. However, the earlier grammar in 6.10 1 describes the directives as “#includepp-tokens new-line”. My interpretation of this is that the directive is parsed as having preprocessor token(s), notably a string literal, and 6.10.2 3 says that if that string literal has the form shown as “"q-char-sequence"”, then it is a#includedirective of the type that paragraph is discussing.)