c - 使用 '.' token 定义常量

Question

如果我这样定义常量，为什么会出现错误？

#‎define‬ F.NAME "First"
#define L.NAME "Second"
printf("%s %s\n", F.NAME, L.NAME);

但如果我这样做，一切都会很顺利

#define F_NAME "First"
#define L_NAME "Second"
printf("%s %s\n", F_NAME, L_NAME);

我得到的错误是。

Warning: missing white space after the macro name
Error: expected expressions before '.' token

为什么我不能用“.”定义常量？

Answer 1

因为规范不允许。不允许使用句点和大多数其他特殊字符(_ 除外)。

An identifier is a sequence of nondigit characters (including the underscore _, the lowercase and uppercase Latin letters, and other characters) and digits, which designates one or more entities as described in 6.2.1. Lowercase and uppercase letters are distinct. There is no specific limit on the maximum length of an identifier.

来自 C99 规范的 §6.4.2.1。