Comments, Identifiers, Keywords, Constants
参考书：
The C Programming Language (K&R, second edition)
Comments
The characters /* introduce a comment, which terminates with characters */.
Comments do not nest, and they do not occur within string or character literals.
Identifiers
An identifier is a sequence of letters and digits, The first character must be a letter; the underscore _ counts as a letter.
Upper and lower case letters are different.
Keywords
The following identifiers are reserved for use as keywords, and may not used otherwise:
Constants
Integer Constants
An integer constant consisting of a sequence of digits is taken to be octal if it begins with (digit zero), decimal otherwise.
A sequence of digits preceded by of (digit zero) is taken to be a hexadecimal integer.
An integer constant may be suffixed by the letter u or U, to specify that it is unsigned. It may also be suffixed by the letter l or L to specify that it is long.
Character constants
A character constants is a sequence of one or more characters enclosed in single quotes, as ‘x’.
The value of a character constant with only one character is the numeric value of the character in the machine’s character set at execution time.
The value of a multi-character constant is implementation-defined.
Escape sequence:
The escape \ooo consists of the backslash followed by 1, 2, or 3 octal digits, which are taken to specify the value of the desired character.
Floating constants
A floating constant consists of an integer part, a decimal point, a fraction part, an e or E, an optional signed integer exponent and an optional type suffix, one of f, F, ;, or L.
The integer and fraction parts both consist of a sequence of digits.
Either the integer and fraction part (not both) may be missing; either the decimal point or the e and the exponent (not both) may be missing.
The type is determined by the suffix; F or f makes it float, L or ; makes it long double; otherwise it is double.
Enumeration Constants
Identifiers declared as enumerators are constants of type int.
