The /* characters introduce a comment; the */ characters terminate a comment. They do not indicate a comment when occurring within a string literal. Comments do not nest. Once the /* introducing a comment is seen, all other characters are ignored until the ending */ is encountered.

An identifier, or name, is a sequence of letters, digits, and underscores (_). The first character cannot be a digit. Uppercase and lowercase letters are distinct. Name length is unlimited. The terms identifier and name are used interchangeably.

The identifiers listed in Table 3-1, are reserved for use as keywords and cannot be used for any other purpose.

Traditional C reserves and ignores the fortran keyword.

The four types of constants are integer, character, floating, and enumeration. Each constant has a type, determined by its form and value.

In this section's discussions of the various types of constants, a unary operator preceding the constant is not considered part of it. Rather, such a construct is a constant-expression (see “Constant Expressions” in Chapter 6). Thus, the integer constant 0xff becomes an integral constant expression by prefixing a minus sign, for instance, -0xff. The effect of the - operator is not considered in the discussion of integer constants.

As an example, the integer constant 0xffffffff has type int in traditional C, with value -1. It has type unsigned in ANSI C, with value 232 - 1. This discrepancy is inconsequential if the constant is assigned to a variable of integral type (for example, int or unsigned), as a conversion occurs. If it is assigned to a double, however, the value differs as indicated between traditional and ANSI C.

12345LL
12345ll

123456ULL
123456ull

A string literal is a sequence of characters surrounded by double quotation marks, as in “...”. A string literal has type array of char and is initialized with the given characters. The compiler places a null byte (\0) at the end of each string literal so that programs that scan the string literal can find its end. A double-quotation character (“) in a string literal must be preceded by a backslash (\). In addition, the same escapes as those described for character constants can be used. (See “Character Constants”, for a list of escapes.) A backslash (\) and the immediately following newline are ignored. Adjacent string literals are concatenated.

In traditional C, all string literals, even when written identically, are distinct.

In ANSI C, identical string literals are not necessarily distinct. Prefixing a string literal with L specifies a wide string literal. Adjacent wide string literals are concatenated.

As an example, consider the sentence “He said, `Hi there.'” This sentence could be written with three adjacent string literals:

"He said, " "\'Hi " "there.\'" 

An operator specifies an operation to be performed. The operators [ ], ( ), and ? : must occur in pairs, possibly separated by expressions. The operators # and ## can occur only in preprocessing directives.

operator can be one of the following:

Individual operations are discussed in Chapter 6, “Expressions and Operators”.

A punctuator is a symbol that has semantic significance but does not specify an operation to be performed. The punctuators [ ], ( ), and { } must occur in pairs, possibly separated by expressions, declarations, or statements. The punctuator # can occur only in preprocessing directives.

Some operators, determined by context, are also punctuators. For example, the array index indicator [ ] is a punctuator in a declaration (see Chapter 7, “Declarations”), but an operator in an expression (see Chapter 6, “Expressions and Operators”).