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26.3 Preprocessing Tokens

Preprocessing divides C code (minus its comments) into tokens that are similar to C tokens, but not exactly the same. Here are the quirks of preprocessing tokens.

The main classes of preprocessing tokens are identifiers, preprocessing numbers, string constants, character constants, and punctuators; there are a few others too.


An identifier preprocessing token is syntactically like an identifier in C: any sequence of letters, digits, or underscores, as well as non-ASCII characters represented using ‘\U’ or ‘\u’, that doesn’t begin with a digit.

During preprocessing, the keywords of C have no special significance; at that stage, they are simply identifiers. Thus, you can define a macro whose name is a keyword. The only identifier that is special during preprocessing is defined (see defined).

preprocessing number

A preprocessing number is something that preprocessing treats textually as a number, including C numeric constants, and other sequences of characters which resemble numeric constants. Preprocessing does not try to verify that a preprocessing number is a valid number in C, and indeed it need not be one.

More precisely, preprocessing numbers begin with an optional period, a required decimal digit, and then continue with any sequence of letters, digits, underscores, periods, and exponents. Exponents are the two-character sequences ‘e+’, ‘e-’, ‘E+’, ‘E-’, ‘p+’, ‘p-’, ‘P+’, and ‘P-’. (The exponents that begin with ‘p’ or ‘P’ are new to C99. They are used for hexadecimal floating-point constants.)

The reason behind this unusual syntactic class is that the full complexity of numeric constants is irrelevant during preprocessing. The distinction between lexically valid and invalid floating-point numbers, for example, doesn’t matter at this stage. The use of preprocessing numbers makes it possible to split an identifier at any position and get exactly two tokens, and reliably paste them together using the ## operator (see Concatenation).


A punctuator is syntactically like an operator. These are the valid punctuators:

[  ]   (  )  {  }  .  ->
++ --  &  *  +  -  ~  !
/  %   << >> <  >  <= >=  ==  !=  ^  |  &&  ||
?  :   ;  ...
=  *=  /=  %=  +=  -=  <<=  >>=  &=  ^=  |=
,  #   ##
<: :>  <% %>  %:  %:%:
string constant

A string constant in the source code is recognized by preprocessing as a single preprocessing token.

character constant

A character constant in the source code is recognized by preprocessing as a single preprocessing token.

header name

Within the #include directive, preprocessing recognizes a header name token. It consists of ‘"name"’, where name is a sequence of source characters other than newline and ‘"’, or ‘<name>’, where name is a sequence of source characters other than newline and ‘>’.

In practice, it is more convenient to think that the #include line is exempt from tokenization.


Any other character that’s valid in a C source program is treated as a separate preprocessing token.

Once the program is broken into preprocessing tokens, they remain separate until the end of preprocessing. Macros that generate two consecutive tokens insert whitespace to keep them separate, if necessary. For example,

#define foo() bar
     → bar baz
     → barbaz

The only exception is with the ## preprocessing operator, which pastes tokens together (see Concatenation).

Preprocessing treats the null character (code 0) as whitespace, but generates a warning for it because it may be invisible to the user (many terminals do not display it at all) and its presence in the file is probably a mistake.

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