6.10 Preprocessing directives

preprocessing-file:
                group_opt

group:
                group-part
                group group-part

group-part:
                if-section
                control-line
                text-line
                # non-directive
if-section:
                if-group elif-groups_opt else-group_opt endif-line

if-group:
                # if     constant-expression new-line group_opt
                # ifdef  identifier new-line group_opt
                # ifndef identifier new-line group_opt

elif-groups:
                elif-group
                elif-groups elif-group

elif-group:
                # elif   constant-expression new-line group_opt

else-group:
                # else   new-line group_opt

endif-line:
                # endif  new-line

control-line:
                # include pp-tokens new-line
                # define  identifier replacement-list new-line
                # define  identifier lparen identifier-list_opt )
                                                    replacement-list new-line
                # define  identifier lparen ... ) replacement-list new-line
                # define  identifier lparen identifier-list , ... )
                                                    replacement-list new-line
                # undef   identifier new-line
                # line    pp-tokens new-line
                # error   pp-tokens_opt new-line
                # pragma  pp-tokens_opt new-line
                #         new-line

text-line:
                pp-tokens_opt new-line

non-directive:
                pp-tokens new-line

lparen:
                a ( character not immediately preceded by white-space

replacement-list:
                pp-tokens_opt

pp-tokens:
                preprocessing-token
                pp-tokens preprocessing-token

new-line:
                the new-line character

1839 A preprocessing directivepreprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that (at the start of translation phase 4) is either the first character in the source file (optionally after white space containing no new-line characters) or that follows white space containing at least one new-line character, and is ended by the next new-line character.¹⁴⁰⁾

1840 A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro.

1841 140) Thus, preprocessing directives are commonly called “lines”.

1842 These “lines” have no other syntactic significance, as all white space is equivalent except in certain situations during preprocessing (see the # character string literal creation operator in 6.10.3.2, for example).

1843 A text line shall not begin with a # preprocessing token.

1844 A non-directive shall not begin with any of the directive names appearing in the syntax.

1845 When in a group that is skipped (6.10.1), the directive syntax is relaxed to allow any sequence of preprocessing tokens to occur between the directive name and the following new-line character.

1846 The only white-space characters that shall appear between preprocessing tokens within a preprocessing directive (from just after the introducing # preprocessing token through just before the terminating new-line character) are space and horizontal-tab (including spaces that have replaced comments or possibly other white-space characters in translation phase 3).

1847 The implementation can process and skip sections of source files conditionally, include other source files, and replace macros.

1848 These capabilities are called preprocessing, because conceptually they occur before translation of the resulting translation unit.

1849 The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.

1850 EXAMPLE In:


        #define EMPTY
        EMPTY # include

the sequence of preprocessing tokens on the second line is not a preprocessing directive, because it does not begin with a # at the start of translation phase 4, even though it will do so after the macro EMPTY has been replaced.