6.10 Preprocessing directives
Syntax
preprocessing-file:
groupopt
group:
group-part
group group-part
group-part:
if-section
control-line
text-line
# non-directive
if-section:
if-group elif-groupsopt else-groupopt endif-line
if-group:
# if constant-expression new-line groupopt
# ifdef identifier new-line groupopt
# ifndef identifier new-line groupopt
elif-groups:
elif-group
elif-groups elif-group
elif-group:
# elif constant-expression new-line groupopt
else-group:
# else new-line groupopt
endif-line:
# endif new-line
control-line:
# include pp-tokens new-line
# define identifier replacement-list new-line
# define identifier lparen identifier-listopt )
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-tokensopt new-line
# pragma pp-tokensopt new-line
# new-line
text-line:
pp-tokensopt new-line
non-directive:
pp-tokens new-line
lparen:
a ( character not immediately preceded by white-space
replacement-list:
pp-tokensopt
pp-tokens:
preprocessing-token
pp-tokens preprocessing-token
new-line:
the new-line character
Description
A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the following constraints: The first token in the sequence is 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. The last token in the sequence is the first new- line character that follows the first token in the sequence.[1] A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro.
A text line shall not begin with a # preprocessing token. A non-directive shall not begin with any of the directive names appearing in the syntax.
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.
Constraints
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).
Semantics
The implementation can process and skip sections of source files conditionally, include other source files, and replace macros. These capabilities are called preprocessing, because conceptually they occur before translation of the resulting translation unit.
The preprocessing tokens within a preprocessing directive are not subject to macro expansion unless otherwise stated.
EXAMPLE In:
#define EMPTY
EMPTY # include <file.h>
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.
Footnotes