(cpp.info) Global Actions
Transformations Made Globally
Most C preprocessor features are inactive unless you give specific
directives to request their use. (Preprocessing directives are lines
starting with `#'; Directives.). But there are three
transformations that the preprocessor always makes on all the input it
receives, even in the absence of directives.
* All C comments are replaced with single spaces.
* Backslash-Newline sequences are deleted, no matter where. This
feature allows you to break long lines for cosmetic purposes
without changing their meaning.
* Predefined macro names are replaced with their expansions (
The first two transformations are done *before* nearly all other
parsing and before preprocessing directives are recognized. Thus, for
example, you can split a line cosmetically with Backslash-Newline
anywhere (except when trigraphs are in use; see below).
*/ # /*
is equivalent into `#define FOO 1020'. You can split even an escape
sequence with Backslash-Newline. For example, you can split `"foo\bar"'
between the `\' and the `b' to get
This behavior is unclean: in all other contexts, a Backslash can be
inserted in a string constant as an ordinary character by writing a
double Backslash, and this creates an exception. But the ANSI C
standard requires it. (Strict ANSI C does not allow Newlines in string
constants, so they do not consider this a problem.)
But there are a few exceptions to all three transformations.
* C comments and predefined macro names are not recognized inside a
`#include' directive in which the file name is delimited with `<'
* C comments and predefined macro names are never recognized within a
character or string constant. (Strictly speaking, this is the
rule, not an exception, but it is worth noting here anyway.)
* Backslash-Newline may not safely be used within an ANSI "trigraph".
Trigraphs are converted before Backslash-Newline is deleted. If
you write what looks like a trigraph with a Backslash-Newline
inside, the Backslash-Newline is deleted as usual, but it is then
too late to recognize the trigraph.
This exception is relevant only if you use the `-trigraphs' option
to enable trigraph processing. Invocation.
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