Most C preprocessor features are inactive unless you give specific directives to request their use. (Preprocessing directives are lines starting with `#'; see 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 (see Predefined).
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).
/* */ # /* */ defi\ ne FO\ O 10\ 20
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
"foo\\ bar"
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 `<' and `>'.
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. See Invocation.