Most C preprocessor features are inactive unless you give specific
directives to request their use. (Preprocessing directives are lines
starting with a @samp{#} token, possibly preceded by whitespace;
-@pxref{Directives}). However, there are three transformations that the
+@pxref{Directives}). However, there are four transformations that the
preprocessor always makes on all the input it receives, even in the
-absence of directives.
+absence of directives. These are, in order:
-@itemize @bullet
+@enumerate
@item
Trigraphs, if enabled, are replaced with the character they represent.
-Conceptually, this is the very first action undertaken, just before
-backslash-newline deletion.
@item
Backslash-newline sequences are deleted, no matter where. This
feature allows you to break long lines for cosmetic purposes without
changing their meaning.
+Recently, the non-traditional preprocessor has relaxed its treatment of
+escaped newlines. Previously, the newline had to immediately follow a
+backslash. The current implementation allows whitespace in the form of
+spaces, horizontal and vertical tabs, and form feeds between the
+backslash and the subsequent newline. The preprocessor issues a
+warning, but treats it as a valid escaped newline and combines the two
+lines to form a single logical line. This works within comments and
+tokens, including multi-line strings, as well as between tokens.
+Comments are @emph{not} treated as whitespace for the purposes of this
+relaxation, since they have not yet been replaced with spaces.
+
@item
-All C comments are replaced with single spaces.
+All comments are replaced with single spaces.
@item
Predefined macro names are replaced with their expansions
(@pxref{Predefined}).
-@end itemize
+@end enumerate
+
+For end-of-line indicators, any of \n, \r\n, \n\r and \r are recognised,
+and treated as ending a single line. As a result, if you mix these in a
+single file you might get incorrect line numbering, because the
+preprocessor would interpret the two-character versions as ending just
+one line. Previous implementations would only handle UNIX-style \n
+correctly, so DOS-style \r\n would need to be passed through a filter
+first.
The first three transformations are done @emph{before} all other parsing
and before preprocessing directives are recognized. Thus, for example,
is equivalent to @code{"foo\bar"}, not to @code{"foo\\bar"}. To avoid
having to worry about this, do not use the GNU extension which permits
-multiline strings. Instead, use string constant concatenation:
+multi-line strings. Instead, use string constant concatenation:
@example
"foo\\"
Your program will be more portable this way, too.
-There are a few exceptions to all three transformations.
+There are a few things to note about the above four transformations.
@itemize @bullet
@item
Comments and predefined macro names (or any macro names, for that
matter) are not recognized inside the argument of an @samp{#include}
-directive, whether it is delimited with quotes or with @samp{<} and
+directive, when it is delimited with quotes or with @samp{<} and
@samp{>}.
@item
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.)
+character or string constant.
@item
ISO ``trigraphs'' are converted before backslash-newlines are 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
+the backslash-newline is deleted as usual, but it is too late to
recognize the trigraph.
This is relevant only if you use the @samp{-trigraphs} option to enable
feature altogether.
Preservation of the form of whitespace between tokens is unlikely to
-change from current behavior (see @ref{Output}), but you are advised not
+change from current behavior (@ref{Output}), but you are advised not
to rely on it.
The following are undocumented and subject to change:-
@itemize @bullet
@item Interpretation of the filename between @samp{<} and @samp{>} tokens
- resulting from a macro-expanded @samp{#include} directive
+ resulting from a macro-expanded filename in a @samp{#include} directive
The text between the @samp{<} and @samp{>} is taken literally if given
-directly within a @samp{#include} or similar directive. If a directive
-of this form is obtained through macro expansion, however, behavior like
-preservation of whitespace, and interpretation of backslashes and quotes
+directly within a @samp{#include} or similar directive. If the
+angle-bracketed filename is obtained through macro expansion, however,
+preservation of whitespace and interpretation of backslashes and quotes
is undefined. @xref{Include Syntax}.
@item Precedence of ## operators with respect to each other
-It is not defined whether a sequence of ## operators are evaluated
-left-to-right, right-to-left or indeed in a consistent direction at all.
-An example of where this might matter is pasting the arguments @samp{1},
-@samp{e} and @samp{-2}. This would be fine for left-to-right pasting,
-but right-to-left pasting would produce an invalid token @samp{e-2}.
+Whether a sequence of ## operators is evaluated left-to-right,
+right-to-left or indeed in a consistent direction at all is not
+specified. An example of where this might matter is pasting the
+arguments @samp{1}, @samp{e} and @samp{-2}. This would be fine for
+left-to-right pasting, but right-to-left pasting would produce an
+invalid token @samp{e-2}. It is possible to guarantee precedence by
+suitable use of nested macros.
@item Precedence of # operator with respect to the ## operator
-It is undefined which of these two operators is evaluated first.
+Which of these two operators is evaluated first is not specified.
@end itemize
@item -Wtrigraphs
@findex -Wtrigraphs
Warn if any trigraphs are encountered. This option used to take effect
-only if @samp{-trigraphs} was also specified, but now works independently.
+only if @samp{-trigraphs} was also specified, but now works
+independently. Warnings are not given for trigraphs within comments, as
+we feel this is obnoxious.
@item -Wwhite-space
@findex -Wwhite-space
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