The preprocessor performs a series of textual transformations on its input. These happen before all other processing. Conceptually, they happen in a rigid order, and the entire file is run through each transformation before the next one begins. GNU CPP actually does them all at once, for performance reasons. These transformations correspond roughly to the first three "phases of translation" described in the C standard.
GNU CPP expects its input to be a text file, that is, an unstructured stream of ASCII characters, with some characters indicating the end of a line of text. Extended ASCII character sets, such as ISO Latin-1 or Unicode encoded in UTF-8, are also acceptable. Character sets that are not strict supersets of seven-bit ASCII will not work. We plan to add complete support for international character sets in a future release.
Different systems use different conventions to indicate the end of a line. GCC accepts the ASCII control sequences LF, CR LF, CR, and LF CR as end-of-line markers. The first three are the canonical sequences used by Unix, DOS and VMS, and the classic Mac OS (before OSX) respectively. You may therefore safely copy source code written on any of those systems to a different one and use it without conversion. (GCC may lose track of the current line number if a file doesn't consistently use one convention, as sometimes happens when it is edited on computers with different conventions that share a network file system.) LF CR is included because it has been reported as an end-of-line marker under exotic conditions.
If the last line of any input file lacks an end-of-line marker, the end of the file is considered to implicitly supply one. The C standard says that this condition provokes undefined behavior, so GCC will emit a warning message.
These are nine three-character sequences, all starting with ??,
that are defined by ISO C to stand for single characters. They permit
obsolete systems that lack some of C's punctuation to use C. For
example, ??/ stands for \, so '??/n' is a character
constant for a newline. By default, GCC ignores trigraphs, but if you
request a strictly conforming mode with the -std option, then
it converts them.
Trigraphs are not popular and many compilers implement them incorrectly.
Portable code should not rely on trigraphs being either converted or
ignored. If you use the -Wall or -Wtrigraphs options,
GCC will warn you when a trigraph would change the meaning of your
program if it were converted.
In a string constant, you can prevent a sequence of question marks from
being confused with a trigraph by inserting a backslash between the
question marks. "(??\?)" is the string (???), not
(?]. Traditional C compilers do not recognize this idiom.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
A continued line is a line which ends with a backslash, \. The
backslash is removed and the following line is joined with the current
one. No space is inserted, so you may split a line anywhere, even in
the middle of a word. (It is generally more readable to split lines
only at white space.)
The trailing backslash on a continued line is commonly referred to as a backslash-newline.
If there is white space between a backslash and the end of a line, that is still a continued line. However, as this is usually the result of an editing mistake, and many compilers will not accept it as a continued line, GCC will warn you about it.
There are two kinds of comments. Block comments begin with
/* and continue until the next */. Block comments do not
nest:
/* this is /* one comment */ text outside comment
Line comments begin with // and continue to the end of the
current line. Line comments do not nest either, but it does not matter,
because they would end in the same place anyway.
// this is // one comment
text outside comment
It is safe to put line comments inside block comments, or vice versa.
/* block comment
// contains line comment
yet more comment
*/ outside comment
// line comment /* contains block comment */
But beware of commenting out one end of a block comment with a line comment.
// l.c. /* block comment begins
oops! this isn't a comment anymore */
Comments are not recognized within string literals. "/* blah */" is the string constant /* blah */, not an empty string.
Line comments are not in the 1989 edition of the C standard, but they are recognized by GCC as an extension. In C++ and in the 1999 edition of the C standard, they are an official part of the language.
Since these transformations happen before all other processing, you can
split a line mechanically with backslash-newline anywhere. You can
comment out the end of a line. You can continue a line comment onto the
next line with backslash-newline. You can even split /*,
*/, and // onto multiple lines with backslash-newline.
For example:
/\
*
*/ # /*
*/ defi\
ne FO\
O 10\
20
is equivalent to #define FOO 1020. All these tricks are
extremely confusing and should not be used in code intended to be
readable.
There is no way to prevent a backslash at the end of a line from being interpreted as a backslash-newline.
"foo\\
bar"
is equivalent to "foo\bar", not to "foo\\bar". To avoid
having to worry about this, do not use the deprecated GNU extension
which permits multi-line strings. Instead, use string literal
concatenation:
"foo\\"
"bar"
Your program will be more portable this way, too.