This file documents the internals of the GNU C Preprocessor.

Copyright 2000 Free Software Foundation, Inc.

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Cpplib - the core of the GNU C Preprocessor
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The GNU C preprocessor in GCC 3.0 has been completely rewritten.  It is now implemented as a library, cpplib, so it can be easily shared between a stand-alone preprocessor, and a preprocessor integrated with the C, C++ and Objective C front ends.  It is also available for use by other programs, though this is not recommended as its exposed interface has not yet reached a point of reasonable stability.

This library has been written to be re-entrant, so that it can be used to preprocess many files simultaneously if necessary.  It has also been written with the preprocessing token as the fundamental unit; the preprocessor in previous versions of GCC would operate on text strings as the fundamental unit.

This brief manual documents some of the internals of cpplib, and a few tricky issues encountered.  It also describes certain behaviour we would like to preserve, such as the format and spacing of its output.

Identifiers, macro expansion, hash nodes, lexing.

cpplib has two interfaces - one is exposed internally only, and the other is for both internal and external use.

The convention is that functions and types that are exposed to multiple files internally are prefixed with _cpp_, and are to be found in the file cpphash.h.  Functions and types exposed to external clients are in cpplib.h, and prefixed with cpp_.

We are striving to reduce the information exposed in cpplib.h to the bare minimum necessary, and then to keep it there.  This makes clear exactly what external clients are entitled to assume, and allows us to change internals in the future without worrying whether library clients are perhaps relying on some kind of undocumented implementation-specific behaviour.

The lexer is contained in the file cpplex.c.  We want to have a lexer that is single-pass, for efficiency reasons.  We would also like the lexer to only step forwards through the input files, and not step back.  This will make future changes to support different character sets, in particular state or shift-dependent ones, much easier.

This file also contains all information needed to spell a token, i.e. to output it either in a diagnostic or to a preprocessed output file.  This information is not exported, but made available to clients through such functions as cpp_spell_token and cpp_token_len.

The most painful aspect of lexing ISO-standard C and C++ is handling trigraphs and backlash-escaped newlines.  Trigraphs are processed before any interpretation of the meaning of a character is made, and unfortunately there is a trigraph representation for a backslash, so it is possible for the trigraph ??/ to introduce an escaped newline.

Escaped newlines are tedious because theoretically they can occur anywhere - between the + and = of the += token, within the characters of an identifier, and even between the * and / that terminates a comment.  Moreover, you cannot be sure there is just one - there might be an arbitrarily long sequence of them.

So the routine parse_identifier, that lexes an identifier, cannot assume that it can scan forwards until the first non-identifier character and be done with it, because this could be the \ introducing an escaped newline, or the ? introducing the trigraph sequence that represents the \ of an escaped newline.  Similarly for the routine that handles numbers, parse_number.  If these routines stumble upon a ? or \, they call skip_escaped_newlines to skip over any potential escaped newlines before checking whether they can finish.

Similarly code in the main body of _cpp_lex_token cannot simply check for a = after a + character to determine whether it has a += token; it needs to be prepared for an escaped newline of some sort.  These cases use the function get_effective_char, which returns the first character after any intervening newlines.

The lexer needs to keep track of the correct column position, including counting tabs as specified by the -ftabstop= option. This should be done even within comments; C-style comments can appear in the middle of a line, and we want to report diagnostics in the correct position for text appearing after the end of the comment.

Some identifiers, such as __VA_ARGS__ and poisoned identifiers, may be invalid and require a diagnostic.  However, if they appear in a macro expansion we don't want to complain with each use of the macro. It is therefore best to catch them during the lexing stage, in parse_identifier.  In both cases, whether a diagnostic is needed or not is dependent upon lexer state.  For example, we don't want to issue a diagnostic for re-poisoning a poisoned identifier, or for using __VA_ARGS__ in the expansion of a variable-argument macro. Therefore parse_identifier makes use of flags to determine whether a diagnostic is appropriate.  Since we change state on a per-token basis, and don't lex whole lines at a time, this is not a problem.

Another place where state flags are used to change behaviour is whilst parsing header names.  Normally, a < would be lexed as a single token.  After a #include directive, though, it should be lexed as a single token as far as the nearest > character.  Note that we don't allow the terminators of header names to be escaped; the first " or > terminates the header name.

Interpretation of some character sequences depends upon whether we are lexing C, C++ or Objective C, and on the revision of the standard in force.  For example, @foo is a single identifier token in objective C, but two separate tokens @ and foo in C or C++.  Such cases are handled in the main function _cpp_lex_token, based upon the flags set in the cpp_options structure.

Note we have almost, but not quite, achieved the goal of not stepping backwards in the input stream.  Currently skip_escaped_newlines does step back, though with care it should be possible to adjust it so that this does not happen.  For example, one tricky issue is if we meet a trigraph, but the command line option -trigraphs is not in force but -Wtrigraphs is, we need to warn about it but then buffer it and continue to treat it as 3 separate characters.

The lexer has been written to treat each of \r, \n, \r\n and \n\r as a single new line indicator.  This allows it to transparently preprocess MS-DOS, Macintosh and Unix files without their needing to pass through a special filter beforehand.

We also decided to treat a backslash, either \ or the trigraph ??/, separated from one of the above newline forms by whitespace only (one or more space, tab, form-feed, vertical tab or NUL characters), as an intended escaped newline.  The library issues a diagnostic in this case.

Handling newlines in this way is made simpler by doing it in one place only.  The function handle_newline takes care of all newline characters, and skip_escaped_newlines takes care of all escaping of newlines, deferring to handle_newline to handle the newlines themselves.