@titlepage
@c @finalout
@title Cpplib Internals
-@subtitle Last revised Jan 2001
-@subtitle for GCC version 3.0
+@subtitle Last revised September 2001
+@subtitle for GCC version 3.1
@author Neil Booth
@page
@vskip 0pt plus 1filll
@node Top, Conventions,, (DIR)
@chapter Cpplib---the core of the GNU C Preprocessor
-The GNU C preprocessor in GCC 3.0 has been completely rewritten. It is
+The GNU C preprocessor in GCC 3.x 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
+The 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
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.
-
@menu
* Conventions:: Conventions used in the code.
* Lexer:: The combined C, C++ and Objective-C Lexer.
@node Lexer, Whitespace, Conventions, Top
@unnumbered The Lexer
@cindex lexer
-@cindex tokens
-
-The lexer is contained in the file @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 @samp{cpp_spell_token} and @samp{cpp_token_len}.
+@section Overview
+The lexer is contained in the file @file{cpplex.c}. It is a hand-coded
+lexer, and not implemented as a state machine. It can understand C, C++
+and Objective-C source code, and has been extended to allow reasonably
+successful preprocessing of assembly language. The lexer does not make
+an initial pass to strip out trigraphs and escaped newlines, but handles
+them as they are encountered in a single pass of the input file. It
+returns preprocessing tokens individually, not a line at a time.
+
+It is mostly transparent to users of the library, since the library's
+interface for obtaining the next token, @code{cpp_get_token}, takes care
+of lexing new tokens, handling directives, and expanding macros as
+necessary. However, the lexer does expose some functionality so that
+clients of the library can easily spell a given token, such as
+@code{cpp_spell_token} and @code{cpp_token_len}. These functions are
+useful when generating diagnostics, and for emitting the preprocessed
+output.
+
+@section Lexing a token
+Lexing of an individual token is handled by @code{_cpp_lex_direct} and
+its subroutines. In its current form the code is quite complicated,
+with read ahead characters and suchlike, since it strives to not step
+back in the character stream in preparation for handling non-ASCII file
+encodings. The current plan is to convert any such files to UTF-8
+before processing them. This complexity is therefore unnecessary and
+will be removed, so I'll not discuss it further here.
+
+The job of @code{_cpp_lex_direct} is simply to lex a token. It is not
+responsible for issues like directive handling, returning lookahead
+tokens directly, multiple-include optimisation, or conditional block
+skipping. It necessarily has a minor r@^ole to play in memory
+management of lexed lines. I discuss these issues in a separate section
+(@pxref{Lexing a line}).
+
+The lexer places the token it lexes into storage pointed to by the
+variable @var{cur_token}, and then increments it. This variable is
+important for correct diagnostic positioning. Unless a specific line
+and column are passed to the diagnostic routines, they will examine the
+@var{line} and @var{col} values of the token just before the location
+that @var{cur_token} points to, and use that location to report the
+diagnostic.
+
+The lexer does not consider whitespace to be a token in its own right.
+If whitespace (other than a new line) precedes a token, it sets the
+@code{PREV_WHITE} bit in the token's flags. Each token has its
+@var{line} and @var{col} variables set to the line and column of the
+first character of the token. This line number is the line number in
+the translation unit, and can be converted to a source (file, line) pair
+using the line map code.
+
+The first token on a logical, i.e.@: unescaped, line has the flag
+@code{BOL} set for beginning-of-line. This flag is intended for
+internal use, both to distinguish a @samp{#} that begins a directive
+from one that doesn't, and to generate a callback to clients that want
+to be notified about the start of every non-directive line with tokens
+on it. Clients cannot reliably determine this for themselves: the first
+token might be a macro, and the tokens of a macro expansion do not have
+the @code{BOL} flag set. The macro expansion may even be empty, and the
+next token on the line certainly won't have the @code{BOL} flag set.
+
+New lines are treated specially; exactly how the lexer handles them is
+context-dependent. The C standard mandates that directives are
+terminated by the first unescaped newline character, even if it appears
+in the middle of a macro expansion. Therefore, if the state variable
+@var{in_directive} is set, the lexer returns a @code{CPP_EOF} token,
+which is normally used to indicate end-of-file, to indicate
+end-of-directive. In a directive a @code{CPP_EOF} token never means
+end-of-file. Conveniently, if the caller was @code{collect_args}, it
+already handles @code{CPP_EOF} as if it were end-of-file, and reports an
+error about an unterminated macro argument list.
+
+The C standard also specifies that a new line in the middle of the
+arguments to a macro is treated as whitespace. This white space is
+important in case the macro argument is stringified. The state variable
+@code{parsing_args} is non-zero when the preprocessor is collecting the
+arguments to a macro call. It is set to 1 when looking for the opening
+parenthesis to a function-like macro, and 2 when collecting the actual
+arguments up to the closing parenthesis, since these two cases need to
+be distinguished sometimes. One such time is here: the lexer sets the
+@code{PREV_WHITE} flag of a token if it meets a new line when
+@code{parsing_args} is set to 2. It doesn't set it if it meets a new
+line when @code{parsing_args} is 1, since then code like
+
+@smallexample
+#define foo() bar
+foo
+baz
+@end smallexample
+
+@noindent would be output with an erroneous space before @samp{baz}:
+
+@smallexample
+foo
+ baz
+@end smallexample
+
+This is a good example of the subtlety of getting token spacing correct
+in the preprocessor; there are plenty of tests in the testsuite for
+corner cases like this.
The most painful aspect of lexing ISO-standard C and C++ is handling
trigraphs and backlash-escaped newlines. Trigraphs are processed before
and @samp{/} 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 @samp{parse_identifier}, that lexes an identifier, cannot
-assume that it can scan forwards until the first non-identifier
+So, for example, the routine that lexes a number, @code{parse_number},
+cannot assume that it can scan forwards until the first non-number
character and be done with it, because this could be the @samp{\}
introducing an escaped newline, or the @samp{?} introducing the trigraph
-sequence that represents the @samp{\} of an escaped newline. Similarly
-for the routine that handles numbers, @samp{parse_number}. If these
-routines stumble upon a @samp{?} or @samp{\}, they call
-@samp{skip_escaped_newlines} to skip over any potential escaped newlines
-before checking whether they can finish.
+sequence that represents the @samp{\} of an escaped newline. If it
+encounters a @samp{?} or @samp{\}, it calls @code{skip_escaped_newlines}
+to skip over any potential escaped newlines before checking whether the
+number has been finished.
-Similarly code in the main body of @samp{_cpp_lex_token} cannot simply
+Similarly code in the main body of @code{_cpp_lex_direct} cannot simply
check for a @samp{=} after a @samp{+} character to determine whether it
has a @samp{+=} token; it needs to be prepared for an escaped newline of
-some sort. These cases use the function @samp{get_effective_char},
-which returns the first character after any intervening newlines.
+some sort. Such cases use the function @code{get_effective_char}, which
+returns the first character after any intervening escaped newlines.
-The lexer needs to keep track of the correct column position,
-including counting tabs as specified by the @option{-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
+The lexer needs to keep track of the correct column position, including
+counting tabs as specified by the @option{-ftabstop=} option. This
+should be done even within C-style comments; they 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 @samp{__VA_ARGS__} and poisoned identifiers,
+Some identifiers, such as @code{__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
-@samp{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
-@samp{__VA_ARGS__} in the expansion of a variable-argument macro.
-Therefore @samp{parse_identifier} makes use of flags to determine
+@code{parse_identifier}. In both cases, whether a diagnostic is needed
+or not is dependent upon the lexer's state. For example, we don't want
+to issue a diagnostic for re-poisoning a poisoned identifier, or for
+using @code{__VA_ARGS__} in the expansion of a variable-argument macro.
+Therefore @code{parse_identifier} makes use of state 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 @samp{<} would be lexed as a single
-token. After a @code{#include} directive, though, it should be lexed
-as a single token as far as the nearest @samp{>} character. Note that
-we don't allow the terminators of header names to be escaped; the first
+lexing header names. Normally, a @samp{<} would be lexed as a single
+token. After a @code{#include} directive, though, it should be lexed as
+a single token as far as the nearest @samp{>} character. Note that we
+don't allow the terminators of header names to be escaped; the first
@samp{"} or @samp{>} 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, @samp{::} is a single token in C++, but two
-separate @samp{:} tokens, and almost certainly a syntax error, in C@.
-Such cases are handled in the main function @samp{_cpp_lex_token}, based
-upon the flags set in the @samp{cpp_options} structure.
-
-Note we have almost, but not quite, achieved the goal of not stepping
-backwards in the input stream. Currently @samp{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 @option{-trigraphs} is not in
-force but @option{-Wtrigraphs} is, we need to warn about it but then
-buffer it and continue to treat it as 3 separate characters.
+force. For example, @samp{::} is a single token in C++, but in C it is
+two separate @samp{:} tokens and almost certainly a syntax error. Such
+cases are handled by @code{_cpp_lex_direct} based upon command-line
+flags stored in the @code{cpp_options} structure.
+
+@anchor{Lexing a line}
+@section Lexing a line
@node Whitespace, Hash Nodes, Lexer, Top
@unnumbered Whitespace
projects / gcc.git / commitdiff