cppinternals.texi

理解和实践操作系统的一本好书

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@setfilename cppinternals.info
@settitle The GNU C Preprocessor Internals

@include gcc-common.texi

@ifinfo
@dircategory Software development
@direntry
* Cpplib: (cppinternals).      Cpplib internals.
@end direntry
@end ifinfo

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@c @finalout
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@ifinfo
This file documents the internals of the GNU C Preprocessor.

Copyright 2000, 2001, 2002, 2004, 2005, 2006, 2007 Free Software
Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through Tex and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@end ifinfo

@titlepage
@title Cpplib Internals
@versionsubtitle
@author Neil Booth
@page
@vskip 0pt plus 1filll
@c man begin COPYRIGHT
Copyright @copyright{} 2000, 2001, 2002, 2004, 2005
Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions.
@c man end
@end titlepage
@contents
@page

@node Top
@top
@chapter Cpplib---the GNU C Preprocessor

The GNU C preprocessor is
implemented as a library, @dfn{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.

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
as the fundamental unit.

This brief manual documents the internals of cpplib, and explains some
of the tricky issues.  It is intended that, along with the comments in
the source code, a reasonably competent C programmer should be able to
figure out what the code is doing, and why things have been implemented
the way they have.

@menu
* Conventions::         Conventions used in the code.
* Lexer::               The combined C, C++ and Objective-C Lexer.
* Hash Nodes::          All identifiers are entered into a hash table.
* Macro Expansion::     Macro expansion algorithm.
* Token Spacing::       Spacing and paste avoidance issues.
* Line Numbering::      Tracking location within files.
* Guard Macros::        Optimizing header files with guard macros.
* Files::               File handling.
* Concept Index::       Index.
@end menu

@node Conventions
@unnumbered Conventions
@cindex interface
@cindex header files

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 @samp{_cpp_}, and are to be found in
the file @file{internal.h}.  Functions and types exposed to external
clients are in @file{cpplib.h}, and prefixed with @samp{cpp_}.  For
historical reasons this is no longer quite true, but we should strive to
stick to it.

We are striving to reduce the information exposed in @file{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
behavior.

@node Lexer
@unnumbered The Lexer
@cindex lexer
@cindex newlines
@cindex escaped newlines

@section Overview
The lexer is contained in the file @file{lex.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 such-like, 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 optimization, 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 @code{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
@code{line} and @code{col} values of the token just before the location
that @code{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
@code{line} and @code{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 call-back 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
@code{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 nonzero 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 lexer is written to treat each of @samp{\r}, @samp{\n}, @samp{\r\n}
and @samp{\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 @samp{\} or the trigraph
@samp{??/}, separated from one of the above newline indicators by
non-comment whitespace only, as intending to escape the newline.  It
tends to be a typing mistake, and cannot reasonably be mistaken for
anything else in any of the C-family grammars.  Since handling it this
way is not strictly conforming to the ISO standard, the library issues a
warning wherever it encounters it.

Handling newlines like this is made simpler by doing it in one place
only.  The function @code{handle_newline} takes care of all newline
characters, and @code{skip_escaped_newlines} takes care of arbitrarily
long sequences of escaped newlines, deferring to @code{handle_newline}
to handle the newlines themselves.

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 @samp{??/} to introduce an escaped newline.

Escaped newlines are tedious because theoretically they can occur
anywhere---between the @samp{+} and @samp{=} of the @samp{+=} token,
within the characters of an identifier, and even between the @samp{*}
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, 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