cppinternals.texi

理解和实践操作系统的一本好书

@code{enter_macro_context} also handles special macros like
@code{__LINE__}.  Although these macros expand to a single token which
cannot contain any further macros, for reasons of token spacing
(@pxref{Token Spacing}) and simplicity of implementation, cpplib
handles these special macros by pushing a context containing just that
one token.

The final thing that @code{enter_macro_context} does before returning
is to mark the macro disabled for expansion (except for special macros
like @code{__TIME__}).  The macro is re-enabled when its context is
later popped from the context stack, as described above.  This strict
ordering ensures that a macro is disabled whilst its expansion is
being scanned, but that it is @emph{not} disabled whilst any arguments
to it are being expanded.

@section Scanning the replacement list for macros to expand
The C standard states that, after any parameters have been replaced
with their possibly-expanded arguments, the replacement list is
scanned for nested macros.  Further, any identifiers in the
replacement list that are not expanded during this scan are never
again eligible for expansion in the future, if the reason they were
not expanded is that the macro in question was disabled.

Clearly this latter condition can only apply to tokens resulting from
argument pre-expansion.  Other tokens never have an opportunity to be
re-tested for expansion.  It is possible for identifiers that are
function-like macros to not expand initially but to expand during a
later scan.  This occurs when the identifier is the last token of an
argument (and therefore originally followed by a comma or a closing
parenthesis in its macro's argument list), and when it replaces its
parameter in the macro's replacement list, the subsequent token
happens to be an opening parenthesis (itself possibly the first token
of an argument).

It is important to note that when cpplib reads the last token of a
given context, that context still remains on the stack.  Only when
looking for the @emph{next} token do we pop it off the stack and drop
to a lower context.  This makes backing up by one token easy, but more
importantly ensures that the macro corresponding to the current
context is still disabled when we are considering the last token of
its replacement list for expansion (or indeed expanding it).  As an
example, which illustrates many of the points above, consider

@smallexample
#define foo(x) bar x
foo(foo) (2)
@end smallexample

@noindent which fully expands to @samp{bar foo (2)}.  During pre-expansion
of the argument, @samp{foo} does not expand even though the macro is
enabled, since it has no following parenthesis [pre-expansion of an
argument only uses tokens from that argument; it cannot take tokens
from whatever follows the macro invocation].  This still leaves the
argument token @samp{foo} eligible for future expansion.  Then, when
re-scanning after argument replacement, the token @samp{foo} is
rejected for expansion, and marked ineligible for future expansion,
since the macro is now disabled.  It is disabled because the
replacement list @samp{bar foo} of the macro is still on the context
stack.

If instead the algorithm looked for an opening parenthesis first and
then tested whether the macro were disabled it would be subtly wrong.
In the example above, the replacement list of @samp{foo} would be
popped in the process of finding the parenthesis, re-enabling
@samp{foo} and expanding it a second time.

@section Looking for a function-like macro's opening parenthesis
Function-like macros only expand when immediately followed by a
parenthesis.  To do this cpplib needs to temporarily disable macros
and read the next token.  Unfortunately, because of spacing issues
(@pxref{Token Spacing}), there can be fake padding tokens in-between,
and if the next real token is not a parenthesis cpplib needs to be
able to back up that one token as well as retain the information in
any intervening padding tokens.

Backing up more than one token when macros are involved is not
permitted by cpplib, because in general it might involve issues like
restoring popped contexts onto the context stack, which are too hard.
Instead, searching for the parenthesis is handled by a special
function, @code{funlike_invocation_p}, which remembers padding
information as it reads tokens.  If the next real token is not an
opening parenthesis, it backs up that one token, and then pushes an
extra context just containing the padding information if necessary.

@section Marking tokens ineligible for future expansion
As discussed above, cpplib needs a way of marking tokens as
unexpandable.  Since the tokens cpplib handles are read-only once they
have been lexed, it instead makes a copy of the token and adds the
flag @code{NO_EXPAND} to the copy.

For efficiency and to simplify memory management by avoiding having to
remember to free these tokens, they are allocated as temporary tokens
from the lexer's current token run (@pxref{Lexing a line}) using the
function @code{_cpp_temp_token}.  The tokens are then re-used once the
current line of tokens has been read in.

This might sound unsafe.  However, tokens runs are not re-used at the
end of a line if it happens to be in the middle of a macro argument
list, and cpplib only wants to back-up more than one lexer token in
situations where no macro expansion is involved, so the optimization
is safe.

@node Token Spacing
@unnumbered Token Spacing
@cindex paste avoidance
@cindex spacing
@cindex token spacing

First, consider an issue that only concerns the stand-alone
preprocessor: there needs to be a guarantee that re-reading its preprocessed
output results in an identical token stream.  Without taking special
measures, this might not be the case because of macro substitution.
For example:

@smallexample
#define PLUS +
#define EMPTY
#define f(x) =x=
+PLUS -EMPTY- PLUS+ f(=)
        @expansion{} + + - - + + = = =
@emph{not}
        @expansion{} ++ -- ++ ===
@end smallexample

One solution would be to simply insert a space between all adjacent
tokens.  However, we would like to keep space insertion to a minimum,
both for aesthetic reasons and because it causes problems for people who
still try to abuse the preprocessor for things like Fortran source and
Makefiles.

For now, just notice that when tokens are added (or removed, as shown by
the @code{EMPTY} example) from the original lexed token stream, we need
to check for accidental token pasting.  We call this @dfn{paste
avoidance}.  Token addition and removal can only occur because of macro
expansion, but accidental pasting can occur in many places: both before
and after each macro replacement, each argument replacement, and
additionally each token created by the @samp{#} and @samp{##} operators.

Look at how the preprocessor gets whitespace output correct
normally.  The @code{cpp_token} structure contains a flags byte, and one
of those flags is @code{PREV_WHITE}.  This is flagged by the lexer, and
indicates that the token was preceded by whitespace of some form other
than a new line.  The stand-alone preprocessor can use this flag to
decide whether to insert a space between tokens in the output.

Now consider the result of the following macro expansion:

@smallexample
#define add(x, y, z) x + y +z;
sum = add (1,2, 3);
        @expansion{} sum = 1 + 2 +3;
@end smallexample

The interesting thing here is that the tokens @samp{1} and @samp{2} are
output with a preceding space, and @samp{3} is output without a
preceding space, but when lexed none of these tokens had that property.
Careful consideration reveals that @samp{1} gets its preceding
whitespace from the space preceding @samp{add} in the macro invocation,
@emph{not} replacement list.  @samp{2} gets its whitespace from the
space preceding the parameter @samp{y} in the macro replacement list,
and @samp{3} has no preceding space because parameter @samp{z} has none
in the replacement list.

Once lexed, tokens are effectively fixed and cannot be altered, since
pointers to them might be held in many places, in particular by
in-progress macro expansions.  So instead of modifying the two tokens
above, the preprocessor inserts a special token, which I call a
@dfn{padding token}, into the token stream to indicate that spacing of
the subsequent token is special.  The preprocessor inserts padding
tokens in front of every macro expansion and expanded macro argument.
These point to a @dfn{source token} from which the subsequent real token
should inherit its spacing.  In the above example, the source tokens are
@samp{add} in the macro invocation, and @samp{y} and @samp{z} in the
macro replacement list, respectively.

It is quite easy to get multiple padding tokens in a row, for example if
a macro's first replacement token expands straight into another macro.

@smallexample
#define foo bar
#define bar baz
[foo]
        @expansion{} [baz]
@end smallexample

Here, two padding tokens are generated with sources the @samp{foo} token
between the brackets, and the @samp{bar} token from foo's replacement
list, respectively.  Clearly the first padding token is the one to
use, so the output code should contain a rule that the first
padding token in a sequence is the one that matters.

But what if a macro expansion is left?  Adjusting the above
example slightly:

@smallexample
#define foo bar
#define bar EMPTY baz
#define EMPTY
[foo] EMPTY;
        @expansion{} [ baz] ;
@end smallexample

As shown, now there should be a space before @samp{baz} and the
semicolon in the output.

The rules we decided above fail for @samp{baz}: we generate three
padding tokens, one per macro invocation, before the token @samp{baz}.
We would then have it take its spacing from the first of these, which
carries source token @samp{foo} with no leading space.

It is vital that cpplib get spacing correct in these examples since any
of these macro expansions could be stringified, where spacing matters.

So, this demonstrates that not just entering macro and argument
expansions, but leaving them requires special handling too.  I made
cpplib insert a padding token with a @code{NULL} source token when
leaving macro expansions, as well as after each replaced argument in a
macro's replacement list.  It also inserts appropriate padding tokens on
either side of tokens created by the @samp{#} and @samp{##} operators.
I expanded the rule so that, if we see a padding token with a
@code{NULL} source token, @emph{and} that source token has no leading
space, then we behave as if we have seen no padding tokens at all.  A
quick check shows this rule will then get the above example correct as
well.

Now a relationship with paste avoidance is apparent: we have to be
careful about paste avoidance in exactly the same locations we have
padding tokens in order to get white space correct.  This makes
implementation of paste avoidance easy: wherever the stand-alone
preprocessor is fixing up spacing because of padding tokens, and it
turns out that no space is needed, it has to take the extra step to
check that a space is not needed after all to avoid an accidental paste.
The function @code{cpp_avoid_paste} advises whether a space is required
between two consecutive tokens.  To avoid excessive spacing, it tries
hard to only require a space if one is likely to be necessary, but for
reasons of efficiency it is slightly conservative and might recommend a
space where one is not strictly needed.

@node Line Numbering
@unnumbered Line numbering
@cindex line numbers

@section Just which line number anyway?

There are three reasonable requirements a cpplib client might have for
the line number of a token passed to it:

@itemize @bullet
@item
The source line it was lexed on.
@item
The line it is output on.  This can be different to the line it was
lexed on if, for example, there are intervening escaped newlines or
C-style comments.  For example:

@smallexample
foo /* @r{A long
comment} */ bar \
baz
@result{}
foo bar baz
@end smallexample

@item
If the token results from a macro expansion, the line of the macro name,
or possibly the line of the closing parenthesis in the case of