@ifinfo
This file documents the internals of the GNU C Preprocessor.
-Copyright 2000, 2001 Free Software Foundation, Inc.
+Copyright 2000, 2001, 2002 Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
@node Top
@top
-@chapter Cpplib---the core of the GNU C Preprocessor
+@chapter Cpplib---the GNU C Preprocessor
The GNU C preprocessor in GCC 3.x has been completely rewritten. It is
now implemented as a library, @dfn{cpplib}, so it can be easily shared between
@unnumbered Macro Expansion Algorithm
@cindex macro expansion
-Macro expansion is a surprisingly tricky operation, fraught with nasty
-corner cases and situations that render what you thought was a nifty
-way to optimize the preprocessor's expansion algorithm wrong in quite
-subtle ways.
+Macro expansion is a tricky operation, fraught with nasty corner cases
+and situations that render what you thought was a nifty way to
+optimize the preprocessor's expansion algorithm wrong in quite subtle
+ways.
I strongly recommend you have a good grasp of how the C and C++
standards require macros to be expanded before diving into this
section, let alone the code!. If you don't have a clear mental
picture of how things like nested macro expansion, stringification and
-token pasting are supposed to work, damage to you sanity can quickly
+token pasting are supposed to work, damage to your sanity can quickly
result.
-@section Internal representation of Macros
+@section Internal representation of macros
@cindex macro representation (internal)
The preprocessor stores macro expansions in tokenized form. This
saves repeated lexing passes during expansion, at the cost of a small
increase in memory consumption on average. The tokens are stored
contiguously in memory, so a pointer to the first one and a token
-count is all we need.
+count is all you need to get the replacement list of a macro.
If the macro is a function-like macro the preprocessor also stores its
parameters, in the form of an ordered list of pointers to the hash
@option{-dD}, and to warn about non-trivial macro redefinitions when
the parameter names have changed.
-@section Nested object-like macros
+@section Macro expansion overview
+The preprocessor maintains a @dfn{context stack}, implemented as a
+linked list of @code{cpp_context} structures, which together represent
+the macro expansion state at any one time. The @code{struct
+cpp_reader} member variable @code{context} points to the current top
+of this stack. The top normally holds the unexpanded replacement list
+of the innermost macro under expansion, except when cpplib is about to
+pre-expand an argument, in which case it holds that argument's
+unexpanded tokens.
+
+When there are no macros under expansion, cpplib is in @dfn{base
+context}. All contexts other than the base context contain a
+contiguous list of tokens delimited by a starting and ending token.
+When not in base context, cpplib obtains the next token from the list
+of the top context. If there are no tokens left in the list, it pops
+that context off the stack, and subsequent ones if necessary, until an
+unexhausted context is found or it returns to base context. In base
+context, cpplib reads tokens directly from the lexer.
+
+If it encounters an identifier that is both a macro and enabled for
+expansion, cpplib prepares to push a new context for that macro on the
+stack by calling the routine @code{enter_macro_context}. When this
+routine returns, the new context will contain the unexpanded tokens of
+the replacement list of that macro. In the case of function-like
+macros, @code{enter_macro_context} also replaces any parameters in the
+replacement list, stored as @code{CPP_MACRO_ARG} tokens, with the
+appropriate macro argument. If the standard requires that the
+parameter be replaced with its expanded argument, the argument will
+have been fully macro expanded first.
+
+@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
-@c TODO
-
-@section Function-like macros
+@smallexample
+#define foo(x) bar x
+foo(foo) (2)
+@end smallexample
-@c TODO
+@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
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