@setfilename cppinternals.info
@settitle The GNU C Preprocessor Internals
+@include gcc-common.texi
+
@ifinfo
-@dircategory Programming
+@dircategory Software development
@direntry
* Cpplib: (cppinternals). Cpplib internals.
@end direntry
@ifinfo
This file documents the internals of the GNU C Preprocessor.
-Copyright 2000, 2001 Free Software Foundation, Inc.
+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
@end ifinfo
@titlepage
-@c @finalout
@title Cpplib Internals
-@subtitle Last revised September 2001
-@subtitle for GCC version 3.1
+@versionsubtitle
@author Neil Booth
@page
@vskip 0pt plus 1filll
@c man begin COPYRIGHT
-Copyright @copyright{} 2000, 2001
+Copyright @copyright{} 2000, 2001, 2002, 2004, 2005
Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
@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, cpplib, so it can be easily shared between
+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
* Line Numbering:: Tracking location within files.
* Guard Macros:: Optimizing header files with guard macros.
* Files:: File handling.
-* Index:: Index.
+* Concept Index:: Index.
@end menu
@node Conventions
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{cpphash.h}. Functions and types exposed to external
+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.
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.
+behavior.
@node Lexer
@unnumbered The Lexer
@cindex escaped newlines
@section Overview
-The lexer is contained in the file @file{cpplex.c}. It is a hand-coded
+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
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
+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 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
+@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
@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 test suite for
+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}
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
+Another place where state flags are used to change behavior is whilst
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
token stream. For example, after the name of a function-like macro, it
wants to check the next token to see if it is an opening parenthesis.
Another example is that, after reading the first few tokens of a
-@code{#pragma} directive and not recognising it as a registered pragma,
+@code{#pragma} directive and not recognizing it as a registered pragma,
it wants to backtrack and allow the user-defined handler for unknown
pragmas to access the full @code{#pragma} token stream. The stand-alone
preprocessor wants to be able to test the current token with the
The tokens forming a macro's replacement list are collected by the
@code{#define} handler, and placed in storage that is only freed by
-@code{cpp_destroy}. So if a macro is expanded in our line of tokens,
-the pointers to the tokens of its expansion that we return will always
+@code{cpp_destroy}. So if a macro is expanded in the line of tokens,
+the pointers to the tokens of its expansion that are returned will always
remain valid. However, macros are a little trickier than that, since
they give rise to three sources of fresh tokens. They are the built-in
macros like @code{__LINE__}, and the @samp{#} and @samp{##} operators
-for stringifcation and token pasting. I handled this by allocating
+for stringification and token pasting. I handled this by allocating
space for these tokens from the lexer's token run chain. This means
they automatically receive the same lifetime guarantees as lexed tokens,
and we don't need to concern ourselves with freeing them.
These have been declared to be macros, either on the command line or
with @code{#define}. A few, such as @code{__TIME__} are built-ins
-entered in the hash table during initialisation. The hash node for a
+entered in the hash table during initialization. The hash node for a
normal macro points to a structure with more information about the
macro, such as whether it is function-like, how many arguments it takes,
and its expansion. Built-in macros are flagged as special, and instead
@node Macro Expansion
@unnumbered Macro Expansion Algorithm
+@cindex macro expansion
+
+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 your sanity can quickly
+result.
+
+@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 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
+table entry of each parameter's identifier. Further, in the macro's
+stored expansion each occurrence of a parameter is replaced with a
+special token of type @code{CPP_MACRO_ARG}. Each such token holds the
+index of the parameter it represents in the parameter list, which
+allows rapid replacement of parameters with their arguments during
+expansion. Despite this optimization it is still necessary to store
+the original parameters to the macro, both for dumping with e.g.,
+@option{-dD}, and to warn about non-trivial macro redefinitions when
+the parameter names have changed.
+
+@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
+
+@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
@cindex spacing
@cindex token spacing
-First, let's look at an issue that only concerns the stand-alone
-preprocessor: we want to guarantee that re-reading its preprocessed
+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:
+measures, this might not be the case because of macro substitution.
+For example:
@smallexample
#define PLUS +
still try to abuse the preprocessor for things like Fortran source and
Makefiles.
-For now, just notice that the only places we need to be careful about
-@dfn{paste avoidance} are when tokens are added (or removed) from the
-original token stream. This only occurs because of macro expansion, but
-care is needed in many places: before @strong{and} after each macro
-replacement, each argument replacement, and additionally each token
-created by the @samp{#} and @samp{##} operators.
+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.
-Let's look at how the preprocessor gets whitespace output correct
+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 following:
+Now consider the result of the following macro expansion:
@smallexample
#define add(x, y, z) x + y +z;
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
-@emph{invocation}, @samp{2} gets its whitespace from the space preceding
-the parameter @samp{y} in the macro @emph{replacement list}, and
-@samp{3} has no preceding space because parameter @samp{z} has none in
-the replacement list.
+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 in front of every macro
-expansion and expanded macro argument, to indicate that the subsequent
-token should assume its @code{PREV_WHITE} flag from a different
-@dfn{source token}. In the above example, the source tokens are
+@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.
@expansion{} [baz]
@end smallexample
-Here, two padding tokens with sources @samp{foo} between the brackets,
-and @samp{bar} from foo's replacement list, are generated. Clearly the
-first padding token is the one that matters. But what if we happen to
-leave a macro expansion? Adjusting the above example slightly:
+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
@expansion{} [ baz] ;
@end smallexample
-As shown, now there should be a space before baz and the semicolon. Our
-initial algorithm fails for the former, because we would see three
-padding tokens, one per macro invocation, followed by @samp{baz}, which
-would have inherit its spacing from the original source, @samp{foo},
-which has no leading space. Note that it is vital that cpplib get
-spacing correct in these examples, since any of these macro expansions
-could be stringified, where spacing matters.
-
-So, I have demonstrated that not just entering macro and argument
-expansions, but leaving them requires special handling too. So cpplib
-inserts a padding token with a @code{NULL} source token when leaving
-macro expansions and 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.
-
-Now we can see the relationship with paste avoidance: we have to be
-careful about paste avoidance in exactly the same locations we take care
-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.
+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
C-style comments. For example:
@smallexample
-foo /* A long
-comment */ bar \
+foo /* @r{A long
+comment} */ bar \
baz
@result{}
foo bar baz
on.
Note that whilst we are inside the conditional block, @code{mi_valid} is
-likely to be reset to @code{false}, but this does not matter since the
+likely to be reset to @code{false}, but this does not matter since
the closing @code{#endif} restores it to @code{true} if appropriate.
Finally, since @code{_cpp_lex_direct} pops the file off the buffer stack
@cindex files
Fairly obviously, the file handling code of cpplib resides in the file
-@file{cppfiles.c}. It takes care of the details of file searching,
+@file{files.c}. It takes care of the details of file searching,
opening, reading and caching, for both the main source file and all the
headers it recursively includes.
Enough information is stored in the splay tree that CPP can immediately
tell whether it can skip the header file because of the multiple include
-optimisation, whether the file didn't exist or couldn't be opened for
+optimization, whether the file didn't exist or couldn't be opened for
some reason, or whether the header was flagged not to be re-used, as it
is with the obsolete @code{#import} directive.
applies. This may be higher up the directory tree than the full path to
the file minus the base name.
-@node Index
-@unnumbered Index
+@node Concept Index
+@unnumbered Concept Index
@printindex cp
@bye
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