--- /dev/null
+This is doc/cppinternals.info, produced by makeinfo version 4.8 from
+/home/dmitriyz/src-lcl/android2/toolchain/android-toolchain/gcc-4.2.1/gcc/doc/cppinternals.texi.
+
+INFO-DIR-SECTION Software development
+START-INFO-DIR-ENTRY
+* Cpplib: (cppinternals). Cpplib internals.
+END-INFO-DIR-ENTRY
+
+ This file documents the internals of the GNU C Preprocessor.
+
+ 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.
+
+\1f
+File: cppinternals.info, Node: Top, Next: Conventions, Up: (dir)
+
+The GNU C Preprocessor Internals
+********************************
+
+1 Cpplib--the GNU C Preprocessor
+********************************
+
+The GNU C preprocessor is 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.
+
+ 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.
+
+\1f
+File: cppinternals.info, Node: Conventions, Next: Lexer, Prev: Top, Up: Top
+
+Conventions
+***********
+
+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 `_cpp_', and are to be
+found in the file `internal.h'. Functions and types exposed to external
+clients are in `cpplib.h', and prefixed with `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 `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.
+
+\1f
+File: cppinternals.info, Node: Lexer, Next: Hash Nodes, Prev: Conventions, Up: Top
+
+The Lexer
+*********
+
+Overview
+========
+
+The lexer is contained in the 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, `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
+`cpp_spell_token' and `cpp_token_len'. These functions are useful when
+generating diagnostics, and for emitting the preprocessed output.
+
+Lexing a token
+==============
+
+Lexing of an individual token is handled by `_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 `_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 ro^le to play in memory
+management of lexed lines. I discuss these issues in a separate section
+(*note Lexing a line::).
+
+ The lexer places the token it lexes into storage pointed to by the
+variable `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
+`line' and `col' values of the token just before the location that
+`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 `PREV_WHITE' bit in the token's flags. Each token has its `line'
+and `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
+`BOL' set for beginning-of-line. This flag is intended for internal
+use, both to distinguish a `#' 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
+`BOL' flag set. The macro expansion may even be empty, and the next
+token on the line certainly won't have the `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
+`in_directive' is set, the lexer returns a `CPP_EOF' token, which is
+normally used to indicate end-of-file, to indicate end-of-directive.
+In a directive a `CPP_EOF' token never means end-of-file.
+Conveniently, if the caller was `collect_args', it already handles
+`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
+`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
+`PREV_WHITE' flag of a token if it meets a new line when `parsing_args'
+is set to 2. It doesn't set it if it meets a new line when
+`parsing_args' is 1, since then code like
+
+ #define foo() bar
+ foo
+ baz
+
+would be output with an erroneous space before `baz':
+
+ foo
+ baz
+
+ 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 `\r', `\n', `\r\n' and `\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 `\' or the trigraph
+`??/', 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 `handle_newline' takes care of all newline
+characters, and `skip_escaped_newlines' takes care of arbitrarily long
+sequences of escaped newlines, deferring to `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 `??/' to introduce an escaped newline.
+
+ Escaped newlines are tedious because theoretically they can occur
+anywhere--between the `+' and `=' of the `+=' token, within the
+characters of an identifier, and even between the `*' and `/' 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, `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 `\'
+introducing an escaped newline, or the `?' introducing the trigraph
+sequence that represents the `\' of an escaped newline. If it
+encounters a `?' or `\', it calls `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 `_cpp_lex_direct' cannot simply
+check for a `=' after a `+' character to determine whether it has a
+`+=' token; it needs to be prepared for an escaped newline of some
+sort. Such cases use the function `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 `-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 `__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
+`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
+`__VA_ARGS__' in the expansion of a variable-argument macro. Therefore
+`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 behavior is whilst
+lexing header names. Normally, a `<' would be lexed as a single token.
+After a `#include' directive, though, it should be lexed as a single
+token as far as the nearest `>' character. Note that we don't allow
+the terminators of header names to be escaped; the first `"' or `>'
+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, `::' is a single token in C++, but in C it is two
+separate `:' tokens and almost certainly a syntax error. Such cases
+are handled by `_cpp_lex_direct' based upon command-line flags stored
+in the `cpp_options' structure.
+
+ Once a token has been lexed, it leads an independent existence. The
+spelling of numbers, identifiers and strings is copied to permanent
+storage from the original input buffer, so a token remains valid and
+correct even if its source buffer is freed with `_cpp_pop_buffer'. The
+storage holding the spellings of such tokens remains until the client
+program calls cpp_destroy, probably at the end of the translation unit.
+
+Lexing a line
+=============
+
+When the preprocessor was changed to return pointers to tokens, one
+feature I wanted was some sort of guarantee regarding how long a
+returned pointer remains valid. This is important to the stand-alone
+preprocessor, the future direction of the C family front ends, and even
+to cpplib itself internally.
+
+ Occasionally the preprocessor wants to be able to peek ahead in the
+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
+`#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 `#pragma' token stream. The stand-alone
+preprocessor wants to be able to test the current token with the
+previous one to see if a space needs to be inserted to preserve their
+separate tokenization upon re-lexing (paste avoidance), so it needs to
+be sure the pointer to the previous token is still valid. The
+recursive-descent C++ parser wants to be able to perform tentative
+parsing arbitrarily far ahead in the token stream, and then to be able
+to jump back to a prior position in that stream if necessary.
+
+ The rule I chose, which is fairly natural, is to arrange that the
+preprocessor lex all tokens on a line consecutively into a token buffer,
+which I call a "token run", and when meeting an unescaped new line
+(newlines within comments do not count either), to start lexing back at
+the beginning of the run. Note that we do _not_ lex a line of tokens
+at once; if we did that `parse_identifier' would not have state flags
+available to warn about invalid identifiers (*note Invalid
+identifiers::).
+
+ In other words, accessing tokens that appeared earlier in the current
+line is valid, but since each logical line overwrites the tokens of the
+previous line, tokens from prior lines are unavailable. In particular,
+since a directive only occupies a single logical line, this means that
+the directive handlers like the `#pragma' handler can jump around in
+the directive's tokens if necessary.
+
+ Two issues remain: what about tokens that arise from macro
+expansions, and what happens when we have a long line that overflows
+the token run?
+
+ Since we promise clients that we preserve the validity of pointers
+that we have already returned for tokens that appeared earlier in the
+line, we cannot reallocate the run. Instead, on overflow it is
+expanded by chaining a new token run on to the end of the existing one.
+
+ The tokens forming a macro's replacement list are collected by the
+`#define' handler, and placed in storage that is only freed by
+`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 `__LINE__', and the `#' and `##' operators 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.
+
+ Lexing into a line of tokens solves some of the token memory
+management issues, but not all. The opening parenthesis after a
+function-like macro name might lie on a different line, and the front
+ends definitely want the ability to look ahead past the end of the
+current line. So cpplib only moves back to the start of the token run
+at the end of a line if the variable `keep_tokens' is zero.
+Line-buffering is quite natural for the preprocessor, and as a result
+the only time cpplib needs to increment this variable is whilst looking
+for the opening parenthesis to, and reading the arguments of, a
+function-like macro. In the near future cpplib will export an
+interface to increment and decrement this variable, so that clients can
+share full control over the lifetime of token pointers too.
+
+ The routine `_cpp_lex_token' handles moving to new token runs,
+calling `_cpp_lex_direct' to lex new tokens, or returning
+previously-lexed tokens if we stepped back in the token stream. It also
+checks each token for the `BOL' flag, which might indicate a directive
+that needs to be handled, or require a start-of-line call-back to be
+made. `_cpp_lex_token' also handles skipping over tokens in failed
+conditional blocks, and invalidates the control macro of the
+multiple-include optimization if a token was successfully lexed outside
+a directive. In other words, its callers do not need to concern
+themselves with such issues.
+
+\1f
+File: cppinternals.info, Node: Hash Nodes, Next: Macro Expansion, Prev: Lexer, Up: Top
+
+Hash Nodes
+**********
+
+When cpplib encounters an "identifier", it generates a hash code for it
+and stores it in the hash table. By "identifier" we mean tokens with
+type `CPP_NAME'; this includes identifiers in the usual C sense, as
+well as keywords, directive names, macro names and so on. For example,
+all of `pragma', `int', `foo' and `__GNUC__' are identifiers and hashed
+when lexed.
+
+ Each node in the hash table contain various information about the
+identifier it represents. For example, its length and type. At any one
+time, each identifier falls into exactly one of three categories:
+
+ * Macros
+
+ These have been declared to be macros, either on the command line
+ or with `#define'. A few, such as `__TIME__' are built-ins
+ 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 contain an enum indicating which
+ of the various built-in macros it is.
+
+ * Assertions
+
+ Assertions are in a separate namespace to macros. To enforce
+ this, cpp actually prepends a `#' character before hashing and
+ entering it in the hash table. An assertion's node points to a
+ chain of answers to that assertion.
+
+ * Void
+
+ Everything else falls into this category--an identifier that is not
+ currently a macro, or a macro that has since been undefined with
+ `#undef'.
+
+ When preprocessing C++, this category also includes the named
+ operators, such as `xor'. In expressions these behave like the
+ operators they represent, but in contexts where the spelling of a
+ token matters they are spelt differently. This spelling
+ distinction is relevant when they are operands of the stringizing
+ and pasting macro operators `#' and `##'. Named operator hash
+ nodes are flagged, both to catch the spelling distinction and to
+ prevent them from being defined as macros.
+
+ The same identifiers share the same hash node. Since each identifier
+token, after lexing, contains a pointer to its hash node, this is used
+to provide rapid lookup of various information. For example, when
+parsing a `#define' statement, CPP flags each argument's identifier
+hash node with the index of that argument. This makes duplicated
+argument checking an O(1) operation for each argument. Similarly, for
+each identifier in the macro's expansion, lookup to see if it is an
+argument, and which argument it is, is also an O(1) operation. Further,
+each directive name, such as `endif', has an associated directive enum
+stored in its hash node, so that directive lookup is also O(1).
+
+\1f
+File: cppinternals.info, Node: Macro Expansion, Next: Token Spacing, Prev: Hash Nodes, Up: Top
+
+Macro Expansion Algorithm
+*************************
+
+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.
+
+Internal representation of macros
+=================================
+
+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 `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., `-dD', and to warn
+about non-trivial macro redefinitions when the parameter names have
+changed.
+
+Macro expansion overview
+========================
+
+The preprocessor maintains a "context stack", implemented as a linked
+list of `cpp_context' structures, which together represent the macro
+expansion state at any one time. The `struct cpp_reader' member
+variable `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 "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 `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,
+`enter_macro_context' also replaces any parameters in the replacement
+list, stored as `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.
+
+ `enter_macro_context' also handles special macros like `__LINE__'.
+Although these macros expand to a single token which cannot contain any
+further macros, for reasons of token spacing (*note Token Spacing::)
+and simplicity of implementation, cpplib handles these special macros
+by pushing a context containing just that one token.
+
+ The final thing that `enter_macro_context' does before returning is
+to mark the macro disabled for expansion (except for special macros
+like `__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 _not_ disabled whilst any arguments to it are
+being expanded.
+
+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 _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
+
+ #define foo(x) bar x
+ foo(foo) (2)
+
+which fully expands to `bar foo (2)'. During pre-expansion of the
+argument, `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 `foo'
+eligible for future expansion. Then, when re-scanning after argument
+replacement, the token `foo' is rejected for expansion, and marked
+ineligible for future expansion, since the macro is now disabled. It
+is disabled because the replacement list `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 `foo' would be popped in
+the process of finding the parenthesis, re-enabling `foo' and expanding
+it a second time.
+
+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 (*note
+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, `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.
+
+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
+`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 (*note Lexing a line::) using the
+function `_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.
+
+\1f
+File: cppinternals.info, Node: Token Spacing, Next: Line Numbering, Prev: Macro Expansion, Up: Top
+
+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:
+
+ #define PLUS +
+ #define EMPTY
+ #define f(x) =x=
+ +PLUS -EMPTY- PLUS+ f(=)
+ ==> + + - - + + = = =
+ _not_
+ ==> ++ -- ++ ===
+
+ 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 `EMPTY' example) from the original lexed token stream, we
+need to check for accidental token pasting. We call this "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 `#' and `##' operators.
+
+ Look at how the preprocessor gets whitespace output correct
+normally. The `cpp_token' structure contains a flags byte, and one of
+those flags is `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:
+
+ #define add(x, y, z) x + y +z;
+ sum = add (1,2, 3);
+ ==> sum = 1 + 2 +3;
+
+ The interesting thing here is that the tokens `1' and `2' are output
+with a preceding space, and `3' is output without a preceding space,
+but when lexed none of these tokens had that property. Careful
+consideration reveals that `1' gets its preceding whitespace from the
+space preceding `add' in the macro invocation, _not_ replacement list.
+`2' gets its whitespace from the space preceding the parameter `y' in
+the macro replacement list, and `3' has no preceding space because
+parameter `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
+"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 "source token" from which the subsequent real token should
+inherit its spacing. In the above example, the source tokens are `add'
+in the macro invocation, and `y' and `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.
+
+ #define foo bar
+ #define bar baz
+ [foo]
+ ==> [baz]
+
+ Here, two padding tokens are generated with sources the `foo' token
+between the brackets, and the `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:
+
+ #define foo bar
+ #define bar EMPTY baz
+ #define EMPTY
+ [foo] EMPTY;
+ ==> [ baz] ;
+
+ As shown, now there should be a space before `baz' and the semicolon
+in the output.
+
+ The rules we decided above fail for `baz': we generate three padding
+tokens, one per macro invocation, before the token `baz'. We would
+then have it take its spacing from the first of these, which carries
+source token `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 `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 `#' and `##' operators. I expanded the
+rule so that, if we see a padding token with a `NULL' source token,
+_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 `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.
+
+\1f
+File: cppinternals.info, Node: Line Numbering, Next: Guard Macros, Prev: Token Spacing, Up: Top
+
+Line numbering
+**************
+
+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:
+
+ * The source line it was lexed on.
+
+ * 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:
+
+ foo /* A long
+ comment */ bar \
+ baz
+ =>
+ foo bar baz
+
+ * 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 function-like macro expansion.
+
+ The `cpp_token' structure contains `line' and `col' members. The
+lexer fills these in with the line and column of the first character of
+the token. Consequently, but maybe unexpectedly, a token from the
+replacement list of a macro expansion carries the location of the token
+within the `#define' directive, because cpplib expands a macro by
+returning pointers to the tokens in its replacement list. The current
+implementation of cpplib assigns tokens created from built-in macros
+and the `#' and `##' operators the location of the most recently lexed
+token. This is a because they are allocated from the lexer's token
+runs, and because of the way the diagnostic routines infer the
+appropriate location to report.
+
+ The diagnostic routines in cpplib display the location of the most
+recently _lexed_ token, unless they are passed a specific line and
+column to report. For diagnostics regarding tokens that arise from
+macro expansions, it might also be helpful for the user to see the
+original location in the macro definition that the token came from.
+Since that is exactly the information each token carries, such an
+enhancement could be made relatively easily in future.
+
+ The stand-alone preprocessor faces a similar problem when determining
+the correct line to output the token on: the position attached to a
+token is fairly useless if the token came from a macro expansion. All
+tokens on a logical line should be output on its first physical line, so
+the token's reported location is also wrong if it is part of a physical
+line other than the first.
+
+ To solve these issues, cpplib provides a callback that is generated
+whenever it lexes a preprocessing token that starts a new logical line
+other than a directive. It passes this token (which may be a `CPP_EOF'
+token indicating the end of the translation unit) to the callback
+routine, which can then use the line and column of this token to
+produce correct output.
+
+Representation of line numbers
+==============================
+
+As mentioned above, cpplib stores with each token the line number that
+it was lexed on. In fact, this number is not the number of the line in
+the source file, but instead bears more resemblance to the number of the
+line in the translation unit.
+
+ The preprocessor maintains a monotonic increasing line count, which
+is incremented at every new line character (and also at the end of any
+buffer that does not end in a new line). Since a line number of zero is
+useful to indicate certain special states and conditions, this variable
+starts counting from one.
+
+ This variable therefore uniquely enumerates each line in the
+translation unit. With some simple infrastructure, it is straight
+forward to map from this to the original source file and line number
+pair, saving space whenever line number information needs to be saved.
+The code the implements this mapping lies in the files `line-map.c' and
+`line-map.h'.
+
+ Command-line macros and assertions are implemented by pushing a
+buffer containing the right hand side of an equivalent `#define' or
+`#assert' directive. Some built-in macros are handled similarly.
+Since these are all processed before the first line of the main input
+file, it will typically have an assigned line closer to twenty than to
+one.
+
+\1f
+File: cppinternals.info, Node: Guard Macros, Next: Files, Prev: Line Numbering, Up: Top
+
+The Multiple-Include Optimization
+*********************************
+
+Header files are often of the form
+
+ #ifndef FOO
+ #define FOO
+ ...
+ #endif
+
+to prevent the compiler from processing them more than once. The
+preprocessor notices such header files, so that if the header file
+appears in a subsequent `#include' directive and `FOO' is defined, then
+it is ignored and it doesn't preprocess or even re-open the file a
+second time. This is referred to as the "multiple include
+optimization".
+
+ Under what circumstances is such an optimization valid? If the file
+were included a second time, it can only be optimized away if that
+inclusion would result in no tokens to return, and no relevant
+directives to process. Therefore the current implementation imposes
+requirements and makes some allowances as follows:
+
+ 1. There must be no tokens outside the controlling `#if'-`#endif'
+ pair, but whitespace and comments are permitted.
+
+ 2. There must be no directives outside the controlling directive
+ pair, but the "null directive" (a line containing nothing other
+ than a single `#' and possibly whitespace) is permitted.
+
+ 3. The opening directive must be of the form
+
+ #ifndef FOO
+
+ or
+
+ #if !defined FOO [equivalently, #if !defined(FOO)]
+
+ 4. In the second form above, the tokens forming the `#if' expression
+ must have come directly from the source file--no macro expansion
+ must have been involved. This is because macro definitions can
+ change, and tracking whether or not a relevant change has been
+ made is not worth the implementation cost.
+
+ 5. There can be no `#else' or `#elif' directives at the outer
+ conditional block level, because they would probably contain
+ something of interest to a subsequent pass.
+
+ First, when pushing a new file on the buffer stack,
+`_stack_include_file' sets the controlling macro `mi_cmacro' to `NULL',
+and sets `mi_valid' to `true'. This indicates that the preprocessor
+has not yet encountered anything that would invalidate the
+multiple-include optimization. As described in the next few
+paragraphs, these two variables having these values effectively
+indicates top-of-file.
+
+ When about to return a token that is not part of a directive,
+`_cpp_lex_token' sets `mi_valid' to `false'. This enforces the
+constraint that tokens outside the controlling conditional block
+invalidate the optimization.
+
+ The `do_if', when appropriate, and `do_ifndef' directive handlers
+pass the controlling macro to the function `push_conditional'. cpplib
+maintains a stack of nested conditional blocks, and after processing
+every opening conditional this function pushes an `if_stack' structure
+onto the stack. In this structure it records the controlling macro for
+the block, provided there is one and we're at top-of-file (as described
+above). If an `#elif' or `#else' directive is encountered, the
+controlling macro for that block is cleared to `NULL'. Otherwise, it
+survives until the `#endif' closing the block, upon which `do_endif'
+sets `mi_valid' to true and stores the controlling macro in `mi_cmacro'.
+
+ `_cpp_handle_directive' clears `mi_valid' when processing any
+directive other than an opening conditional and the null directive.
+With this, and requiring top-of-file to record a controlling macro, and
+no `#else' or `#elif' for it to survive and be copied to `mi_cmacro' by
+`do_endif', we have enforced the absence of directives outside the main
+conditional block for the optimization to be on.
+
+ Note that whilst we are inside the conditional block, `mi_valid' is
+likely to be reset to `false', but this does not matter since the
+closing `#endif' restores it to `true' if appropriate.
+
+ Finally, since `_cpp_lex_direct' pops the file off the buffer stack
+at `EOF' without returning a token, if the `#endif' directive was not
+followed by any tokens, `mi_valid' is `true' and `_cpp_pop_file_buffer'
+remembers the controlling macro associated with the file. Subsequent
+calls to `stack_include_file' result in no buffer being pushed if the
+controlling macro is defined, effecting the optimization.
+
+ A quick word on how we handle the
+
+ #if !defined FOO
+
+case. `_cpp_parse_expr' and `parse_defined' take steps to see whether
+the three stages `!', `defined-expression' and `end-of-directive' occur
+in order in a `#if' expression. If so, they return the guard macro to
+`do_if' in the variable `mi_ind_cmacro', and otherwise set it to `NULL'.
+`enter_macro_context' sets `mi_valid' to false, so if a macro was
+expanded whilst parsing any part of the expression, then the
+top-of-file test in `push_conditional' fails and the optimization is
+turned off.
+
+\1f
+File: cppinternals.info, Node: Files, Next: Concept Index, Prev: Guard Macros, Up: Top
+
+File Handling
+*************
+
+Fairly obviously, the file handling code of cpplib resides in the 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.
+
+ The basic strategy is to minimize the number of system calls. On
+many systems, the basic `open ()' and `fstat ()' system calls can be
+quite expensive. For every `#include'-d file, we need to try all the
+directories in the search path until we find a match. Some projects,
+such as glibc, pass twenty or thirty include paths on the command line,
+so this can rapidly become time consuming.
+
+ For a header file we have not encountered before we have little
+choice but to do this. However, it is often the case that the same
+headers are repeatedly included, and in these cases we try to avoid
+repeating the filesystem queries whilst searching for the correct file.
+
+ For each file we try to open, we store the constructed path in a
+splay tree. This path first undergoes simplification by the function
+`_cpp_simplify_pathname'. For example, `/usr/include/bits/../foo.h' is
+simplified to `/usr/include/foo.h' before we enter it in the splay tree
+and try to `open ()' the file. CPP will then find subsequent uses of
+`foo.h', even as `/usr/include/foo.h', in the splay tree and save
+system calls.
+
+ Further, it is likely the file contents have also been cached,
+saving a `read ()' system call. We don't bother caching the contents of
+header files that are re-inclusion protected, and whose re-inclusion
+macro is defined when we leave the header file for the first time. If
+the host supports it, we try to map suitably large files into memory,
+rather than reading them in directly.
+
+ The include paths are internally stored on a null-terminated
+singly-linked list, starting with the `"header.h"' directory search
+chain, which then links into the `<header.h>' directory chain.
+
+ Files included with the `<foo.h>' syntax start the lookup directly
+in the second half of this chain. However, files included with the
+`"foo.h"' syntax start at the beginning of the chain, but with one
+extra directory prepended. This is the directory of the current file;
+the one containing the `#include' directive. Prepending this directory
+on a per-file basis is handled by the function `search_from'.
+
+ Note that a header included with a directory component, such as
+`#include "mydir/foo.h"' and opened as
+`/usr/local/include/mydir/foo.h', will have the complete path minus the
+basename `foo.h' as the current directory.
+
+ 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 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 `#import' directive.
+
+ For the benefit of MS-DOS filesystems with an 8.3 filename
+limitation, CPP offers the ability to treat various include file names
+as aliases for the real header files with shorter names. The map from
+one to the other is found in a special file called `header.gcc', stored
+in the command line (or system) include directories to which the mapping
+applies. This may be higher up the directory tree than the full path to
+the file minus the base name.
+
+\1f
+File: cppinternals.info, Node: Concept Index, Prev: Files, Up: Top
+
+Concept Index
+*************
+
+\0\b[index\0\b]
+* Menu:
+
+* assertions: Hash Nodes. (line 6)
+* controlling macros: Guard Macros. (line 6)
+* escaped newlines: Lexer. (line 6)
+* files: Files. (line 6)
+* guard macros: Guard Macros. (line 6)
+* hash table: Hash Nodes. (line 6)
+* header files: Conventions. (line 6)
+* identifiers: Hash Nodes. (line 6)
+* interface: Conventions. (line 6)
+* lexer: Lexer. (line 6)
+* line numbers: Line Numbering. (line 6)
+* macro expansion: Macro Expansion. (line 6)
+* macro representation (internal): Macro Expansion. (line 19)
+* macros: Hash Nodes. (line 6)
+* multiple-include optimization: Guard Macros. (line 6)
+* named operators: Hash Nodes. (line 6)
+* newlines: Lexer. (line 6)
+* paste avoidance: Token Spacing. (line 6)
+* spacing: Token Spacing. (line 6)
+* token run: Lexer. (line 192)
+* token spacing: Token Spacing. (line 6)
+
+
+\1f
+Tag Table:
+Node: Top\7f1005
+Node: Conventions\7f2690
+Node: Lexer\7f3632
+Ref: Invalid identifiers\7f11545
+Ref: Lexing a line\7f13494
+Node: Hash Nodes\7f18267
+Node: Macro Expansion\7f21146
+Node: Token Spacing\7f30093
+Node: Line Numbering\7f35953
+Node: Guard Macros\7f40038
+Node: Files\7f44829
+Node: Concept Index\7f48295
+\1f
+End Tag Table
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