+++ /dev/null
-This is doc/cppinternals.info, produced by makeinfo version 4.13 from
-/d/gcc-4.4.3/gcc-4.4.3/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, 2006, 2007 Free Software
-Foundation, Inc.
-
- Permission is granted to make and distribute verbatim copies of this
-manual provided the copyright notice and this permission notice are
-preserved on all copies.
-
- 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\7f971
-Node: Conventions\7f2656
-Node: Lexer\7f3598
-Ref: Invalid identifiers\7f11511
-Ref: Lexing a line\7f13460
-Node: Hash Nodes\7f18233
-Node: Macro Expansion\7f21112
-Node: Token Spacing\7f30059
-Node: Line Numbering\7f35919
-Node: Guard Macros\7f40004
-Node: Files\7f44795
-Node: Concept Index\7f48261
-\1f
-End Tag Table
projects / msp430-gcc.git / blobdiff