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diff --git a/contrib/gcc/doc/cppinternals.texi b/contrib/gcc/doc/cppinternals.texi new file mode 100644 index 000000000000..3f3d9af00517 --- /dev/null +++ b/contrib/gcc/doc/cppinternals.texi @@ -0,0 +1,1065 @@ +\input texinfo +@setfilename cppinternals.info +@settitle The GNU C Preprocessor Internals + +@ifinfo +@dircategory Programming +@direntry +* Cpplib: (cppinternals). Cpplib internals. +@end direntry +@end ifinfo + +@c @smallbook +@c @cropmarks +@c @finalout +@setchapternewpage odd +@ifinfo +This file documents the internals of the GNU C Preprocessor. + +Copyright 2000, 2001, 2002 Free Software Foundation, Inc. + +Permission is granted to make and distribute verbatim copies of +this manual provided the copyright notice and this permission notice +are preserved on all copies. + +@ignore +Permission is granted to process this file through Tex and print the +results, provided the printed document carries copying permission +notice identical to this one except for the removal of this paragraph +(this paragraph not being relevant to the printed manual). + +@end ignore +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. +@end ifinfo + +@titlepage +@c @finalout +@title Cpplib Internals +@subtitle Last revised January 2002 +@subtitle for GCC version 3.1 +@author Neil Booth +@page +@vskip 0pt plus 1filll +@c man begin COPYRIGHT +Copyright @copyright{} 2000, 2001, 2002 +Free Software Foundation, Inc. + +Permission is granted to make and distribute verbatim copies of +this manual provided the copyright notice and this permission notice +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. +@c man end +@end titlepage +@contents +@page + +@node Top +@top +@chapter Cpplib---the GNU C Preprocessor + +The GNU C preprocessor in GCC 3.x has been completely rewritten. It is +now implemented as a library, @dfn{cpplib}, so it can be easily shared between +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. +* Index:: Index. +@end menu + +@node Conventions +@unnumbered Conventions +@cindex interface +@cindex header files + +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 @samp{_cpp_}, and are to be found in +the file @file{cpphash.h}. Functions and types exposed to external +clients are in @file{cpplib.h}, and prefixed with @samp{cpp_}. For +historical reasons this is no longer quite true, but we should strive to +stick to it. + +We are striving to reduce the information exposed in @file{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. + +@node Lexer +@unnumbered The Lexer +@cindex lexer +@cindex newlines +@cindex escaped newlines + +@section Overview +The lexer is contained in the file @file{cpplex.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, @code{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 +@code{cpp_spell_token} and @code{cpp_token_len}. These functions are +useful when generating diagnostics, and for emitting the preprocessed +output. + +@section Lexing a token +Lexing of an individual token is handled by @code{_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 @code{_cpp_lex_direct} is simply to lex a token. It is not +responsible for issues like directive handling, returning lookahead +tokens directly, multiple-include optimization, or conditional block +skipping. It necessarily has a minor r@^ole to play in memory +management of lexed lines. I discuss these issues in a separate section +(@pxref{Lexing a line}). + +The lexer places the token it lexes into storage pointed to by the +variable @code{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 +@code{line} and @code{col} values of the token just before the location +that @code{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 +@code{PREV_WHITE} bit in the token's flags. Each token has its +@code{line} and @code{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 +@code{BOL} set for beginning-of-line. This flag is intended for +internal use, both to distinguish a @samp{#} 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 @code{BOL} flag set. The macro expansion may even be empty, and the +next token on the line certainly won't have the @code{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 +@code{in_directive} is set, the lexer returns a @code{CPP_EOF} token, +which is normally used to indicate end-of-file, to indicate +end-of-directive. In a directive a @code{CPP_EOF} token never means +end-of-file. Conveniently, if the caller was @code{collect_args}, it +already handles @code{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 +@code{parsing_args} is nonzero when the preprocessor is collecting the +arguments to a macro call. It is set to 1 when looking for the opening +parenthesis to a function-like macro, and 2 when collecting the actual +arguments up to the closing parenthesis, since these two cases need to +be distinguished sometimes. One such time is here: the lexer sets the +@code{PREV_WHITE} flag of a token if it meets a new line when +@code{parsing_args} is set to 2. It doesn't set it if it meets a new +line when @code{parsing_args} is 1, since then code like + +@smallexample +#define foo() bar +foo +baz +@end smallexample + +@noindent would be output with an erroneous space before @samp{baz}: + +@smallexample +foo + baz +@end smallexample + +This is a good example of the subtlety of getting token spacing correct +in the preprocessor; there are plenty of tests in the test suite for +corner cases like this. + +The lexer is written to treat each of @samp{\r}, @samp{\n}, @samp{\r\n} +and @samp{\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 @samp{\} or the trigraph +@samp{??/}, 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 @code{handle_newline} takes care of all newline +characters, and @code{skip_escaped_newlines} takes care of arbitrarily +long sequences of escaped newlines, deferring to @code{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 @samp{??/} to introduce an escaped newline. + +Escaped newlines are tedious because theoretically they can occur +anywhere---between the @samp{+} and @samp{=} of the @samp{+=} token, +within the characters of an identifier, and even between the @samp{*} +and @samp{/} 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, @code{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 @samp{\} +introducing an escaped newline, or the @samp{?} introducing the trigraph +sequence that represents the @samp{\} of an escaped newline. If it +encounters a @samp{?} or @samp{\}, it calls @code{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 @code{_cpp_lex_direct} cannot simply +check for a @samp{=} after a @samp{+} character to determine whether it +has a @samp{+=} token; it needs to be prepared for an escaped newline of +some sort. Such cases use the function @code{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 @option{-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. + +@anchor{Invalid identifiers} +Some identifiers, such as @code{__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 +@code{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 @code{__VA_ARGS__} in the expansion of a variable-argument macro. +Therefore @code{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 @samp{<} would be lexed as a single +token. After a @code{#include} directive, though, it should be lexed as +a single token as far as the nearest @samp{>} character. Note that we +don't allow the terminators of header names to be escaped; the first +@samp{"} or @samp{>} 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, @samp{::} is a single token in C++, but in C it is +two separate @samp{:} tokens and almost certainly a syntax error. Such +cases are handled by @code{_cpp_lex_direct} based upon command-line +flags stored in the @code{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 @code{_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. + +@anchor{Lexing a line} +@section Lexing a line +@cindex token run + +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 +@code{#pragma} directive and not recognizing it as a registered pragma, +it wants to backtrack and allow the user-defined handler for unknown +pragmas to access the full @code{#pragma} token stream. The stand-alone +preprocessor wants to be able to test the current token with the +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 @dfn{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 @emph{not} lex a line of +tokens at once; if we did that @code{parse_identifier} would not have +state flags available to warn about invalid identifiers (@pxref{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 @code{#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 +@code{#define} handler, and placed in storage that is only freed by +@code{cpp_destroy}. So if a macro is expanded in our line of tokens, +the pointers to the tokens of its expansion that we return will always +remain valid. However, macros are a little trickier than that, since +they give rise to three sources of fresh tokens. They are the built-in +macros like @code{__LINE__}, and the @samp{#} and @samp{##} operators +for 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 @code{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 @code{_cpp_lex_token} handles moving to new token runs, +calling @code{_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 @code{BOL} flag, which might indicate a +directive that needs to be handled, or require a start-of-line call-back +to be made. @code{_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. + +@node Hash Nodes +@unnumbered Hash Nodes +@cindex hash table +@cindex identifiers +@cindex macros +@cindex assertions +@cindex named operators + +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 @code{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 @code{pragma}, @code{int}, @code{foo} and +@code{__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: + +@itemize @bullet +@item Macros + +These have been declared to be macros, either on the command line or +with @code{#define}. A few, such as @code{__TIME__} are built-ins +entered in the hash table during 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. + +@item Assertions + +Assertions are in a separate namespace to macros. To enforce this, cpp +actually prepends a @code{#} character before hashing and entering it in +the hash table. An assertion's node points to a chain of answers to +that assertion. + +@item Void + +Everything else falls into this category---an identifier that is not +currently a macro, or a macro that has since been undefined with +@code{#undef}. + +When preprocessing C++, this category also includes the named operators, +such as @code{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 @code{#} and +@code{##}. Named operator hash nodes are flagged, both to catch the +spelling distinction and to prevent them from being defined as macros. +@end itemize + +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 @code{#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 @code{endif}, has an associated directive +enum stored in its hash node, so that directive lookup is also O(1). + +@node Macro Expansion +@unnumbered Macro Expansion Algorithm +@cindex macro expansion + +Macro expansion is a tricky operation, fraught with nasty corner cases +and situations that render what you thought was a nifty way to +optimize the preprocessor's expansion algorithm wrong in quite subtle +ways. + +I strongly recommend you have a good grasp of how the C and C++ +standards require macros to be expanded before diving into this +section, let alone the code!. If you don't have a clear mental +picture of how things like nested macro expansion, stringification and +token pasting are supposed to work, damage to your sanity can quickly +result. + +@section Internal representation of macros +@cindex macro representation (internal) + +The preprocessor stores macro expansions in tokenized form. This +saves repeated lexing passes during expansion, at the cost of a small +increase in memory consumption on average. The tokens are stored +contiguously in memory, so a pointer to the first one and a token +count is all you need to get the replacement list of a macro. + +If the macro is a function-like macro the preprocessor also stores its +parameters, in the form of an ordered list of pointers to the hash +table entry of each parameter's identifier. Further, in the macro's +stored expansion each occurrence of a parameter is replaced with a +special token of type @code{CPP_MACRO_ARG}. Each such token holds the +index of the parameter it represents in the parameter list, which +allows rapid replacement of parameters with their arguments during +expansion. Despite this optimization it is still necessary to store +the original parameters to the macro, both for dumping with e.g., +@option{-dD}, and to warn about non-trivial macro redefinitions when +the parameter names have changed. + +@section Macro expansion overview +The preprocessor maintains a @dfn{context stack}, implemented as a +linked list of @code{cpp_context} structures, which together represent +the macro expansion state at any one time. The @code{struct +cpp_reader} member variable @code{context} points to the current top +of this stack. The top normally holds the unexpanded replacement list +of the innermost macro under expansion, except when cpplib is about to +pre-expand an argument, in which case it holds that argument's +unexpanded tokens. + +When there are no macros under expansion, cpplib is in @dfn{base +context}. All contexts other than the base context contain a +contiguous list of tokens delimited by a starting and ending token. +When not in base context, cpplib obtains the next token from the list +of the top context. If there are no tokens left in the list, it pops +that context off the stack, and subsequent ones if necessary, until an +unexhausted context is found or it returns to base context. In base +context, cpplib reads tokens directly from the lexer. + +If it encounters an identifier that is both a macro and enabled for +expansion, cpplib prepares to push a new context for that macro on the +stack by calling the routine @code{enter_macro_context}. When this +routine returns, the new context will contain the unexpanded tokens of +the replacement list of that macro. In the case of function-like +macros, @code{enter_macro_context} also replaces any parameters in the +replacement list, stored as @code{CPP_MACRO_ARG} tokens, with the +appropriate macro argument. If the standard requires that the +parameter be replaced with its expanded argument, the argument will +have been fully macro expanded first. + +@code{enter_macro_context} also handles special macros like +@code{__LINE__}. Although these macros expand to a single token which +cannot contain any further macros, for reasons of token spacing +(@pxref{Token Spacing}) and simplicity of implementation, cpplib +handles these special macros by pushing a context containing just that +one token. + +The final thing that @code{enter_macro_context} does before returning +is to mark the macro disabled for expansion (except for special macros +like @code{__TIME__}). The macro is re-enabled when its context is +later popped from the context stack, as described above. This strict +ordering ensures that a macro is disabled whilst its expansion is +being scanned, but that it is @emph{not} disabled whilst any arguments +to it are being expanded. + +@section Scanning the replacement list for macros to expand +The C standard states that, after any parameters have been replaced +with their possibly-expanded arguments, the replacement list is +scanned for nested macros. Further, any identifiers in the +replacement list that are not expanded during this scan are never +again eligible for expansion in the future, if the reason they were +not expanded is that the macro in question was disabled. + +Clearly this latter condition can only apply to tokens resulting from +argument pre-expansion. Other tokens never have an opportunity to be +re-tested for expansion. It is possible for identifiers that are +function-like macros to not expand initially but to expand during a +later scan. This occurs when the identifier is the last token of an +argument (and therefore originally followed by a comma or a closing +parenthesis in its macro's argument list), and when it replaces its +parameter in the macro's replacement list, the subsequent token +happens to be an opening parenthesis (itself possibly the first token +of an argument). + +It is important to note that when cpplib reads the last token of a +given context, that context still remains on the stack. Only when +looking for the @emph{next} token do we pop it off the stack and drop +to a lower context. This makes backing up by one token easy, but more +importantly ensures that the macro corresponding to the current +context is still disabled when we are considering the last token of +its replacement list for expansion (or indeed expanding it). As an +example, which illustrates many of the points above, consider + +@smallexample +#define foo(x) bar x +foo(foo) (2) +@end smallexample + +@noindent which fully expands to @samp{bar foo (2)}. During pre-expansion +of the argument, @samp{foo} does not expand even though the macro is +enabled, since it has no following parenthesis [pre-expansion of an +argument only uses tokens from that argument; it cannot take tokens +from whatever follows the macro invocation]. This still leaves the +argument token @samp{foo} eligible for future expansion. Then, when +re-scanning after argument replacement, the token @samp{foo} is +rejected for expansion, and marked ineligible for future expansion, +since the macro is now disabled. It is disabled because the +replacement list @samp{bar foo} of the macro is still on the context +stack. + +If instead the algorithm looked for an opening parenthesis first and +then tested whether the macro were disabled it would be subtly wrong. +In the example above, the replacement list of @samp{foo} would be +popped in the process of finding the parenthesis, re-enabling +@samp{foo} and expanding it a second time. + +@section Looking for a function-like macro's opening parenthesis +Function-like macros only expand when immediately followed by a +parenthesis. To do this cpplib needs to temporarily disable macros +and read the next token. Unfortunately, because of spacing issues +(@pxref{Token Spacing}), there can be fake padding tokens in-between, +and if the next real token is not a parenthesis cpplib needs to be +able to back up that one token as well as retain the information in +any intervening padding tokens. + +Backing up more than one token when macros are involved is not +permitted by cpplib, because in general it might involve issues like +restoring popped contexts onto the context stack, which are too hard. +Instead, searching for the parenthesis is handled by a special +function, @code{funlike_invocation_p}, which remembers padding +information as it reads tokens. If the next real token is not an +opening parenthesis, it backs up that one token, and then pushes an +extra context just containing the padding information if necessary. + +@section Marking tokens ineligible for future expansion +As discussed above, cpplib needs a way of marking tokens as +unexpandable. Since the tokens cpplib handles are read-only once they +have been lexed, it instead makes a copy of the token and adds the +flag @code{NO_EXPAND} to the copy. + +For efficiency and to simplify memory management by avoiding having to +remember to free these tokens, they are allocated as temporary tokens +from the lexer's current token run (@pxref{Lexing a line}) using the +function @code{_cpp_temp_token}. The tokens are then re-used once the +current line of tokens has been read in. + +This might sound unsafe. However, tokens runs are not re-used at the +end of a line if it happens to be in the middle of a macro argument +list, and cpplib only wants to back-up more than one lexer token in +situations where no macro expansion is involved, so the optimization +is safe. + +@node Token Spacing +@unnumbered Token Spacing +@cindex paste avoidance +@cindex spacing +@cindex token spacing + +First, let's look at an issue that only concerns the stand-alone +preprocessor: we want to guarantee that re-reading its preprocessed +output results in an identical token stream. Without taking special +measures, this might not be the case because of macro substitution. +For example: + +@smallexample +#define PLUS + +#define EMPTY +#define f(x) =x= ++PLUS -EMPTY- PLUS+ f(=) + @expansion{} + + - - + + = = = +@emph{not} + @expansion{} ++ -- ++ === +@end smallexample + +One solution would be to simply insert a space between all adjacent +tokens. However, we would like to keep space insertion to a minimum, +both for aesthetic reasons and because it causes problems for people who +still try to abuse the preprocessor for things like Fortran source and +Makefiles. + +For now, just notice that when tokens are added (or removed, as shown by +the @code{EMPTY} example) from the original lexed token stream, we need +to check for accidental token pasting. We call this @dfn{paste +avoidance}. Token addition and removal can only occur because of macro +expansion, but accidental pasting can occur in many places: both before +and after each macro replacement, each argument replacement, and +additionally each token created by the @samp{#} and @samp{##} operators. + +Let's look at how the preprocessor gets whitespace output correct +normally. The @code{cpp_token} structure contains a flags byte, and one +of those flags is @code{PREV_WHITE}. This is flagged by the lexer, and +indicates that the token was preceded by whitespace of some form other +than a new line. The stand-alone preprocessor can use this flag to +decide whether to insert a space between tokens in the output. + +Now consider the result of the following macro expansion: + +@smallexample +#define add(x, y, z) x + y +z; +sum = add (1,2, 3); + @expansion{} sum = 1 + 2 +3; +@end smallexample + +The interesting thing here is that the tokens @samp{1} and @samp{2} are +output with a preceding space, and @samp{3} is output without a +preceding space, but when lexed none of these tokens had that property. +Careful consideration reveals that @samp{1} gets its preceding +whitespace from the space preceding @samp{add} in the macro invocation, +@emph{not} replacement list. @samp{2} gets its whitespace from the +space preceding the parameter @samp{y} in the macro replacement list, +and @samp{3} has no preceding space because parameter @samp{z} has none +in the replacement list. + +Once lexed, tokens are effectively fixed and cannot be altered, since +pointers to them might be held in many places, in particular by +in-progress macro expansions. So instead of modifying the two tokens +above, the preprocessor inserts a special token, which I call a +@dfn{padding token}, into the token stream to indicate that spacing of +the subsequent token is special. The preprocessor inserts padding +tokens in front of every macro expansion and expanded macro argument. +These point to a @dfn{source token} from which the subsequent real token +should inherit its spacing. In the above example, the source tokens are +@samp{add} in the macro invocation, and @samp{y} and @samp{z} in the +macro replacement list, respectively. + +It is quite easy to get multiple padding tokens in a row, for example if +a macro's first replacement token expands straight into another macro. + +@smallexample +#define foo bar +#define bar baz +[foo] + @expansion{} [baz] +@end smallexample + +Here, two padding tokens are generated with sources the @samp{foo} token +between the brackets, and the @samp{bar} token from foo's replacement +list, respectively. Clearly the first padding token is the one we +should use, so our output code should contain a rule that the first +padding token in a sequence is the one that matters. + +But what if we happen to leave a macro expansion? Adjusting the above +example slightly: + +@smallexample +#define foo bar +#define bar EMPTY baz +#define EMPTY +[foo] EMPTY; + @expansion{} [ baz] ; +@end smallexample + +As shown, now there should be a space before @samp{baz} and the +semicolon in the output. + +The rules we decided above fail for @samp{baz}: we generate three +padding tokens, one per macro invocation, before the token @samp{baz}. +We would then have it take its spacing from the first of these, which +carries source token @samp{foo} with no leading space. + +It is vital that cpplib get spacing correct in these examples since any +of these macro expansions could be stringified, where spacing matters. + +So, this demonstrates that not just entering macro and argument +expansions, but leaving them requires special handling too. I made +cpplib insert a padding token with a @code{NULL} source token when +leaving macro expansions, as well as after each replaced argument in a +macro's replacement list. It also inserts appropriate padding tokens on +either side of tokens created by the @samp{#} and @samp{##} operators. +I expanded the rule so that, if we see a padding token with a +@code{NULL} source token, @emph{and} that source token has no leading +space, then we behave as if we have seen no padding tokens at all. A +quick check shows this rule will then get the above example correct as +well. + +Now a relationship with paste avoidance is apparent: we have to be +careful about paste avoidance in exactly the same locations we have +padding tokens in order to get white space correct. This makes +implementation of paste avoidance easy: wherever the stand-alone +preprocessor is fixing up spacing because of padding tokens, and it +turns out that no space is needed, it has to take the extra step to +check that a space is not needed after all to avoid an accidental paste. +The function @code{cpp_avoid_paste} advises whether a space is required +between two consecutive tokens. To avoid excessive spacing, it tries +hard to only require a space if one is likely to be necessary, but for +reasons of efficiency it is slightly conservative and might recommend a +space where one is not strictly needed. + +@node Line Numbering +@unnumbered Line numbering +@cindex line numbers + +@section Just which line number anyway? + +There are three reasonable requirements a cpplib client might have for +the line number of a token passed to it: + +@itemize @bullet +@item +The source line it was lexed on. +@item +The line it is output on. This can be different to the line it was +lexed on if, for example, there are intervening escaped newlines or +C-style comments. For example: + +@smallexample +foo /* A long +comment */ bar \ +baz +@result{} +foo bar baz +@end smallexample + +@item +If the token results from a macro expansion, the line of the macro name, +or possibly the line of the closing parenthesis in the case of +function-like macro expansion. +@end itemize + +The @code{cpp_token} structure contains @code{line} and @code{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 @code{#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 @samp{#} and @samp{##} 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 @emph{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 +@code{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. + +@section 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 @file{line-map.c} and +@file{line-map.h}. + +Command-line macros and assertions are implemented by pushing a buffer +containing the right hand side of an equivalent @code{#define} or +@code{#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. + +@node Guard Macros +@unnumbered The Multiple-Include Optimization +@cindex guard macros +@cindex controlling macros +@cindex multiple-include optimization + +Header files are often of the form + +@smallexample +#ifndef FOO +#define FOO +@dots{} +#endif +@end smallexample + +@noindent +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 @code{#include} directive and @code{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 @dfn{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: + +@enumerate +@item +There must be no tokens outside the controlling @code{#if}-@code{#endif} +pair, but whitespace and comments are permitted. + +@item +There must be no directives outside the controlling directive pair, but +the @dfn{null directive} (a line containing nothing other than a single +@samp{#} and possibly whitespace) is permitted. + +@item +The opening directive must be of the form + +@smallexample +#ifndef FOO +@end smallexample + +or + +@smallexample +#if !defined FOO [equivalently, #if !defined(FOO)] +@end smallexample + +@item +In the second form above, the tokens forming the @code{#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. + +@item +There can be no @code{#else} or @code{#elif} directives at the outer +conditional block level, because they would probably contain something +of interest to a subsequent pass. +@end enumerate + +First, when pushing a new file on the buffer stack, +@code{_stack_include_file} sets the controlling macro @code{mi_cmacro} to +@code{NULL}, and sets @code{mi_valid} to @code{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, +@code{_cpp_lex_token} sets @code{mi_valid} to @code{false}. This +enforces the constraint that tokens outside the controlling conditional +block invalidate the optimization. + +The @code{do_if}, when appropriate, and @code{do_ifndef} directive +handlers pass the controlling macro to the function +@code{push_conditional}. cpplib maintains a stack of nested conditional +blocks, and after processing every opening conditional this function +pushes an @code{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 @code{#elif} or +@code{#else} directive is encountered, the controlling macro for that +block is cleared to @code{NULL}. Otherwise, it survives until the +@code{#endif} closing the block, upon which @code{do_endif} sets +@code{mi_valid} to true and stores the controlling macro in +@code{mi_cmacro}. + +@code{_cpp_handle_directive} clears @code{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 @code{#else} or @code{#elif} for it to survive and be copied to +@code{mi_cmacro} by @code{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, @code{mi_valid} is +likely to be reset to @code{false}, but this does not matter since the +the closing @code{#endif} restores it to @code{true} if appropriate. + +Finally, since @code{_cpp_lex_direct} pops the file off the buffer stack +at @code{EOF} without returning a token, if the @code{#endif} directive +was not followed by any tokens, @code{mi_valid} is @code{true} and +@code{_cpp_pop_file_buffer} remembers the controlling macro associated +with the file. Subsequent calls to @code{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 + +@smallexample +#if !defined FOO +@end smallexample + +@noindent +case. @code{_cpp_parse_expr} and @code{parse_defined} take steps to see +whether the three stages @samp{!}, @samp{defined-expression} and +@samp{end-of-directive} occur in order in a @code{#if} expression. If +so, they return the guard macro to @code{do_if} in the variable +@code{mi_ind_cmacro}, and otherwise set it to @code{NULL}. +@code{enter_macro_context} sets @code{mi_valid} to false, so if a macro +was expanded whilst parsing any part of the expression, then the +top-of-file test in @code{push_conditional} fails and the optimization +is turned off. + +@node Files +@unnumbered File Handling +@cindex files + +Fairly obviously, the file handling code of cpplib resides in the file +@file{cppfiles.c}. It takes care of the details of file searching, +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 @code{open ()} and @code{fstat ()} system calls can +be quite expensive. For every @code{#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 +@code{_cpp_simplify_pathname}. For example, +@file{/usr/include/bits/../foo.h} is simplified to +@file{/usr/include/foo.h} before we enter it in the splay tree and try +to @code{open ()} the file. CPP will then find subsequent uses of +@file{foo.h}, even as @file{/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 +@code{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 @code{"header.h"} directory search +chain, which then links into the @code{<header.h>} directory chain. + +Files included with the @code{<foo.h>} syntax start the lookup directly +in the second half of this chain. However, files included with the +@code{"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 @code{#include} directive. Prepending this +directory on a per-file basis is handled by the function +@code{search_from}. + +Note that a header included with a directory component, such as +@code{#include "mydir/foo.h"} and opened as +@file{/usr/local/include/mydir/foo.h}, will have the complete path minus +the basename @samp{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 @code{#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 @samp{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. + +@node Index +@unnumbered Index +@printindex cp + +@bye |