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diff --git a/gcc-4.9/gcc/doc/cppinternals.info b/gcc-4.9/gcc/doc/cppinternals.info new file mode 100644 index 000000000..391787ef2 --- /dev/null +++ b/gcc-4.9/gcc/doc/cppinternals.info @@ -0,0 +1,1029 @@ +This is cppinternals.info, produced by makeinfo version 5.1 from +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 (C) 2000-2014 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. + + +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. + + +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. + + +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. + + +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). + + +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. + + +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. + + +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. + + +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. + + +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. + + +File: cppinternals.info, Node: Concept Index, Prev: Files, Up: Top + +Concept Index +************* + + +* Menu: + +* assertions: Hash Nodes. (line 6) +* controlling macros: Guard Macros. (line 6) +* escaped newlines: Lexer. (line 5) +* 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 5) +* 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 191) +* token spacing: Token Spacing. (line 6) + + + +Tag Table: +Node: Top905 +Node: Conventions2590 +Node: Lexer3532 +Ref: Invalid identifiers11447 +Ref: Lexing a line13397 +Node: Hash Nodes18170 +Node: Macro Expansion21049 +Node: Token Spacing29997 +Node: Line Numbering35854 +Node: Guard Macros39939 +Node: Files44730 +Node: Concept Index48196 + +End Tag Table |