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@setfilename cppinternals.info

@settitle The GNU C Preprocessor Internals

@include gcc-common.texi

@ifinfo

@dircategory Software development

@direntry

* Cpplib: (cppinternals).      Cpplib internals.

@end direntry

@end ifinfo

@c @smallbook

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@c @finalout

@setchapternewpage odd

@ifinfo

This file documents the internals of the GNU C Preprocessor.

Copyright 2000, 2001, 2002, 2004, 2005 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of

this manual provided the copyright notice and this permission notice

are preserved on all copies.

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@end ifinfo

@titlepage

@title Cpplib Internals

@versionsubtitle

@author Neil Booth

@page

@vskip 0pt plus 1filll

@c man begin COPYRIGHT

Copyright @copyright{} 2000, 2001, 2002, 2004, 2005

Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of

this manual provided the copyright notice and this permission notice

are preserved on all copies.

Permission is granted to copy and distribute modified versions of this

manual under the conditions for verbatim copying, provided also that

the entire resulting derived work is distributed under the terms of a

permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual

into another language, under the above conditions for modified versions.

@c man end

@end titlepage

@contents

@page

@node Top

@top

@chapter Cpplib---the GNU C Preprocessor

The GNU C preprocessor is

implemented as a library, @dfn{cpplib}, so it can be easily shared between

a stand-alone preprocessor, and a preprocessor integrated with the C,

C++ and Objective-C front ends.  It is also available for use by other

programs, though this is not recommended as its exposed interface has

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.

@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{internal.h}.  Functions and types exposed to external

clients are in @file{cpplib.h}, and prefixed with @samp{cpp_}.  For

historical reasons this is no longer quite true, but we should strive to

stick to it.

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{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, @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 testsuite 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 the line of tokens,

the pointers to the tokens of its expansion that are returned will always

remain valid.  However, macros are a little trickier than that, since

they give rise to three sources of fresh tokens.  They are the built-in

macros like @code{__LINE__}, and the @samp{#} and @samp{##} operators

for 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, 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:

@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.

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 to

use, so the output code should contain a rule that the first

padding token in a sequence is the one that matters.

But what if a macro expansion is left?  Adjusting the above

example slightly:

@smallexample

#define foo bar

#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 /* @r{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