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diff --git a/share/doc/cppinternals/Macro-Expansion.html b/share/doc/cppinternals/Macro-Expansion.html new file mode 100644 index 0000000..3ae799b --- /dev/null +++ b/share/doc/cppinternals/Macro-Expansion.html @@ -0,0 +1,241 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>The GNU C Preprocessor Internals: Macro Expansion</title> + +<meta name="description" content="The GNU C Preprocessor Internals: Macro Expansion"> +<meta name="keywords" content="The GNU C Preprocessor Internals: Macro Expansion"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="index.html#Top" rel="up" title="Top"> +<link href="Token-Spacing.html#Token-Spacing" rel="next" title="Token Spacing"> +<link href="Hash-Nodes.html#Hash-Nodes" rel="previous" title="Hash Nodes"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Macro-Expansion"></a> +<div class="header"> +<p> +Next: <a href="Token-Spacing.html#Token-Spacing" accesskey="n" rel="next">Token Spacing</a>, Previous: <a href="Hash-Nodes.html#Hash-Nodes" accesskey="p" rel="previous">Hash Nodes</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Macro-Expansion-Algorithm"></a> +<h2 class="unnumbered">Macro Expansion Algorithm</h2> +<a name="index-macro-expansion"></a> + +<p>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. +</p> +<p>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, stringizing and +token pasting are supposed to work, damage to your sanity can quickly +result. +</p> +<a name="Internal-representation-of-macros"></a> +<h3 class="section">Internal representation of macros</h3> +<a name="index-macro-representation-_0028internal_0029"></a> + +<p>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. +</p> +<p>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</code>. 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., +<samp>-dD</samp>, and to warn about non-trivial macro redefinitions when +the parameter names have changed. +</p> +<a name="Macro-expansion-overview"></a> +<h3 class="section">Macro expansion overview</h3> +<p>The preprocessor maintains a <em>context stack</em>, implemented as a +linked list of <code>cpp_context</code> structures, which together represent +the macro expansion state at any one time. The <code>struct +cpp_reader</code> member variable <code>context</code> 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. +</p> +<p>When there are no macros under expansion, cpplib is in <em>base +context</em>. 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. +</p> +<p>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</code>. 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</code> also replaces any parameters in the +replacement list, stored as <code>CPP_MACRO_ARG</code> 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. +</p> +<p><code>enter_macro_context</code> also handles special macros like +<code>__LINE__</code>. Although these macros expand to a single token which +cannot contain any further macros, for reasons of token spacing +(see <a href="Token-Spacing.html#Token-Spacing">Token Spacing</a>) and simplicity of implementation, cpplib +handles these special macros by pushing a context containing just that +one token. +</p> +<p>The final thing that <code>enter_macro_context</code> does before returning +is to mark the macro disabled for expansion (except for special macros +like <code>__TIME__</code>). 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 <em>not</em> disabled whilst any arguments +to it are being expanded. +</p> +<a name="Scanning-the-replacement-list-for-macros-to-expand"></a> +<h3 class="section">Scanning the replacement list for macros to expand</h3> +<p>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. +</p> +<p>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). +</p> +<p>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 <em>next</em> 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 +</p> +<div class="smallexample"> +<pre class="smallexample">#define foo(x) bar x +foo(foo) (2) +</pre></div> + +<p>which fully expands to ‘<samp>bar foo (2)</samp>’. During pre-expansion +of the argument, ‘<samp>foo</samp>’ 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</samp>’ eligible for future expansion. Then, when +re-scanning after argument replacement, the token ‘<samp>foo</samp>’ 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</samp>’ of the macro is still on the context +stack. +</p> +<p>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</samp>’ would be +popped in the process of finding the parenthesis, re-enabling +‘<samp>foo</samp>’ and expanding it a second time. +</p> +<a name="Looking-for-a-function_002dlike-macro_0027s-opening-parenthesis"></a> +<h3 class="section">Looking for a function-like macro’s opening parenthesis</h3> +<p>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 +(see <a href="Token-Spacing.html#Token-Spacing">Token Spacing</a>), 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. +</p> +<p>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</code>, 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. +</p> +<a name="Marking-tokens-ineligible-for-future-expansion"></a> +<h3 class="section">Marking tokens ineligible for future expansion</h3> +<p>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</code> to the copy. +</p> +<p>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 (see <a href="Lexer.html#Lexing-a-line">Lexing a line</a>) using the +function <code>_cpp_temp_token</code>. The tokens are then re-used once the +current line of tokens has been read in. +</p> +<p>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. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Token-Spacing.html#Token-Spacing" accesskey="n" rel="next">Token Spacing</a>, Previous: <a href="Hash-Nodes.html#Hash-Nodes" accesskey="p" rel="previous">Hash Nodes</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |