You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
94 lines
4.4 KiB
HTML
94 lines
4.4 KiB
HTML
4 years ago
|
<html lang="en">
|
||
|
|
<head>
|
||
|
|
<title>Hash Nodes - The GNU C Preprocessor Internals</title>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html">
|
||
|
|
<meta name="description" content="The GNU C Preprocessor Internals">
|
||
|
|
<meta name="generator" content="makeinfo 4.13">
|
||
|
|
<link title="Top" rel="start" href="index.html#Top">
|
||
|
|
<link rel="prev" href="Lexer.html#Lexer" title="Lexer">
|
||
|
|
<link rel="next" href="Macro-Expansion.html#Macro-Expansion" title="Macro Expansion">
|
||
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
||
|
|
<meta http-equiv="Content-Style-Type" content="text/css">
|
||
|
|
<style type="text/css"><!--
|
||
|
|
pre.display { font-family:inherit }
|
||
|
|
pre.format { font-family:inherit }
|
||
|
|
pre.smalldisplay { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallformat { font-family:inherit; font-size:smaller }
|
||
|
|
pre.smallexample { font-size:smaller }
|
||
|
|
pre.smalllisp { font-size:smaller }
|
||
|
|
span.sc { font-variant:small-caps }
|
||
|
|
span.roman { font-family:serif; font-weight:normal; }
|
||
|
|
span.sansserif { font-family:sans-serif; font-weight:normal; }
|
||
|
|
--></style>
|
||
|
|
</head>
|
||
|
|
<body>
|
||
|
|
<div class="node">
|
||
|
|
<a name="Hash-Nodes"></a>
|
||
|
|
<p>
|
||
|
|
Next: <a rel="next" accesskey="n" href="Macro-Expansion.html#Macro-Expansion">Macro Expansion</a>,
|
||
|
|
Previous: <a rel="previous" accesskey="p" href="Lexer.html#Lexer">Lexer</a>,
|
||
|
|
Up: <a rel="up" accesskey="u" href="index.html#Top">Top</a>
|
||
|
|
<hr>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
<h2 class="unnumbered">Hash Nodes</h2>
|
||
|
|
|
||
|
|
<p><a name="index-hash-table-7"></a><a name="index-identifiers-8"></a><a name="index-macros-9"></a><a name="index-assertions-10"></a><a name="index-named-operators-11"></a>
|
||
|
|
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</code>; 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>, <code>int</code>, <code>foo</code> and
|
||
|
|
<code>__GNUC__</code> are identifiers and hashed when lexed.
|
||
|
|
|
||
|
|
<p>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:
|
||
|
|
|
||
|
|
<ul>
|
||
|
|
<li>Macros
|
||
|
|
|
||
|
|
<p>These have been declared to be macros, either on the command line or
|
||
|
|
with <code>#define</code>. A few, such as <code>__TIME__</code> 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.
|
||
|
|
|
||
|
|
<li>Assertions
|
||
|
|
|
||
|
|
<p>Assertions are in a separate namespace to macros. To enforce this, cpp
|
||
|
|
actually prepends a <code>#</code> character before hashing and entering it in
|
||
|
|
the hash table. An assertion's node points to a chain of answers to
|
||
|
|
that assertion.
|
||
|
|
|
||
|
|
<li>Void
|
||
|
|
|
||
|
|
<p>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</code>.
|
||
|
|
|
||
|
|
<p>When preprocessing C++, this category also includes the named operators,
|
||
|
|
such as <code>xor</code>. 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>#</code> and
|
||
|
|
<code>##</code>. Named operator hash nodes are flagged, both to catch the
|
||
|
|
spelling distinction and to prevent them from being defined as macros.
|
||
|
|
</ul>
|
||
|
|
|
||
|
|
<p>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</code> 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</code>, has an associated directive
|
||
|
|
enum stored in its hash node, so that directive lookup is also O(1).
|
||
|
|
|
||
|
|
</body></html>
|
||
|
|
|