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.
180 lines
8.2 KiB
HTML
180 lines
8.2 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|
<html>
|
|
<!-- Copyright (C) 1987-2018 Free Software Foundation, Inc.
|
|
|
|
Permission is granted to copy, distribute and/or modify this document
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
|
any later version published by the Free Software Foundation. A copy of
|
|
the license is included in the
|
|
section entitled "GNU Free Documentation License".
|
|
|
|
This manual contains no Invariant Sections. The Front-Cover Texts are
|
|
(a) (see below), and the Back-Cover Texts are (b) (see below).
|
|
|
|
(a) The FSF's Front-Cover Text is:
|
|
|
|
A GNU Manual
|
|
|
|
(b) The FSF's Back-Cover Text is:
|
|
|
|
You have freedom to copy and modify this GNU Manual, like GNU
|
|
software. Copies published by the Free Software Foundation raise
|
|
funds for GNU development. -->
|
|
<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
|
|
<head>
|
|
<title>Traditional macros (The C Preprocessor)</title>
|
|
|
|
<meta name="description" content="Traditional macros (The C Preprocessor)">
|
|
<meta name="keywords" content="Traditional macros (The C Preprocessor)">
|
|
<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="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|
<link href="Traditional-Mode.html#Traditional-Mode" rel="up" title="Traditional Mode">
|
|
<link href="Traditional-miscellany.html#Traditional-miscellany" rel="next" title="Traditional miscellany">
|
|
<link href="Traditional-lexical-analysis.html#Traditional-lexical-analysis" rel="prev" title="Traditional lexical analysis">
|
|
<style type="text/css">
|
|
<!--
|
|
a.summary-letter {text-decoration: none}
|
|
blockquote.indentedblock {margin-right: 0em}
|
|
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
|
|
blockquote.smallquotation {font-size: smaller}
|
|
div.display {margin-left: 3.2em}
|
|
div.example {margin-left: 3.2em}
|
|
div.lisp {margin-left: 3.2em}
|
|
div.smalldisplay {margin-left: 3.2em}
|
|
div.smallexample {margin-left: 3.2em}
|
|
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.nolinebreak {white-space: nowrap}
|
|
span.roman {font-family: initial; font-weight: normal}
|
|
span.sansserif {font-family: sans-serif; font-weight: normal}
|
|
ul.no-bullet {list-style: none}
|
|
-->
|
|
</style>
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<a name="Traditional-macros"></a>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Traditional-miscellany.html#Traditional-miscellany" accesskey="n" rel="next">Traditional miscellany</a>, Previous: <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis" accesskey="p" rel="prev">Traditional lexical analysis</a>, Up: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="u" rel="up">Traditional Mode</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
|
|
</div>
|
|
<hr>
|
|
<a name="Traditional-macros-1"></a>
|
|
<h3 class="section">10.2 Traditional macros</h3>
|
|
|
|
<p>The major difference between traditional and ISO macros is that the
|
|
former expand to text rather than to a token sequence. CPP removes
|
|
all leading and trailing horizontal whitespace from a macro’s
|
|
replacement text before storing it, but preserves the form of internal
|
|
whitespace.
|
|
</p>
|
|
<p>One consequence is that it is legitimate for the replacement text to
|
|
contain an unmatched quote (see <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis">Traditional lexical analysis</a>). An
|
|
unclosed string or character constant continues into the text
|
|
following the macro call. Similarly, the text at the end of a macro’s
|
|
expansion can run together with the text after the macro invocation to
|
|
produce a single token.
|
|
</p>
|
|
<p>Normally comments are removed from the replacement text after the
|
|
macro is expanded, but if the <samp>-CC</samp> option is passed on the
|
|
command-line comments are preserved. (In fact, the current
|
|
implementation removes comments even before saving the macro
|
|
replacement text, but it careful to do it in such a way that the
|
|
observed effect is identical even in the function-like macro case.)
|
|
</p>
|
|
<p>The ISO stringizing operator ‘<samp>#</samp>’ and token paste operator
|
|
‘<samp>##</samp>’ have no special meaning. As explained later, an effect
|
|
similar to these operators can be obtained in a different way. Macro
|
|
names that are embedded in quotes, either from the main file or after
|
|
macro replacement, do not expand.
|
|
</p>
|
|
<p>CPP replaces an unquoted object-like macro name with its replacement
|
|
text, and then rescans it for further macros to replace. Unlike
|
|
standard macro expansion, traditional macro expansion has no provision
|
|
to prevent recursion. If an object-like macro appears unquoted in its
|
|
replacement text, it will be replaced again during the rescan pass,
|
|
and so on <em>ad infinitum</em>. GCC detects when it is expanding
|
|
recursive macros, emits an error message, and continues after the
|
|
offending macro invocation.
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define PLUS +
|
|
#define INC(x) PLUS+x
|
|
INC(foo);
|
|
→ ++foo;
|
|
</pre></div>
|
|
|
|
<p>Function-like macros are similar in form but quite different in
|
|
behavior to their ISO counterparts. Their arguments are contained
|
|
within parentheses, are comma-separated, and can cross physical lines.
|
|
Commas within nested parentheses are not treated as argument
|
|
separators. Similarly, a quote in an argument cannot be left
|
|
unclosed; a following comma or parenthesis that comes before the
|
|
closing quote is treated like any other character. There is no
|
|
facility for handling variadic macros.
|
|
</p>
|
|
<p>This implementation removes all comments from macro arguments, unless
|
|
the <samp>-C</samp> option is given. The form of all other horizontal
|
|
whitespace in arguments is preserved, including leading and trailing
|
|
whitespace. In particular
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">f( )
|
|
</pre></div>
|
|
|
|
<p>is treated as an invocation of the macro ‘<samp>f</samp>’ with a single
|
|
argument consisting of a single space. If you want to invoke a
|
|
function-like macro that takes no arguments, you must not leave any
|
|
whitespace between the parentheses.
|
|
</p>
|
|
<p>If a macro argument crosses a new line, the new line is replaced with
|
|
a space when forming the argument. If the previous line contained an
|
|
unterminated quote, the following line inherits the quoted state.
|
|
</p>
|
|
<p>Traditional preprocessors replace parameters in the replacement text
|
|
with their arguments regardless of whether the parameters are within
|
|
quotes or not. This provides a way to stringize arguments. For
|
|
example
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define str(x) "x"
|
|
str(/* <span class="roman">A comment</span> */some text )
|
|
→ "some text "
|
|
</pre></div>
|
|
|
|
<p>Note that the comment is removed, but that the trailing space is
|
|
preserved. Here is an example of using a comment to effect token
|
|
pasting.
|
|
</p>
|
|
<div class="smallexample">
|
|
<pre class="smallexample">#define suffix(x) foo_/**/x
|
|
suffix(bar)
|
|
→ foo_bar
|
|
</pre></div>
|
|
|
|
<hr>
|
|
<div class="header">
|
|
<p>
|
|
Next: <a href="Traditional-miscellany.html#Traditional-miscellany" accesskey="n" rel="next">Traditional miscellany</a>, Previous: <a href="Traditional-lexical-analysis.html#Traditional-lexical-analysis" accesskey="p" rel="prev">Traditional lexical analysis</a>, Up: <a href="Traditional-Mode.html#Traditional-Mode" accesskey="u" rel="up">Traditional Mode</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
|
|
</div>
|
|
|
|
|
|
|
|
</body>
|
|
</html>
|