177 lines
8.7 KiB
HTML
177 lines
8.7 KiB
HTML
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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section entitled "GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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<head>
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<title>The C Preprocessor: Differences from previous versions</title>
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<meta name="description" content="The C Preprocessor: Differences from previous versions">
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<meta name="keywords" content="The C Preprocessor: Differences from previous versions">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Implementation-Details.html#Implementation-Details" rel="up" title="Implementation Details">
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<link href="Invocation.html#Invocation" rel="next" title="Invocation">
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<link href="Obsolete-Features.html#Obsolete-Features" rel="prev" title="Obsolete Features">
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Differences-from-previous-versions"></a>
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<div class="header">
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<p>
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Previous: <a href="Obsolete-Features.html#Obsolete-Features" accesskey="p" rel="prev">Obsolete Features</a>, Up: <a href="Implementation-Details.html#Implementation-Details" accesskey="u" rel="up">Implementation Details</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>
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</div>
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<hr>
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<a name="Differences-from-previous-versions-1"></a>
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<h3 class="section">11.4 Differences from previous versions</h3>
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<a name="index-differences-from-previous-versions"></a>
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<p>This section details behavior which has changed from previous versions
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of CPP. We do not plan to change it again in the near future, but
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we do not promise not to, either.
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</p>
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<p>The “previous versions” discussed here are 2.95 and before. The
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behavior of GCC 3.0 is mostly the same as the behavior of the widely
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used 2.96 and 2.97 development snapshots. Where there are differences,
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they generally represent bugs in the snapshots.
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</p>
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<ul>
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<li> -I- deprecated
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<p>This option has been deprecated in 4.0. <samp>-iquote</samp> is meant to
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replace the need for this option.
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</p>
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</li><li> Order of evaluation of ‘<samp>#</samp>’ and ‘<samp>##</samp>’ operators
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<p>The standard does not specify the order of evaluation of a chain of
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‘<samp>##</samp>’ operators, nor whether ‘<samp>#</samp>’ is evaluated before, after, or
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at the same time as ‘<samp>##</samp>’. You should therefore not write any code
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which depends on any specific ordering. It is possible to guarantee an
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ordering, if you need one, by suitable use of nested macros.
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</p>
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<p>An example of where this might matter is pasting the arguments ‘<samp>1</samp>’,
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‘<samp>e</samp>’ and ‘<samp>-2</samp>’. This would be fine for left-to-right pasting,
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but right-to-left pasting would produce an invalid token ‘<samp>e-2</samp>’.
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</p>
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<p>GCC 3.0 evaluates ‘<samp>#</samp>’ and ‘<samp>##</samp>’ at the same time and strictly
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left to right. Older versions evaluated all ‘<samp>#</samp>’ operators first,
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then all ‘<samp>##</samp>’ operators, in an unreliable order.
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</p>
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</li><li> The form of whitespace between tokens in preprocessor output
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<p>See <a href="Preprocessor-Output.html#Preprocessor-Output">Preprocessor Output</a>, for the current textual format. This is
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also the format used by stringification. Normally, the preprocessor
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communicates tokens directly to the compiler’s parser, and whitespace
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does not come up at all.
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</p>
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<p>Older versions of GCC preserved all whitespace provided by the user and
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inserted lots more whitespace of their own, because they could not
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accurately predict when extra spaces were needed to prevent accidental
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token pasting.
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</p>
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</li><li> Optional argument when invoking rest argument macros
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<p>As an extension, GCC permits you to omit the variable arguments entirely
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when you use a variable argument macro. This is forbidden by the 1999 C
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standard, and will provoke a pedantic warning with GCC 3.0. Previous
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versions accepted it silently.
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</p>
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</li><li> ‘<samp>##</samp>’ swallowing preceding text in rest argument macros
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<p>Formerly, in a macro expansion, if ‘<samp>##</samp>’ appeared before a variable
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arguments parameter, and the set of tokens specified for that argument
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in the macro invocation was empty, previous versions of CPP would
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back up and remove the preceding sequence of non-whitespace characters
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(<strong>not</strong> the preceding token). This extension is in direct
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conflict with the 1999 C standard and has been drastically pared back.
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</p>
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<p>In the current version of the preprocessor, if ‘<samp>##</samp>’ appears between
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a comma and a variable arguments parameter, and the variable argument is
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omitted entirely, the comma will be removed from the expansion. If the
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variable argument is empty, or the token before ‘<samp>##</samp>’ is not a
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comma, then ‘<samp>##</samp>’ behaves as a normal token paste.
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</p>
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</li><li> ‘<samp>#line</samp>’ and ‘<samp>#include</samp>’
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<p>The ‘<samp>#line</samp>’ directive used to change GCC’s notion of the
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“directory containing the current file”, used by ‘<samp>#include</samp>’ with
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a double-quoted header file name. In 3.0 and later, it does not.
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See <a href="Line-Control.html#Line-Control">Line Control</a>, for further explanation.
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</p>
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</li><li> Syntax of ‘<samp>#line</samp>’
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<p>In GCC 2.95 and previous, the string constant argument to ‘<samp>#line</samp>’
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was treated the same way as the argument to ‘<samp>#include</samp>’: backslash
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escapes were not honored, and the string ended at the second ‘<samp>"</samp>’.
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This is not compliant with the C standard. In GCC 3.0, an attempt was
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made to correct the behavior, so that the string was treated as a real
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string constant, but it turned out to be buggy. In 3.1, the bugs have
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been fixed. (We are not fixing the bugs in 3.0 because they affect
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relatively few people and the fix is quite invasive.)
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</p>
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</li></ul>
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<hr>
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<div class="header">
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<p>
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Previous: <a href="Obsolete-Features.html#Obsolete-Features" accesskey="p" rel="prev">Obsolete Features</a>, Up: <a href="Implementation-Details.html#Implementation-Details" accesskey="u" rel="up">Implementation Details</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>
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</div>
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</body>
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</html>
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