<html lang="en">
<head>
<title>Differences from previous versions - The C Preprocessor</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="The C Preprocessor">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Implementation-Details.html#Implementation-Details" title="Implementation Details">
<link rel="prev" href="Obsolete-Features.html#Obsolete-Features" title="Obsolete Features">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
Copyright (C) 1987, 1989, 1991, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
2008, 2009, 2010
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 1.2 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.
-->
<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>
<link rel="stylesheet" type="text/css" href="../cs.css">
</head>
<body>
<div class="node">
<a name="Differences-from-previous-versions"></a>
<p>
Previous:&nbsp;<a rel="previous" accesskey="p" href="Obsolete-Features.html#Obsolete-Features">Obsolete Features</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Implementation-Details.html#Implementation-Details">Implementation Details</a>
<hr>
</div>

<h3 class="section">11.4 Differences from previous versions</h3>

<p><a name="index-differences-from-previous-versions-117"></a>
This section details behavior which has changed from previous versions
of CPP.  We do not plan to change it again in the near future, but
we do not promise not to, either.

   <p>The &ldquo;previous versions&rdquo; discussed here are 2.95 and before.  The
behavior of GCC 3.0 is mostly the same as the behavior of the widely
used 2.96 and 2.97 development snapshots.  Where there are differences,
they generally represent bugs in the snapshots.

     <ul>
<li>-I- deprecated

     <p>This option has been deprecated in 4.0.  <samp><span class="option">-iquote</span></samp> is meant to
replace the need for this option.

     <li>Order of evaluation of &lsquo;<samp><span class="samp">#</span></samp>&rsquo; and &lsquo;<samp><span class="samp">##</span></samp>&rsquo; operators

     <p>The standard does not specify the order of evaluation of a chain of
&lsquo;<samp><span class="samp">##</span></samp>&rsquo; operators, nor whether &lsquo;<samp><span class="samp">#</span></samp>&rsquo; is evaluated before, after, or
at the same time as &lsquo;<samp><span class="samp">##</span></samp>&rsquo;.  You should therefore not write any code
which depends on any specific ordering.  It is possible to guarantee an
ordering, if you need one, by suitable use of nested macros.

     <p>An example of where this might matter is pasting the arguments &lsquo;<samp><span class="samp">1</span></samp>&rsquo;,
&lsquo;<samp><span class="samp">e</span></samp>&rsquo; and &lsquo;<samp><span class="samp">-2</span></samp>&rsquo;.  This would be fine for left-to-right pasting,
but right-to-left pasting would produce an invalid token &lsquo;<samp><span class="samp">e-2</span></samp>&rsquo;.

     <p>GCC 3.0 evaluates &lsquo;<samp><span class="samp">#</span></samp>&rsquo; and &lsquo;<samp><span class="samp">##</span></samp>&rsquo; at the same time and strictly
left to right.  Older versions evaluated all &lsquo;<samp><span class="samp">#</span></samp>&rsquo; operators first,
then all &lsquo;<samp><span class="samp">##</span></samp>&rsquo; operators, in an unreliable order.

     <li>The form of whitespace between tokens in preprocessor output

     <p>See <a href="Preprocessor-Output.html#Preprocessor-Output">Preprocessor Output</a>, for the current textual format.  This is
also the format used by stringification.  Normally, the preprocessor
communicates tokens directly to the compiler's parser, and whitespace
does not come up at all.

     <p>Older versions of GCC preserved all whitespace provided by the user and
inserted lots more whitespace of their own, because they could not
accurately predict when extra spaces were needed to prevent accidental
token pasting.

     <li>Optional argument when invoking rest argument macros

     <p>As an extension, GCC permits you to omit the variable arguments entirely
when you use a variable argument macro.  This is forbidden by the 1999 C
standard, and will provoke a pedantic warning with GCC 3.0.  Previous
versions accepted it silently.

     <li>&lsquo;<samp><span class="samp">##</span></samp>&rsquo; swallowing preceding text in rest argument macros

     <p>Formerly, in a macro expansion, if &lsquo;<samp><span class="samp">##</span></samp>&rsquo; appeared before a variable
arguments parameter, and the set of tokens specified for that argument
in the macro invocation was empty, previous versions of CPP would
back up and remove the preceding sequence of non-whitespace characters
(<strong>not</strong> the preceding token).  This extension is in direct
conflict with the 1999 C standard and has been drastically pared back.

     <p>In the current version of the preprocessor, if &lsquo;<samp><span class="samp">##</span></samp>&rsquo; appears between
a comma and a variable arguments parameter, and the variable argument is
omitted entirely, the comma will be removed from the expansion.  If the
variable argument is empty, or the token before &lsquo;<samp><span class="samp">##</span></samp>&rsquo; is not a
comma, then &lsquo;<samp><span class="samp">##</span></samp>&rsquo; behaves as a normal token paste.

     <li>&lsquo;<samp><span class="samp">#line</span></samp>&rsquo; and &lsquo;<samp><span class="samp">#include</span></samp>&rsquo;

     <p>The &lsquo;<samp><span class="samp">#line</span></samp>&rsquo; directive used to change GCC's notion of the
&ldquo;directory containing the current file&rdquo;, used by &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; with
a double-quoted header file name.  In 3.0 and later, it does not. 
See <a href="Line-Control.html#Line-Control">Line Control</a>, for further explanation.

     <li>Syntax of &lsquo;<samp><span class="samp">#line</span></samp>&rsquo;

     <p>In GCC 2.95 and previous, the string constant argument to &lsquo;<samp><span class="samp">#line</span></samp>&rsquo;
was treated the same way as the argument to &lsquo;<samp><span class="samp">#include</span></samp>&rsquo;: backslash
escapes were not honored, and the string ended at the second &lsquo;<samp><span class="samp">"</span></samp>&rsquo;. 
This is not compliant with the C standard.  In GCC 3.0, an attempt was
made to correct the behavior, so that the string was treated as a real
string constant, but it turned out to be buggy.  In 3.1, the bugs have
been fixed.  (We are not fixing the bugs in 3.0 because they affect
relatively few people and the fix is quite invasive.)

   </ul>

   </body></html>