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<a name="Other-Incompatibilities"></a>
<div class="header">
<p>
Previous: <a href="Incompatibilities.html#Incompatibilities" accesskey="p" rel="prev">Incompatibilities</a>, Up: <a href="Compatibility.html#Compatibility" accesskey="u" rel="up">Compatibility</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Other-incompatibilities"></a>
<h3 class="section">16.3 Other incompatibilities</h3>

<p>There are a few other incompatibilities between this implementation of
<code>m4</code>, and the System V version.
</p>
<ul>
<li> GNU <code>m4</code> implements sync lines differently from System V
<code>m4</code>, when text is being diverted.  GNU <code>m4</code> outputs
the sync lines when the text is being diverted, and System V <code>m4</code>
when the diverted text is being brought back.

<p>The problem is which lines and file names should be attached to text
that is being, or has been, diverted.  System V <code>m4</code> regards all
the diverted text as being generated by the source line containing the
<code>undivert</code> call, whereas GNU <code>m4</code> regards the
diverted text as being generated at the time it is diverted.
</p>
<p>The sync line option is used mostly when using <code>m4</code> as
a front end to a compiler.  If a diverted line causes a compiler error,
the error messages should most probably refer to the place where the
diversion was made, and not where it was inserted again.
</p>
<div class="example">
<pre class="example">divert(2)2
divert(1)1
divert`'0
&rArr;#line 3 &quot;stdin&quot;
&rArr;0
^D
&rArr;#line 2 &quot;stdin&quot;
&rArr;1
&rArr;#line 1 &quot;stdin&quot;
&rArr;2
</pre></div>

<p>The current <code>m4</code> implementation has a limitation that the syncline
output at the start of each diversion occurs no matter what, even if the
previous diversion did not end with a newline.  This goes contrary to
the claim that synclines appear on a line by themselves, so this
limitation may be corrected in a future version of <code>m4</code>.  In the
meantime, when using <samp>-s</samp>, it is wisest to make sure all
diversions end with newline.
</p>
</li><li> GNU <code>m4</code> makes no attempt at prohibiting self-referential
definitions like:

<div class="example">
<pre class="example">define(`x', `x')
&rArr;
define(`x', `x ')
&rArr;
</pre></div>

<a name="index-rescanning-4"></a>
<p>There is nothing inherently wrong with defining &lsquo;<samp>x</samp>&rsquo; to
return &lsquo;<samp>x</samp>&rsquo;.  The wrong thing is to expand &lsquo;<samp>x</samp>&rsquo; unquoted,
because that would cause an infinite rescan loop.
In <code>m4</code>, one might use macros to hold strings, as we do for
variables in other programming languages, further checking them with:
</p>
<div class="example">
<pre class="example">ifelse(defn(`<var>holder</var>'), `<var>value</var>', &hellip;)
</pre></div>

<p>In cases like this one, an interdiction for a macro to hold its own name
would be a useless limitation.  Of course, this leaves more rope for the
GNU <code>m4</code> user to hang himself!  Rescanning hangs may be
avoided through careful programming, a little like for endless loops in
traditional programming languages.
</p></li></ul>

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<p>
Previous: <a href="Incompatibilities.html#Incompatibilities" accesskey="p" rel="prev">Incompatibilities</a>, Up: <a href="Compatibility.html#Compatibility" accesskey="u" rel="up">Compatibility</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p>
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