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<a name="Trace"></a>
<div class="header">
<p>
Next: <a href="Debug-Levels.html#Debug-Levels" accesskey="n" rel="next">Debug Levels</a>, Previous: <a href="Dumpdef.html#Dumpdef" accesskey="p" rel="prev">Dumpdef</a>, Up: <a href="Debugging.html#Debugging" accesskey="u" rel="up">Debugging</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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<hr>
<a name="Tracing-macro-calls"></a>
<h3 class="section">7.2 Tracing macro calls</h3>

<a name="index-tracing-macro-expansion"></a>
<a name="index-macro-expansion_002c-tracing"></a>
<a name="index-expansion_002c-tracing-macro"></a>
<a name="index-standard-error_002c-output-to-1"></a>
<p>It is possible to trace macro calls and expansions through the builtins
<code>traceon</code> and <code>traceoff</code>:
</p>
<dl>
<dt><a name="index-traceon"></a>Builtin: <strong>traceon</strong> <em>(<span class="roman">[</span><var>names&hellip;</var><span class="roman">]</span>)</em></dt>
<dt><a name="index-traceoff"></a>Builtin: <strong>traceoff</strong> <em>(<span class="roman">[</span><var>names&hellip;</var><span class="roman">]</span>)</em></dt>
<dd><p>When called without any arguments, <code>traceon</code> and <code>traceoff</code>
will turn tracing on and off, respectively, for all currently defined
macros.
</p>
<p>When called with arguments, only the macros listed in <var>names</var> are
affected, whether or not they are currently defined.
</p>
<p>The expansion of <code>traceon</code> and <code>traceoff</code> is void.
</p></dd></dl>

<p>Whenever a traced macro is called and the arguments have been collected,
the call is displayed.  If the expansion of the macro call is not void,
the expansion can be displayed after the call.  The output is printed
to the current debug file (defaulting to standard error, see <a href="Debug-Output.html#Debug-Output">Debug Output</a>).
</p>
<div class="example">
<pre class="example">$ <kbd>m4 -d</kbd>
define(`foo', `Hello World.')
&rArr;
define(`echo', `$@')
&rArr;
traceon(`foo', `echo')
&rArr;
foo
error&rarr;m4trace: -1- foo -&gt; `Hello World.'
&rArr;Hello World.
echo(`gnus', `and gnats')
error&rarr;m4trace: -1- echo(`gnus', `and gnats') -&gt; ``gnus',`and gnats''
&rArr;gnus,and gnats
</pre></div>

<p>The number between dashes is the depth of the expansion.  It is one most
of the time, signifying an expansion at the outermost level, but it
increases when macro arguments contain unquoted macro calls.  The
maximum number that will appear between dashes is controlled by the
option <samp>--nesting-limit</samp> (or <samp>-L</samp>, see <a href="Limits-control.html#Limits-control">Invoking m4</a>).  Additionally, the option <samp>--trace</samp> (or
<samp>-t</samp>) can be used to invoke <code>traceon(<var>name</var>)</code> before
parsing input.
</p>
<div class="example">
<pre class="example">$ <kbd>m4 -L 3 -t ifelse</kbd>
ifelse(`one level')
error&rarr;m4trace: -1- ifelse
&rArr;
ifelse(ifelse(ifelse(`three levels')))
error&rarr;m4trace: -3- ifelse
error&rarr;m4trace: -2- ifelse
error&rarr;m4trace: -1- ifelse
&rArr;
ifelse(ifelse(ifelse(ifelse(`four levels'))))
error&rarr;m4:stdin:3: recursion limit of 3 exceeded, use -L&lt;N&gt; to change it
</pre></div>

<p>Tracing by name is an attribute that is preserved whether the macro is
defined or not.  This allows the selection of macros to trace before
those macros are defined.
</p>
<div class="example">
<pre class="example">$ <kbd>m4 -d</kbd>
traceoff(`foo')
&rArr;
traceon(`foo')
&rArr;
foo
&rArr;foo
defn(`foo')
&rArr;
define(`foo', `bar')
&rArr;
foo
error&rarr;m4trace: -1- foo -&gt; `bar'
&rArr;bar
undefine(`foo')
&rArr;
ifdef(`foo', `yes', `no')
&rArr;no
indir(`foo')
error&rarr;m4:stdin:9: undefined macro `foo'
&rArr;
define(`foo', `blah')
&rArr;
foo
error&rarr;m4trace: -1- foo -&gt; `blah'
&rArr;blah
traceoff
&rArr;
foo
&rArr;blah
</pre></div>

<p>Tracing even works on builtins.  However, <code>defn</code> (see <a href="Defn.html#Defn">Defn</a>)
does not transfer tracing status.
</p>
<div class="example">
<pre class="example">$ <kbd>m4 -d</kbd>
traceon(`traceon')
&rArr;
traceon(`traceoff')
error&rarr;m4trace: -1- traceon(`traceoff')
&rArr;
traceoff(`traceoff')
error&rarr;m4trace: -1- traceoff(`traceoff')
&rArr;
traceoff(`traceon')
&rArr;
traceon(`eval', `m4_divnum')
&rArr;
define(`m4_eval', defn(`eval'))
&rArr;
define(`m4_divnum', defn(`divnum'))
&rArr;
eval(divnum)
error&rarr;m4trace: -1- eval(`0') -&gt; `0'
&rArr;0
m4_eval(m4_divnum)
error&rarr;m4trace: -2- m4_divnum -&gt; `0'
&rArr;0
</pre></div>

<p>See <a href="Debug-Levels.html#Debug-Levels">Debug Levels</a>, for information on controlling the details of the
display.  The format of the trace output is not specified by
POSIX, and varies between implementations of <code>m4</code>.
</p>

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Next: <a href="Debug-Levels.html#Debug-Levels" accesskey="n" rel="next">Debug Levels</a>, Previous: <a href="Dumpdef.html#Dumpdef" accesskey="p" rel="prev">Dumpdef</a>, Up: <a href="Debugging.html#Debugging" accesskey="u" rel="up">Debugging</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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