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<h5 class="subsubsection">3.2.3.1 How to install the tracing functionality</h5>

<!-- mcheck.h -->
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<div class="defun">
&mdash; Function: void <b>mtrace</b> (<var>void</var>)<var><a name="index-mtrace-298"></a></var><br>
<blockquote><p>When the <code>mtrace</code> function is called it looks for an environment
variable named <code>MALLOC_TRACE</code>.  This variable is supposed to
contain a valid file name.  The user must have write access.  If the
file already exists it is truncated.  If the environment variable is not
set or it does not name a valid file which can be opened for writing
nothing is done.  The behavior of <code>malloc</code> etc. is not changed. 
For obvious reasons this also happens if the application is installed
with the SUID or SGID bit set.

        <p>If the named file is successfully opened, <code>mtrace</code> installs special
handlers for the functions <code>malloc</code>, <code>realloc</code>, and
<code>free</code> (see <a href="Hooks-for-Malloc.html#Hooks-for-Malloc">Hooks for Malloc</a>).  From then on, all uses of these
functions are traced and protocolled into the file.  There is now of
course a speed penalty for all calls to the traced functions so tracing
should not be enabled during normal use.

        <p>This function is a GNU extension and generally not available on other
systems.  The prototype can be found in <samp><span class="file">mcheck.h</span></samp>. 
</p></blockquote></div>

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&mdash; Function: void <b>muntrace</b> (<var>void</var>)<var><a name="index-muntrace-299"></a></var><br>
<blockquote><p>The <code>muntrace</code> function can be called after <code>mtrace</code> was used
to enable tracing the <code>malloc</code> calls.  If no (successful) call of
<code>mtrace</code> was made <code>muntrace</code> does nothing.

        <p>Otherwise it deinstalls the handlers for <code>malloc</code>, <code>realloc</code>,
and <code>free</code> and then closes the protocol file.  No calls are
protocolled anymore and the program runs again at full speed.

        <p>This function is a GNU extension and generally not available on other
systems.  The prototype can be found in <samp><span class="file">mcheck.h</span></samp>. 
</p></blockquote></div>

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