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   4   Support
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   6 </th><td width="20%" align="right"> <a accesskey="n" href="diagnostics.html">Next</a></td></tr></table><hr /></div><div class="sect1" title="Termination"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.support.termination"></a>Termination</h2></div></div></div><div class="sect2" title="Termination Handlers"><div class="titlepage"><div><div><h3 class="title"><a id="support.termination.handlers"></a>Termination Handlers</h3></div></div></div><p>
   7       Not many changes here to <code class="filename">cstdlib</code>.  You should note that the
   8       <code class="function">abort()</code> function does not call the
   9       destructors of automatic nor static objects, so if you're
  10       depending on those to do cleanup, it isn't going to happen.
  11       (The functions registered with <code class="function">atexit()</code>
  12       don't get called either, so you can forget about that
  13       possibility, too.)
  14     </p><p>
  15       The good old <code class="function">exit()</code> function can be a bit
  16       funky, too, until you look closer.  Basically, three points to
  17       remember are:
  18     </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
  19         Static objects are destroyed in reverse order of their creation.
  20         </p></li><li class="listitem"><p>
  21         Functions registered with <code class="function">atexit()</code> are called in
  22         reverse order of registration, once per registration call.
  23         (This isn't actually new.)
  24         </p></li><li class="listitem"><p>
  25         The previous two actions are <span class="quote">“<span class="quote">interleaved,</span>”</span> that is,
  26         given this pseudocode:
  27         </p><pre class="programlisting">
  28   extern "C or C++" void  f1 (void);
  29   extern "C or C++" void  f2 (void);
  30
  31   static Thing obj1;
  32   atexit(f1);
  33   static Thing obj2;
  34   atexit(f2);
  35 </pre><p>
  36         then at a call of <code class="function">exit()</code>,
  37         <code class="varname">f2</code> will be called, then
  38         <code class="varname">obj2</code> will be destroyed, then
  39         <code class="varname">f1</code> will be called, and finally
  40         <code class="varname">obj1</code> will be destroyed. If
  41         <code class="varname">f1</code> or <code class="varname">f2</code> allow an
  42         exception to propagate out of them, Bad Things happen.
  43         </p></li></ol></div><p>
  44       Note also that <code class="function">atexit()</code> is only required to store 32
  45       functions, and the compiler/library might already be using some of
  46       those slots.  If you think you may run out, we recommend using
  47       the <code class="function">xatexit</code>/<code class="function">xexit</code> combination from <code class="literal">libiberty</code>, which has no such limit.
  48     </p></div><div class="sect2" title="Verbose Terminate Handler"><div class="titlepage"><div><div><h3 class="title"><a id="support.termination.verbose"></a>Verbose Terminate Handler</h3></div></div></div><p>
  49       If you are having difficulty with uncaught exceptions and want a
  50       little bit of help debugging the causes of the core dumps, you can
  51       make use of a GNU extension, the verbose terminate handler.
  52     </p><pre class="programlisting">
  53 #include &lt;exception&gt;
  54
  55 int main()
  56 {
  57   std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
  58   ...
  59
  60   throw <em class="replaceable"><code>anything</code></em>;
  61 }
  62 </pre><p>
  63      The <code class="function">__verbose_terminate_handler</code> function
  64      obtains the name of the current exception, attempts to demangle
  65      it, and prints it to stderr.  If the exception is derived from
  66      <code class="classname">exception</code> then the output from
  67      <code class="function">what()</code> will be included.
  68    </p><p>
  69      Any replacement termination function is required to kill the
  70      program without returning; this one calls abort.
  71    </p><p>
  72      For example:
  73    </p><pre class="programlisting">
  74 #include &lt;exception&gt;
  75 #include &lt;stdexcept&gt;
  76
  77 struct argument_error : public std::runtime_error
  78 {
  79   argument_error(const std::string&amp; s): std::runtime_error(s) { }
  80 };
  81
  82 int main(int argc)
  83 {
  84   std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
  85   if (argc &gt; 5)
  86     throw argument_error(<span class="quote">“<span class="quote">argc is greater than 5!</span>”</span>);
  87   else
  88     throw argc;
  89 }
  90 </pre><p>
  91      With the verbose terminate handler active, this gives:
  92    </p><pre class="screen">
  93    <code class="computeroutput">
  94    % ./a.out
  95    terminate called after throwing a `int'
  96    Aborted
  97    % ./a.out f f f f f f f f f f f
  98    terminate called after throwing an instance of `argument_error'
  99    what(): argc is greater than 5!
 100    Aborted
 101    </code>
 102    </pre><p>
 103      The 'Aborted' line comes from the call to
 104      <code class="function">abort()</code>, of course.
 105    </p><p>
 106      This is the default termination handler; nothing need be done to
 107      use it.  To go back to the previous <span class="quote">“<span class="quote">silent death</span>”</span>
 108      method, simply include <code class="filename">exception</code> and
 109      <code class="filename">cstdlib</code>, and call
 110    </p><pre class="programlisting">
 111      std::set_terminate(std::abort);
 112    </pre><p>
 113      After this, all calls to <code class="function">terminate</code> will use
 114      <code class="function">abort</code> as the terminate handler.
 115    </p><p>
 116      Note: the verbose terminate handler will attempt to write to
 117      stderr.  If your application closes stderr or redirects it to an
 118      inappropriate location,
 119      <code class="function">__verbose_terminate_handler</code> will behave in
 120      an unspecified manner.
 121    </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="dynamic_memory.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="support.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="diagnostics.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Dynamic Memory </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 5.
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