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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Termination</title><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="support.html" title="Chapter 4.  Support" /><link rel="prev" href="dynamic_memory.html" title="Dynamic Memory" /><link rel="next" href="diagnostics.html" title="Chapter 5.  Diagnostics" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Termination</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="dynamic_memory.html">Prev</a> </td><th width="60%" align="center">Chapter 4. 
  Support
  
</th><td width="20%" align="right"> <a accesskey="n" href="diagnostics.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="std.support.termination"></a>Termination</h2></div></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="support.termination.handlers"></a>Termination Handlers</h3></div></div></div><p>
      Not many changes here to <code class="filename">cstdlib</code>.  You should note that the
      <code class="function">abort()</code> function does not call the
      destructors of automatic nor static objects, so if you're
      depending on those to do cleanup, it isn't going to happen.
      (The functions registered with <code class="function">atexit()</code>
      don't get called either, so you can forget about that
      possibility, too.)
    </p><p>
      The good old <code class="function">exit()</code> function can be a bit
      funky, too, until you look closer.  Basically, three points to
      remember are:
    </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
	Static objects are destroyed in reverse order of their creation.
	</p></li><li class="listitem"><p>
	Functions registered with <code class="function">atexit()</code> are called in
	reverse order of registration, once per registration call.
	(This isn't actually new.)
	</p></li><li class="listitem"><p>
	The previous two actions are <span class="quote">“<span class="quote">interleaved,</span>”</span> that is,
	given this pseudocode:
	</p><pre class="programlisting">
  extern "C or C++" void  f1 (void);
  extern "C or C++" void  f2 (void);

  static Thing obj1;
  atexit(f1);
  static Thing obj2;
  atexit(f2);
</pre><p>
	then at a call of <code class="function">exit()</code>,
	<code class="varname">f2</code> will be called, then
	<code class="varname">obj2</code> will be destroyed, then
	<code class="varname">f1</code> will be called, and finally
	<code class="varname">obj1</code> will be destroyed. If
	<code class="varname">f1</code> or <code class="varname">f2</code> allow an
	exception to propagate out of them, Bad Things happen.
	</p></li></ol></div><p>
      Note also that <code class="function">atexit()</code> is only required to store 32
      functions, and the compiler/library might already be using some of
      those slots.  If you think you may run out, we recommend using
      the <code class="function">xatexit</code>/<code class="function">xexit</code> combination from <code class="literal">libiberty</code>, which has no such limit.
    </p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="support.termination.verbose"></a>Verbose Terminate Handler</h3></div></div></div><p>
      If you are having difficulty with uncaught exceptions and want a
      little bit of help debugging the causes of the core dumps, you can
      make use of a GNU extension, the verbose terminate handler.
    </p><pre class="programlisting">
#include &lt;exception&gt;

int main()
{
  std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
  ...

  throw <em class="replaceable"><code>anything</code></em>;
}
</pre><p>
     The <code class="function">__verbose_terminate_handler</code> function
     obtains the name of the current exception, attempts to demangle
     it, and prints it to stderr.  If the exception is derived from
     <code class="classname">exception</code> then the output from
     <code class="function">what()</code> will be included.
   </p><p>
     Any replacement termination function is required to kill the
     program without returning; this one calls abort.
   </p><p>
     For example:
   </p><pre class="programlisting">
#include &lt;exception&gt;
#include &lt;stdexcept&gt;

struct argument_error : public std::runtime_error
{
  argument_error(const std::string&amp; s): std::runtime_error(s) { }
};

int main(int argc)
{
  std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
  if (argc &gt; 5)
    throw argument_error("argc is greater than 5!");
  else
    throw argc;
}
</pre><p>
     With the verbose terminate handler active, this gives:
   </p><pre class="screen">
   <code class="computeroutput">
   % ./a.out
   terminate called after throwing a `int'
   Aborted
   % ./a.out f f f f f f f f f f f
   terminate called after throwing an instance of `argument_error'
   what(): argc is greater than 5!
   Aborted
   </code>
   </pre><p>
     The 'Aborted' line comes from the call to
     <code class="function">abort()</code>, of course.
   </p><p>
     This is the default termination handler; nothing need be done to
     use it.  To go back to the previous <span class="quote">“<span class="quote">silent death</span>”</span>
     method, simply include <code class="filename">exception</code> and
     <code class="filename">cstdlib</code>, and call
   </p><pre class="programlisting">
     std::set_terminate(std::abort);
   </pre><p>
     After this, all calls to <code class="function">terminate</code> will use
     <code class="function">abort</code> as the terminate handler.
   </p><p>
     Note: the verbose terminate handler will attempt to write to
     stderr.  If your application closes stderr or redirects it to an
     inappropriate location,
     <code class="function">__verbose_terminate_handler</code> will behave in
     an unspecified manner.
   </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="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 5. 
  Diagnostics
  
</td></tr></table></div></body></html>