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125 lines
7.0 KiB
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml"><head><title>Termination</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.76.1"/><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 align="left"><a accesskey="p" href="dynamic_memory.html">Prev</a> </td><th width="60%" align="center">Chapter 4.
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Support
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</th><td align="right"> <a accesskey="n" href="diagnostics.html">Next</a></td></tr></table><hr/></div><div class="section" title="Termination"><div class="titlepage"><div><div><h2 class="title"><a id="std.support.termination"/>Termination</h2></div></div></div><div class="section" title="Termination Handlers"><div class="titlepage"><div><div><h3 class="title"><a id="support.termination.handlers"/>Termination Handlers</h3></div></div></div><p>
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Not many changes here to <code class="filename">cstdlib</code>. You should note that the
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<code class="function">abort()</code> function does not call the
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destructors of automatic nor static objects, so if you're
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depending on those to do cleanup, it isn't going to happen.
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(The functions registered with <code class="function">atexit()</code>
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don't get called either, so you can forget about that
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possibility, too.)
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</p><p>
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The good old <code class="function">exit()</code> function can be a bit
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funky, too, until you look closer. Basically, three points to
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remember are:
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</p><div class="orderedlist"><ol class="orderedlist"><li class="listitem"><p>
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Static objects are destroyed in reverse order of their creation.
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</p></li><li class="listitem"><p>
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Functions registered with <code class="function">atexit()</code> are called in
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reverse order of registration, once per registration call.
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(This isn't actually new.)
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</p></li><li class="listitem"><p>
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The previous two actions are <span class="quote">“<span class="quote">interleaved,</span>”</span> that is,
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given this pseudocode:
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</p><pre class="programlisting">
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extern "C or C++" void f1 (void);
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extern "C or C++" void f2 (void);
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static Thing obj1;
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atexit(f1);
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static Thing obj2;
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atexit(f2);
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</pre><p>
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then at a call of <code class="function">exit()</code>,
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<code class="varname">f2</code> will be called, then
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<code class="varname">obj2</code> will be destroyed, then
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<code class="varname">f1</code> will be called, and finally
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<code class="varname">obj1</code> will be destroyed. If
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<code class="varname">f1</code> or <code class="varname">f2</code> allow an
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exception to propagate out of them, Bad Things happen.
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</p></li></ol></div><p>
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Note also that <code class="function">atexit()</code> is only required to store 32
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functions, and the compiler/library might already be using some of
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those slots. If you think you may run out, we recommend using
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the <code class="function">xatexit</code>/<code class="function">xexit</code> combination from <code class="literal">libiberty</code>, which has no such limit.
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</p></div><div class="section" title="Verbose Terminate Handler"><div class="titlepage"><div><div><h3 class="title"><a id="support.termination.verbose"/>Verbose Terminate Handler</h3></div></div></div><p>
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If you are having difficulty with uncaught exceptions and want a
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little bit of help debugging the causes of the core dumps, you can
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make use of a GNU extension, the verbose terminate handler.
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</p><pre class="programlisting">
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#include <exception>
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int main()
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{
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std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
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...
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throw <em class="replaceable"><code>anything</code></em>;
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}
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</pre><p>
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The <code class="function">__verbose_terminate_handler</code> function
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obtains the name of the current exception, attempts to demangle
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it, and prints it to stderr. If the exception is derived from
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<code class="classname">exception</code> then the output from
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<code class="function">what()</code> will be included.
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</p><p>
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Any replacement termination function is required to kill the
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program without returning; this one calls abort.
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</p><p>
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For example:
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</p><pre class="programlisting">
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#include <exception>
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#include <stdexcept>
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struct argument_error : public std::runtime_error
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{
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argument_error(const std::string& s): std::runtime_error(s) { }
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};
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int main(int argc)
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{
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std::set_terminate(__gnu_cxx::__verbose_terminate_handler);
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if (argc > 5)
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throw argument_error(<span class="quote">“<span class="quote">argc is greater than 5!</span>”</span>);
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else
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throw argc;
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}
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</pre><p>
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With the verbose terminate handler active, this gives:
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</p><pre class="screen">
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<code class="computeroutput">
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% ./a.out
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terminate called after throwing a `int'
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Aborted
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% ./a.out f f f f f f f f f f f
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terminate called after throwing an instance of `argument_error'
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what(): argc is greater than 5!
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Aborted
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</code>
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</pre><p>
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The 'Aborted' line comes from the call to
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<code class="function">abort()</code>, of course.
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</p><p>
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This is the default termination handler; nothing need be done to
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use it. To go back to the previous <span class="quote">“<span class="quote">silent death</span>”</span>
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method, simply include <code class="filename">exception</code> and
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<code class="filename">cstdlib</code>, and call
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</p><pre class="programlisting">
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std::set_terminate(std::abort);
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</pre><p>
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After this, all calls to <code class="function">terminate</code> will use
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<code class="function">abort</code> as the terminate handler.
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</p><p>
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Note: the verbose terminate handler will attempt to write to
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stderr. If your application closes stderr or redirects it to an
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inappropriate location,
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<code class="function">__verbose_terminate_handler</code> will behave in
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an unspecified manner.
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</p></div></div><div class="navfooter"><hr/><table width="100%" summary="Navigation footer"><tr><td align="left"><a accesskey="p" href="dynamic_memory.html">Prev</a> </td><td align="center"><a accesskey="u" href="support.html">Up</a></td><td align="right"> <a accesskey="n" href="diagnostics.html">Next</a></td></tr><tr><td align="left" valign="top">Dynamic Memory </td><td align="center"><a accesskey="h" href="../index.html">Home</a></td><td align="right" valign="top"> Chapter 5.
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Diagnostics
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</td></tr></table></div></body></html>
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