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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="sec-exceptions-in-signal-handlers"></a>Exceptions in signal handlers</h2></div></div></div>
<p>
When a program is aborted because of an unhandled C++ exception, it's sometimes
possible to use a debugger to find the location where the exception was thrown.
This is more difficult than usual if the exception was thrown from a signal handler.
</p>
<p>
This section describes primarily what you can expect on a Linux system, when you
use <a class="ulink" href="http://www.gnu.org/software/gdb/" target="_top">the gdb debugger</a>.
</p>
<p>
First, let's look at a simple example where an exception is thrown from a normal
function (no signal handler).
</p>
<pre class="programlisting">
// without_signal.cc
#include <gtkmm.h>
bool throwSomething()
{
throw "Something";
return true;
}
int main(int argc, char** argv)
{
throwSomething();
Glib::RefPtr<Gtk::Application> app =
Gtk::Application::create(argc, argv, "org.gtkmm.without_signal");
return app->run();
}
</pre>
<p>
</p>
<p>
Here is an excerpt from a <span class="application">gdb</span> session. Only the most
interesting parts of the output are shown.
</p>
<pre class="programlisting">
> gdb without_signal
(gdb) run
terminate called after throwing an instance of 'char const*'
Program received signal SIGABRT, Aborted.
(gdb) backtrace
#7 0x08048864 in throwSomething () at without_signal.cc:6
#8 0x0804887d in main (argc=1, argv=0xbfffecd4) at without_signal.cc:12
</pre>
<p>
You can see that the exception was thrown from <code class="filename">without_signal.cc</code>,
line 6 (<code class="code">throw "Something";</code>).
</p>
<p>
Now let's see what happens when an exception is thrown from a signal handler.
Here's the source code.
</p>
<pre class="programlisting">
// with_signal.cc
#include <gtkmm.h>
bool throwSomething()
{
throw "Something";
return true;
}
int main(int argc, char** argv)
{
Glib::signal_timeout().connect(sigc::ptr_fun(throwSomething), 500);
Glib::RefPtr<Gtk::Application> app =
Gtk::Application::create(argc, argv, "org.gtkmm.with_signal");
app->hold();
return app->run();
}
</pre>
<p>
</p>
<p>
And here's an excerpt from a <span class="application">gdb</span> session.
</p>
<pre class="programlisting">
> gdb with_signal
(gdb) run
(with_signal:2703): glibmm-ERROR **:
unhandled exception (type unknown) in signal handler
Program received signal SIGTRAP, Trace/breakpoint trap.
(gdb) backtrace
#2 0x0063c6ab in glibmm_unexpected_exception () at exceptionhandler.cc:77
#3 Glib::exception_handlers_invoke () at exceptionhandler.cc:150
#4 0x0063d370 in glibmm_source_callback (data=0x804d620) at main.cc:212
#13 0x002e1b31 in Gtk::Application::run (this=0x804f300) at application.cc:178
#14 0x08048ccc in main (argc=1, argv=0xbfffecd4) at with_signal.cc:16
</pre>
<p>
The exception is caught in <span class="application">glibmm</span>, and the program
ends with a call to <code class="function">g_error()</code>. Other exceptions may result
in different behaviour, but in any case the exception from a signal handler is
caught in <span class="application">glibmm</span> or <span class="application">gtkmm</span>, and
<span class="application">gdb</span> can't see where it was thrown.
</p>
<p>
To see where the exception is thrown, you can use the <span class="application">gdb</span>
command <strong class="userinput"><code>catch throw</code></strong>.
</p>
<pre class="programlisting">
> gdb with_signal
(gdb) catch throw
Catchpoint 1 (throw)
(gdb) run
Catchpoint 1 (exception thrown), 0x00714ff0 in __cxa_throw ()
(gdb) backtrace
#0 0x00714ff0 in __cxa_throw () from /usr/lib/i386-linux-gnu/libstdc++.so.6
#1 0x08048bd4 in throwSomething () at with_signal.cc:6
(gdb) continue
Continuing.
(with_signal:2375): glibmm-ERROR **
unhandled exception (type unknown) in signal handler
Program received signal SIGTRAP, Trace/breakpoint trap.
</pre>
<p>
</p>
<p>
If there are many caught exceptions before the interesting uncaught one, this
method can be tedious. It can be automated with the following
<span class="application">gdb</span> commands.
</p>
<pre class="programlisting">
(gdb) catch throw
(gdb) commands
(gdb) backtrace
(gdb) continue
(gdb) end
(gdb) set pagination off
(gdb) run
</pre>
<p>
These commands will print a backtrace from each <code class="code">throw</code> and continue.
The backtrace from the last (or possibly the last but one) <code class="code">throw</code>
before the program stops, is the interesting one.
</p>
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