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<!-- $Id: ACE-FMM.html 80826 2008-03-04 14:51:23Z wotte $ -->
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
<html>
<head>
<title>ACE FMM</title>
</head>
<body bgcolor=#ffffff>
<center>
<font face=helvetica size=5>ACE Frequently Made Mistakes</font>
<br>
<br>
<table border=0 cellpadding=3 cellspacing=1 width=550>
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
ACE_Task::getq() returns the error
<b>resource temporarily unavailable</b>
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
Your Task is a subclass of ACE_Task<ACE_NULL_SYNCH> and
you are using it in a multithreaded program.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
Try using ACE_Task<ACE_MT_SYNCH>
instead so that the associated Message_Queue
is configured for access by multiple threads.
</td>
<tr><td colspan=2><hr noshade></td></tr>
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
ACE_Task::wait() throws an assert violation
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
When you activate()d your Task, you specified
THR_DETACHED, which causes wait() to be unable to perform what you
want it to.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
Make sure you specify the flag THR_JOINABLE when activating
your ACE_Task object.
</td>
<tr><td colspan=2><hr noshade></td></tr>
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
Apparent race conditions when spawning threads (or activating Tasks)
from within a constructor.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
You are not guaranteed to have a valid <b>this</b> pointer
until the constructor has exited. Threads spawned from
a constructor are free to run
immediately, and may attempt to use an invalid <b>this</b> pointer.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
Move your Task activations and other thread-spawning activites
<b>out</b> of the constructor.
</td>
<tr><td colspan=2><hr noshade></td></tr>
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
Compiler issues warnings/erros regarding using too few template
arguments, such as "'ACE_Svc_Handler' : too few template arguments".
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
Instead of using the appropriate macro, you supplied an actual class
name as a parameter. This will fail depending upon platform and compiler,
due to the way templates are handled.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
Instead of instantiating a template class like <b>ACE_Svc_Handler<<u>ACE_SOCK_Stream</u>, ACE_NULL_SYNCH></b>, use the form of <b>ACE_Svc_Handler<<u>ACE_SOCK_STREAM</u>, ACE_NULL_SYNCH></b> which circumvents the platform peculiarities by using the macro. This also applies to some other template classes.
</td>
<tr><td colspan=2><hr noshade></td></tr>
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
Unable to compare ACE_thread_t variables (such as ACE_Thread::self())
using operator== ().
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
On some platforms, thread ids are numeric, and on some, they aren't. On some
implementations, simple a == b comparisons
are legal and sane. Some are not.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
Use the <b>ACE_OS::thr_equal()</b> function to reliably compare thread
ids, regardless of platform.
</td>
<tr><td colspan=2><hr noshade></td></tr>
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
ACE_Reactor::run_event_loop() does not seem to function correctly
for a Reactor created in your application.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
You have not set the ACE_Reactor::instance() to refer to your new reactor.
run_event_loop only functions on the reactor currently installed as the
global Singleton.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
Use the <b>ACE_Reactor::instance(ACE_Reactor *,
int delete_reactor = 0)</b> static method to install your reactor as the global
Singleton before calling run_event_loop().
</td>
<tr><td colspan=2><hr noshade></td></tr>
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
Infinite recursion when you invoke ACE_Reactor::remove_handler()
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
You are invoking remove_handler() from within handle_close() (or a
method invoked by handle_close()) but you have not specified the
DONT_CALL flag.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
Be sure to <b>OR</b> in the DONT_CALL flag in this situation.<br>
e.g. --<br>
<ul><pre>
int MyHandler::handle_close (ACE_HANDLE handle,
ACE_Reactor_Mask close_mask)
{
...
my_reactor_->remove_handler( this,
ACE_Event_Handler::READ_MASK |
ACE_Event_Handler::DONT_CALL );
...
return 0;
}
</pre></ul>
</td>
<tr><td colspan=2><hr noshade></td></tr>
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
Application crashes after deleting Event_Handler.
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
You left a dangling pointer to the Event_Handler in the Reactor.
It is the application's responsibility to remove all pending notifications,
timer events and completely remove the event handler I/O registrations
before removing the event handler.
Also, the application should remove the event handler from the reactor
<b>before</b> closing the underlying file descriptor / handle.
Otherwise:
<ul>
<li>The reactor does not know how to remove the event handler, because the
handle is used as the identifier for the event handlers</li>
<li>The file descriptor / handle may be reused by another thread, leading to
nasty race conditions.</li>
</ul>
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
<ul>
<li>Use reference counted event handlers. The reactor and the application
cooperate to remove the event handler when the last reference goes away.
</li>
<li>Remember to call <tt>purge_pending_notifications()</tt>,
<tt>remove_handler()</tt> and <tt>cancel_timer()</tt> before deleting the
event handler.
</li>
</ul>
</td>
<td align=left valign=top>
</td>
<tr><td colspan=2><hr noshade></td></tr>
<!--
<tr>
<td align=right valign=top>
<b>symptom</b>
</td>
<td align=left valign=top>
</td>
</tr>
<tr>
<td align=right valign=top>
<b>probable cause</b>
</td>
<td align=left valign=top>
</td>
</tr>
<tr>
<td align=right valign=top>
<b>solution</b>
</td>
<td align=left valign=top>
</td>
<tr><td colspan=2><hr noshade></td></tr>
-->
<tr>
<td align=center colspan=2>
<font size=2>maintained by <a href="mailto:bob@werken.com">bob@werken.com</a></font>
</td>
</tr>
</table>
</center>
Back to <A HREF="index.html">ACE Documentation Home</A>.
</body>
</html>
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