ACE/docs/ACE-FMM.html

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   4 <html>
   5   <head>
   6     <title>ACE FMM</title>
   7   </head>
   8
   9   <body bgcolor=#ffffff>
  10 <center>
  11 <font face=helvetica size=5>ACE Frequently Made Mistakes</font>
  12
  13 <br>
  14 <br>
  15 <table border=0 cellpadding=3 cellspacing=1 width=550>
  16
  17 <tr>
  18 <td align=right valign=top>
  19         <b>symptom</b>
  20 </td>
  21 <td align=left valign=top>
  22         ACE_Task::getq() returns the error
  23         <b>resource temporarily unavailable</b>
  24 </td>
  25 </tr>
  26 <tr>
  27 <td align=right valign=top>
  28         <b>probable cause</b>
  29 </td>
  30 <td align=left valign=top>
  31         Your Task is a subclass of ACE_Task&lt;ACE_NULL_SYNCH&gt; and
  32         you are using it in a multithreaded program.
  33 </td>
  34 </tr>
  35 <tr>
  36 <td align=right valign=top>
  37         <b>solution</b>
  38 </td>
  39 <td align=left valign=top>
  40         Try using ACE_Task&lt;ACE_MT_SYNCH&gt;
  41         instead so that the associated Message_Queue
  42         is configured for access by multiple threads.
  43 </td>
  44 <tr><td colspan=2><hr noshade></td></tr>
  45
  46 <tr>
  47 <td align=right valign=top>
  48         <b>symptom</b>
  49 </td>
  50 <td align=left valign=top>
  51         ACE_Task::wait() throws an assert violation
  52 </td>
  53 </tr>
  54 <tr>
  55 <td align=right valign=top>
  56         <b>probable cause</b>
  57 </td>
  58 <td align=left valign=top>
  59         When you activate()d your Task, you specified
  60         THR_DETACHED, which causes wait() to be unable to perform what you
  61         want it to.
  62 </td>
  63 </tr>
  64 <tr>
  65 <td align=right valign=top>
  66         <b>solution</b>
  67 </td>
  68 <td align=left valign=top>
  69         Make sure you specify the flag THR_JOINABLE when activating
  70          your ACE_Task object.
  71 </td>
  72 <tr><td colspan=2><hr noshade></td></tr>
  73
  74
  75
  76 <tr>
  77 <td align=right valign=top>
  78         <b>symptom</b>
  79 </td>
  80 <td align=left valign=top>
  81          Apparent race conditions when spawning threads (or activating Tasks)
  82         from within a constructor.
  83 </td>
  84 </tr>
  85 <tr>
  86 <td align=right valign=top>
  87         <b>probable cause</b>
  88 </td>
  89 <td align=left valign=top>
  90         You are not guaranteed to have a valid <b>this</b> pointer
  91         until the constructor has exited.  Threads spawned from
  92         a constructor are free to run
  93         immediately, and may attempt to use an invalid <b>this</b> pointer.
  94
  95 </td>
  96 </tr>
  97 <tr>
  98 <td align=right valign=top>
  99         <b>solution</b>
 100 </td>
 101 <td align=left valign=top>
 102         Move your Task activations and other thread-spawning activities
 103         <b>out</b> of the constructor.
 104 </td>
 105 <tr><td colspan=2><hr noshade></td></tr>
 106
 107
 108
 109 <tr>
 110 <td align=right valign=top>
 111         <b>symptom</b>
 112 </td>
 113 <td align=left valign=top>
 114         Compiler issues warnings/erros regarding using too few template
 115         arguments, such as "'ACE_Svc_Handler' : too few template arguments".
 116 </td>
 117 </tr>
 118 <tr>
 119 <td align=right valign=top>
 120         <b>probable cause</b>
 121 </td>
 122 <td align=left valign=top>
 123         Instead of using the appropriate macro, you supplied an actual class
 124         name as a parameter.  This will fail depending upon platform and compiler,
 125         due to the way templates are handled.
 126 </td>
 127 </tr>
 128 <tr>
 129 <td align=right valign=top>
 130         <b>solution</b>
 131 </td>
 132 <td align=left valign=top>
 133         Instead of instantiating a template class like <b>ACE_Svc_Handler&lt;<u>ACE_SOCK_Stream</u>, ACE_NULL_SYNCH&gt;</b>, use the form of <b>ACE_Svc_Handler&lt;<u>ACE_SOCK_STREAM</u>, ACE_NULL_SYNCH&gt;</b> which circumvents the platform peculiarities by using the macro. This also applies to some other template classes.
 134 </td>
 135 <tr><td colspan=2><hr noshade></td></tr>
 136
 137
 138
 139 <tr>
 140 <td align=right valign=top>
 141         <b>symptom</b>
 142 </td>
 143 <td align=left valign=top>
 144         Unable to compare ACE_thread_t variables (such as ACE_Thread::self())
 145         using operator== ().
 146 </td>
 147 </tr>
 148 <tr>
 149 <td align=right valign=top>
 150         <b>probable cause</b>
 151 </td>
 152 <td align=left valign=top>
 153         On some platforms, thread ids are numeric, and on some, they aren't. On some
 154         implementations, simple a == b comparisons
 155         are legal and sane.  Some are not.
 156
 157 </td>
 158 </tr>
 159 <tr>
 160 <td align=right valign=top>
 161         <b>solution</b>
 162 </td>
 163 <td align=left valign=top>
 164         Use the <b>ACE_OS::thr_equal()</b> function to reliably compare thread
 165         ids, regardless of platform.
 166 </td>
 167 <tr><td colspan=2><hr noshade></td></tr>
 168
 169 <tr>
 170 <td align=right valign=top>
 171         <b>symptom</b>
 172 </td>
 173 <td align=left valign=top>
 174         ACE_Reactor::run_event_loop() does not seem to function correctly
 175         for a Reactor created in your application.
 176 </td>
 177 </tr>
 178 <tr>
 179 <td align=right valign=top>
 180         <b>probable cause</b>
 181 </td>
 182 <td align=left valign=top>
 183         You have not set the ACE_Reactor::instance() to refer to your new reactor.
 184         run_event_loop only functions on the reactor currently installed as the
 185         global Singleton.
 186 </td>
 187 </tr>
 188 <tr>
 189 <td align=right valign=top>
 190         <b>solution</b>
 191 </td>
 192 <td align=left valign=top>
 193         Use the <b>ACE_Reactor::instance(ACE_Reactor *,
 194         int delete_reactor = 0)</b> static method to install your reactor as the global
 195         Singleton before calling run_event_loop().
 196 </td>
 197 <tr><td colspan=2><hr noshade></td></tr>
 198
 199
 200
 201
 202
 203 <tr>
 204 <td align=right valign=top>
 205         <b>symptom</b>
 206 </td>
 207 <td align=left valign=top>
 208 Infinite recursion when you invoke ACE_Reactor::remove_handler()
 209 </td>
 210 </tr>
 211 <tr>
 212 <td align=right valign=top>
 213         <b>probable cause</b>
 214 </td>
 215 <td align=left valign=top>
 216 You are invoking remove_handler() from within handle_close() (or a
 217 method  invoked by handle_close()) but you have not specified the
 218 DONT_CALL flag.
 219 </td>
 220 </tr>
 221 <tr>
 222 <td align=right valign=top>
 223         <b>solution</b>
 224 </td>
 225 <td align=left valign=top>
 226 Be sure to <b>OR</b> in the DONT_CALL flag in this situation.<br>
 227 e.g. --<br>
 228 <ul><pre>
 229 int MyHandler::handle_close (ACE_HANDLE handle,
 230                              ACE_Reactor_Mask close_mask)
 231 {
 232     ...
 233     my_reactor_-&gt;remove_handler( this,
 234                                 ACE_Event_Handler::READ_MASK |
 235                                 ACE_Event_Handler::DONT_CALL );
 236     ...
 237     return 0;
 238 }
 239 </pre></ul>
 240
 241 </td>
 242 <tr><td colspan=2><hr noshade></td></tr>
 243
 244 <tr>
 245 <td align=right valign=top>
 246         <b>symptom</b>
 247 </td>
 248 <td align=left valign=top>
 249   Application crashes after deleting Event_Handler.
 250 </td>
 251 </tr>
 252 <tr>
 253 <td align=right valign=top>
 254         <b>probable cause</b>
 255 </td>
 256 <td align=left valign=top>
 257   You left a dangling pointer to the Event_Handler in the Reactor.
 258   It is the application's responsibility to remove all pending notifications,
 259   timer events and completely remove the event handler I/O registrations
 260   before removing the event handler.
 261
 262   Also, the application should remove the event handler from the reactor
 263   <b>before</b> closing the underlying file descriptor / handle.
 264   Otherwise:
 265   <ul>
 266   <li>The reactor does not know how to remove the event handler, because the
 267   handle is used as the identifier for the event handlers</li>
 268   <li>The file descriptor / handle may be reused by another thread, leading to
 269   nasty race conditions.</li>
 270   </ul>
 271 </td>
 272 </tr>
 273 <tr>
 274 <td align=right valign=top>
 275         <b>solution</b>
 276 </td>
 277 <td align=left valign=top>
 278   <ul>
 279     <li>Use reference counted event handlers.  The reactor and the application
 280       cooperate to remove the event handler when the last reference goes away.
 281     </li>
 282     <li>Remember to call <tt>purge_pending_notifications()</tt>,
 283      <tt>remove_handler()</tt> and <tt>cancel_timer()</tt> before deleting the
 284   event handler.
 285     </li>
 286   </ul>
 287 </td>
 288 <td align=left valign=top>
 289
 290 </td>
 291 <tr><td colspan=2><hr noshade></td></tr>
 292
 293 <!--
 294
 295 <tr>
 296 <td align=right valign=top>
 297         <b>symptom</b>
 298 </td>
 299 <td align=left valign=top>
 300
 301 </td>
 302 </tr>
 303 <tr>
 304 <td align=right valign=top>
 305         <b>probable cause</b>
 306 </td>
 307 <td align=left valign=top>
 308
 309 </td>
 310 </tr>
 311 <tr>
 312 <td align=right valign=top>
 313         <b>solution</b>
 314 </td>
 315 <td align=left valign=top>
 316
 317 </td>
 318 <tr><td colspan=2><hr noshade></td></tr>
 319
 320 -->
 321 <tr>
 322 <td align=center colspan=2>
 323 <font size=2>maintained by <a href="mailto:bob@werken.com">bob@werken.com</a></font>
 324 </td>
 325 </tr>
 326
 327 </table>
 328 </center>
 329 Back to <A HREF="index.html">ACE Documentation Home</A>.
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