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authorGravatar Chris Xiong <chirs241097@gmail.com> 2024-06-22 19:59:16 -0400
committerGravatar Chris Xiong <chirs241097@gmail.com> 2024-06-22 19:59:16 -0400
commit4e1a29cab8d31cff30d88d2dfc0f526372fc33bd (patch)
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parente81928f76ab105c65a0a0997c289f0f521589128 (diff)
downloadweb-4e1a29cab8d31cff30d88d2dfc0f526372fc33bd.tar.xz
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- <li><a href="/"><h1>Chrisoft</h1></a></li>
- <li><a href="/blog"><h2>Blog</h2></a></li>
- <li><a href="#"><h3 id="title">2-in-1</h3></a></li>
- <li><span>Tags</span>
- <ul id="tagslist">
- <li><a href="/blog/list/devel/">devel</a></li><li><a href="/blog/list/qmidiplayer/">qmidiplayer</a></li><li><a href="/blog/list/c++/">c++</a></li><li><a href="/blog/list/qt/">qt</a></li></ul>
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- <span>Table of Contents</span>
- <ul id="tocroot">
- <li><a class="toctarg" href="#tocanch0">UPD 2016-10-28</a></li><li><ul class="tocnode"><li><a class="toctarg" href="#tocanch1">BUT THAT'S NOT THE END OF THE STORY.</a></li></ul></li><li><a class="toctarg" href="#tocanch2">UPD@2016-12-28</a></li></ul>
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- <li style="margin-left:-0.5em"><a id="prevp" href="2016-06-21.html">Prev post</a></li>
- <li style="margin-left:-0.5em"><a id="nextp" href="2017-01-09.html">Next post</a></li>
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- <div id="content">
- <h2 id="titleh" class="TText" style="font-wight:normal;">2-in-1</h2>
- <div id="datetags" class="TText" style="margin-bottom:1em;">2016-09-24<br>#devel #qmidiplayer #c++ #qt</div>
- <hr><div id="article" class="TText">
-<article>
-<p>
-The visualization plugin for QMidiPlayer in Linux has suffered from a dbus-related bug for a long time:
-after you have opened the visualization window, close it and try to bring it up again, it will either
-result in several lines of dbus warning which suggests there's a bug in the program (on Debian sid),
-or simply a crash (on Arch Linux).
-</p>
-<p>
-Well, I have to admit the design of QMidiPlayer's visualization architecture is pretty weird:
-the visualization plugin spawns its own thread and do almost everything in it. After doing some
-research on the Internet, I realized that everyone suggests that Qt and SDL <i>shouldn't</i> be
-used together, "use QtOpenGL instead". However I thought that will hang the main UI thread and
-I just don't believe in the myth.
-</p>
-<p>
-So I started my investigation in early May. I even opened an related issue on github. Unfortunately nobody
-offered help. So I had to solve the issue myself. First I had a look into the dbus warnings:
-</p><pre style="white-space: pre-wrap;">process &lt;PID&gt;: arguments to dbus_connection_unref() were incorrect, assertion "connection-&gt;generation == _dbus_current_generation" failed in file ../../dbus/dbus-connection.c line 2822.
-This is normally a bug in some application using the D-Bus library.
-</pre>
-<p></p>
-<p>
-Qt definitely uses dbus. After spitting these into stderr, QMidiPlayer crashes instantly in the
-QDBusConnection thread. Googling the warning turned out nothing. Then I came across with this blog post
-from a KDE developer:<br>
-<a href="https://blogs.kde.org/2009/03/26/how-crash-almost-every-qtkde-application-and-how-fix-it-0">https://blogs.kde.org/2009/03/26/how-crash-almost-every-qtkde-application-and-how-fix-it-0</a>
-</p>
-<p>
-Unfortunately, after applying the "fix", the bug didn't change even a little bit. So I had to move on.
-</p>
-<h3 id="tocanch0" class="tvis">UPD 2016-10-28</h3>
-<p>
-After several months' idleness of the investigation due to the lack of idea, I suddenly decided to have
-a look at all dbus calls when QMidiPlayer crashes. Then I found the following dbus call:
-</p><pre style="white-space: pre-wrap;">path=/org/freedesktop/ScreenSaver; interface=org.freedesktop.ScreenSaver; method=Inhibit;
-string "My SDL application"
-string "Playing a game"
-</pre>
-<p></p>
-<p>
-Apparently SDL is also using dbus to do some dirty work -- disabling the screen saver! So I appended
-SDL_EnableScreenSaver in SMELT's init function. Unfortunately, nothing happened again. However I've
-guessed out the cause to the problem: <b>both Qt and SDL try to initialize a dbus connection in <i>one</i>
-process, which is not allowed. So I have to get rid of either one of them.</b>
-</p>
-<p>
-Therefore I began to port the SMELT engine to GLFW. After two days of work, the port was finished and it worked
-like a charm -- I have solved the problem!
-</p>
-<h4 id="tocanch1" class="tvis">BUT THAT'S NOT THE END OF THE STORY.</h4>
-<p>
-Driven by the unstoppable merriness after fixing a long-existed bug, I started to build GLFW version of
-SMELT for Windows. The process was painful but finally I managed to do it successfully. After that
-it's time to compile the visualization library. A beautiful DLL file, sized 1096192 bytes, was produced.
-This build is almost 4x smaller than the previous failed mingw build and I regarded this as a omen of
-fortune.
-</p>
-<p>
-I put the DLL file into the plugin folder of QMidiPlayer and launched the application. No errors occured
-as the mingw build once caused and the plugin showed up correctly in the plugin manager. I was so excited
-that I turned on the plugin immediately and I couldn't wait to announce the result to my friends. However
-when I hit the visualization I was greeted by a déjà vu scene.
-</p>
-<img src="//filestorage.chrisoft.org/blog/img/qmpcrash.png" alt="" width="70%" decoding="async">
-<p>
-And this nasty little dialog from the Windows 2000 era.
-</p>
-<img src="//filestorage.chrisoft.org/blog/img/couldntbewritten.png" alt="" width="40%" decoding="async">
-<p>
-So another tale of a battle between a fierce bug and me is due to happen. <s>Stay tuned and watch the fight!</s>
-</p>
-<a id="n1" href="#note1" class="note">[1]</a>
-<h3 id="tocanch2" class="tvis">UPD@2016-12-28</h3>
-<p>
-So the bug has been (mostly) fixed by the following two commits!<br>
-<a href="https://github.com/chirs241097/QMidiPlayer/commit/b79c4b7e3cab3711e87ba9e28fa8423a84ea7efa">b79c4b7e3cab3711e87ba9e28fa8423a84ea7efa@QMidiPlayer</a><br>
-<a href="https://github.com/BearKidsTeam/SMELT/commit/b13f72857af93489f535b84d62882f681dc84a73">b13f72857af93489f535b84d62882f681dc84a73@SMELT</a><br>
-</p>
-<p>
-Basically the most tough part to debug is caused by the stupidest typo...
-</p><pre style="padding-left:20px;border:1px white solid;">@@ -62,7 +62,7 @@ bool SMELT_IMPL::smInit()
- else if(i==0x80000004)
- memcpy(cpuName+32, CPUInfo, sizeof(CPUInfo));
- }
- - while(*cpuName=' ')++cpuName;
- + while(*cpuName==' ')++cpuName;
- smLog("%s:" SLINE ": CPU: %s\n", SYS_SDL_SRCFN,cpuName);
- free(loced);
-</pre>
-<p></p>
-<p>
-This naïve mistake screwed up the memory and this should be the cause that made the debugger report different stack traces in different build modes...
-(that reminds me of the experience of screwing up the call stack, in which the backtrace was filled up by "0x0000000000000000 in ??()"...)
-</p>
-<p>
-After having fixed this, it was much easier to debug the application. Soon afterwards the first working visualization plugin for QMP on Windows
-came out.
-</p>
-<p>
-Finally I have to make more complaints about the develop environment for a cross-platform C++ application developer in Microsoft Windows.
-It's bloated, complicated and confusing. I wonder whether Microsoft is deliberately making it difficult in order to force more developers
-to migrate to C#/.Net. What a big ambition Microsoft is bearing!
-</p>
-<article>
-</article></article></div><br><hr>
- <div class="TText" id="notediv" style="font-size:80%;"><span class="TText"><a id="note1" href="#n1">[1]</a>:
- Current status of the bug in Windows:<br>
- Release build QMidiPlayer+Release build plugin=crash at somewhere in visualization.dll.<br>
- Release build QMidiPlayer+Debug build plugin=crash at somewhere in Qt5Core.dll.<br>
- Debug build QMidiPlayer+Debug build plugin=crash instantly on launch at plugin initialization.<br>
-I have totally no idea about what to do next.
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