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| Question regarding multithreading |
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ryancoboy
Member #14,468
July 2012
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I have never worked with multi-threading before (started learning about it a few days ago). it looked interesting and like a powerful tool, so i started messing around with it. Making a few test programs here and there. In my latest test I have hit a roadblock that I can't seem to get myself around. I have located the problem, but i am unaware as to why it is giving me a run-time-error. here is the code for the thread that is running and causing the error (note: this function works if not using it in a thread): 1void *Load(ALLEGRO_THREAD *thr, void *arg)
2{
3 std::cout << "Load thread: Started." << std::endl;
4
5 ThreadInfo *Info = (ThreadInfo*) arg;
6
7 al_get_bitmap_format(al_get_backbuffer(al_get_current_display()));
8 //LoadMap();
9
10 al_lock_mutex(Info->mutex);
11 Info->State->CurState = Playing;
12 al_unlock_mutex(Info->mutex);
13
14 std::cout << "Load thread: Ended." << std::endl;
15
16 return NULL;
17}
The line that it gives the error on is: I'm guessing it is because the call to al_get_bitmap(...) is not thread safe. But I'm still new to this multi-threading stuff. So if someone could confirm or deny, that be great. And any ideas on how to make this work using threads would also be appreciated. Thanks in advanced, -Ryan |
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Todd Cope
Member #998
November 2000
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The first thing you should do is check the return value of each function before passing it to the next. ALLEGRO_DISPLAY * display; ALLEGRO_BITMAP * bitmap; int format; display = al_get_current_display(); if(display) { bitmap = al_get_backbuffer(display); if(bitmap) { format = al_get_bitmap_format(bitmap); } } Most likely, one of these functions is returning NULL and causing a crash when that value is passed to a function expecting a valid pointer. Also, in your code you aren't storing the format anywhere. What exactly are you trying to do there? |
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ryancoboy
Member #14,468
July 2012
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@Todd Cope: Thanks for the tips I'll try that here in a couple moments and post back here (dinner time EDIT: I just tested it real quick. And "format" ended up being equal to 23. So it looks to be running fine there as well. -Ryan |
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Trent Gamblin
Member #261
April 2000
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The manual states that each thread has it's own "current display", and new threads have a default NULL current display, so you're passing NULL into those functions and that's probably why it's crashing. If you had done what Todd said, you would have figured that out.
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ryancoboy
Member #14,468
July 2012
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@Trent Gamblin: your right. I must of missed that part in the manual or forgot about it. Thanks for clearing that up for me. And i was in a rush for my dinner so i didn't test the return value of my display in my "Load" thread. I just did it in my main. I was rushing and didn't read the post completely. Sorry, for that. That pretty much solves my problem. Thanks guys. -Ryan |
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). As you can see, I have a function commented out "LoadMap()" and deep in that function is the al_get_bitmap_format(...) code. i just moved it up front so that i wouldn't have to post a billion function calls, etc. Once a solution is found to prevent the error I will attempt to fix it where it is actually intended to be used.