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Debugging memory leak(?) or logic problem


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#1 polyfrag   Crossbones+   -  Reputation: 2163

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Posted 07 August 2014 - 06:55 PM

Since I've added a data structure with lists that is multiply copied and modified, for each tile and each building type, I've noticed that the program grows to and exceeds 1.8 GB of RAM until it reaches my computer's limit and crashes. But there isn't more than 30 items in the lists and I've tried Dr. memory but it doesn't show any leak. It crashes on SDL_SetWindowIcon for some reason under debugger and AppVerifier saying there's a problem accessing memory location in sdl2.dll or something. I've tried checking if memory is freed using GetProcessMemoryInfo and checking working set size but it doesn't decrease, even when I free an int pointer. Should it decrease?



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#2 flodihn   Members   -  Reputation: 254

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Posted 08 August 2014 - 01:30 AM

Try running valgrind on your code, it is a static analyzer and might detect if you allocate memory, but fail to de-allocate it.



#3 Bacterius   Crossbones+   -  Reputation: 10834

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Posted 08 August 2014 - 01:47 AM

Try running valgrind on your code, it is a static analyzer and might detect if you allocate memory, but fail to de-allocate it.

 

That's not what a static analyzer is. Valgrind simply extends malloc to keep track of allocated/deallocated blocks (as well as tracking the stack and other memory regions) so that it can know for sure when you read or write memory that you shouldn't be able to and knowing exactly how much memory you have allocated at any given point during your program. A static analyzer doesn't run your program and hence does not rely on any specific input to your program (that is actually the whole point, being able to make a statement on a static piece of code rather than on its execution and result given some input).

 

Valgrind is also not well-supported on Windows as far as I know, though there were some efforts towards porting it last time I checked. If you're on a platform that Valgrind supports though it is a must-have in your toolbox.


“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”


#4 Álvaro   Crossbones+   -  Reputation: 15560

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Posted 08 August 2014 - 04:30 AM

Putting crap in a standard container and never removing it is not technically a leak, but it could very well be what you are experiencing.

See if you can simplify your program a lot and still have the behavior you observe, then try to understand what's going on in the simpler program.

#5 flodihn   Members   -  Reputation: 254

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Posted 08 August 2014 - 05:08 AM

 

Try running valgrind on your code, it is a static analyzer and might detect if you allocate memory, but fail to de-allocate it.

 

That's not what a static analyzer is.

 

That is true, I mixed it up with another tool called Dializer.

 

I would vote up Álvaro idea of simplifying your code, for example I built a game engine a few years ago and I can still remember the only 4 places I used the new keyword (allocaed on the heap). When a new game object was created, I called new in my logic, rendering, data once and my network part had an allocated network buffer on the heap.



#6 Álvaro   Crossbones+   -  Reputation: 15560

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Posted 08 August 2014 - 06:29 AM

I would vote up Álvaro idea of simplifying your code, for example I built a game engine a few years ago and I can still remember the only 4 places I used the new keyword (allocaed on the heap). When a new game object was created, I called new in my logic, rendering, data once and my network part had an allocated network buffer on the heap.


That's a very good idea, but it's not what I was talking about. What I was trying to say is that one can debug a problem of this type by making a copy of the code and stripping it out of everything that you can while still observing the bug. For instance, you can probably get rid of all the sound and the graphics, you can replace the input with some simple scripted behavior... Keep hacking at it until anything you remove makes the bug go away. Now you have a much simpler program to reason about, and you have a better chance of figuring out what's wrong.

Edited by Álvaro, 08 August 2014 - 06:31 AM.


#7 Bregma   Crossbones+   -  Reputation: 5939

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Posted 08 August 2014 - 06:34 AM


one can debug a problem of this type by making a copy of the code and stripping it out of everything that you can while still observing the bug.

This is the essence of unit tests.

 

The question should be: do your unit tests show similar behaviour?

 

Can you draw any lessons from that?


Stephen M. Webb
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#8 Ohforf sake   Members   -  Reputation: 1959

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Posted 08 August 2014 - 07:03 AM

I've tried checking if memory is freed using GetProcessMemoryInfo and checking working set size but it doesn't decrease, even when I free an int pointer. Should it decrease?


On x86, memory gets mapped into processes in quantities of at least 4KB. I'm not familiar with GetProcessMemoryInfo, but it might just report how many of those 4KB pages are mapped to the process, and not how many bytes of those you are actually using. If so, allocating or freeing small data structures should not directly change the amount of memory your process is holding on to.

If this is a crossplatform game (seeing as you are using SDL), then valgrind is definitely an option.




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