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How to handle a lack of memory [message #46272]

Wed, 06 April 2016 10:47

koldo
Messages: 3435
Registered: August 2008

Senior Veteran

Hello all

Now String::Cat() as many U++ functions calls SysAllocRaw() to allocate a String.

However if requested size is too big, SysAllocRaw() unconditionally shows a panic message and stops the program.

I propose you to throw an exception in this case, so that the main program can handle this properly.

In the old days we did this Smile

char *a = malloc(size);
if (a == 0) {
	printf("\nNot enough memory available");
	... code that handles this properly ...
	... program can follow running without crashing ...
}

Best regards
Iñaki

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Re: How to handle a lack of memory [message #46281 is a reply to message #46272]

Thu, 07 April 2016 22:59

mirek
Messages: 14257
Registered: November 2005

Ultimate Member

It is not so easy...

Frankly, it all boils down to VectorMap::Add implementation as example:

T& Add(const K& k, const T& x) { key.Add(k); return value.Add(x); }

Now if value.Add runs out of memory, VectorMap is left in inconsistent state. Not that this is just example, practical code is full of examples like this. E.g. what is wrong with this code:

struct Foo {
   Bar *ptr;

   Foo() { ptr = new Bar; }
   ~Foo() { delete ptr; }
}

It is certainly possible to solve that by adding exception block into Add, but correctly resolving this issue everywhere would result in significant increase in codebase (and probably decreased performace). And it is virtually untestable.

Plus, in 64 bit platform (but often with 32-bits too), you are rather going to freeze system first (because of swap trashing) that actually get to out-of-memory.

In practice, I do not remember getting out-of-memory in other situations that program bugs. Have you ever got out-of-mem in theide?

Mirek

[Updated on: Thu, 07 April 2016 23:00]

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Re: How to handle a lack of memory [message #46285 is a reply to message #46281]

Fri, 08 April 2016 09:00

koldo
Messages: 3435
Registered: August 2008

Senior Veteran

Hello Mirek

For example there is a program with a virtual ArrayCtrl that takes data from a huge file.
If user selects and copy the data to the Clipboard, program may crash.

Best regards
Iñaki

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Re: How to handle a lack of memory [message #46288 is a reply to message #46285]

Fri, 08 April 2016 09:09

mirek
Messages: 14257
Registered: November 2005

Ultimate Member

koldo wrote on Fri, 08 April 2016 09:00

Hello Mirek

For example there is a program with a virtual ArrayCtrl that takes data from a huge file.
If user selects and copy the data to the Clipboard, program may crash.

Well, I have to say that generally, this is one of areas where I am not really that much sure that used approach (Panic) is correct.

However, somewhat I suspect that here the sensible solution is rather checking the size of selection before starting the process. The advantage is that it would solve "freezing" problem too.

I know that it is not always possible. And sometimes such check is difficult - but making the code "memory exception safe" reliably is much more difficult than that, I am afraid...

Mirek

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Re: How to handle a lack of memory [message #46298 is a reply to message #46288]

Mon, 11 April 2016 08:33

koldo
Messages: 3435
Registered: August 2008

Senior Veteran

Hello Mirek

I agree with you. However I think that this issue should have to be solved some time.
Maybe it is not possible to solve it soon for all cases, but maybe it is possible to solve it for a big percentage of them.
Remember the Pareto principle (for example, with 20% of the effort you may solve the 80% of the problem) Smile

Best regards
Iñaki

[Updated on: Mon, 11 April 2016 08:35]

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Re: How to handle a lack of memory [message #46300 is a reply to message #46298]

Mon, 11 April 2016 12:06

mirek
Messages: 14257
Registered: November 2005

Ultimate Member

BTW, for reference, Qt is not much advanced in handling this problem either:

http://doc.qt.io/qt-4.8/exceptionsafety.html

IMO: Really not worth spending time with this. I could imagine replacement function for Panic, but I do not think that it is worth the effort to make the code "bad_alloc exception safe".

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