Conditions that may lead to memory corruption

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I just read an article which stated the following conditions that may cause memory corruption. But no proper expalantion was given there. Can anyone explain that how does these conditions lead to memory corruption and what happens internally.

1. Using an address before memory is allocated and set

void x()
{
  struct *abc_ptr;
  p = abc_ptr->name;
}

2. Using a pointer which is already freed

int *p = new int;
//code
delete p;
//use of p here may lead to memory corruption

3. Freeing memory which has already been freed.

char *a = malloc(128*sizeof(char));
free(a);
...   Do stuff
free(a);  // A check for NULL would indicate nothing.
          // This memory space may be reallocated and thus we may be freeing
          // memory we do not intend to free or portions of another block of 
          // memory. The size of the block of memory allocated is often held 
          // just before the memory block itself..
                  // some code
}
delete i;

4. Freeing memory which was not dynamically allocated

void main()
{
struct ABC abc;
struct ABC *abc_ptr = &abc;
free(abc_ptr);
}

5. Incorrect use of delete: The delete must match the use of new

void x()
{
  abc *abc_ptr = new abc[100];
  delete [] abc_ptr;
}

http://www.yolinux.com/TUTORIALS/C++MemoryCorruptionAndMemoryLeaks.html[^]

Posted 15-Mar-12 3:11am

Member 8576081

Updated 15-Mar-12 23:23pm

v2

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CPallini 15-Mar-12 9:52am

What's wrong in sample 5?

JackDingler 15-Mar-12 11:43am

Trick question from the professor.

And a good one. It's not uncommon to be assigned to fix a problem, and be given bad information about the cause, because the assignee only sees the symptoms.

Bugs must be verified before they can be properly fixed.

CPallini 15-Mar-12 16:29pm

I see nothing wrong in the code sample.

JackDingler 15-Mar-12 11:34am

Smells like homework.

JackDingler 15-Mar-12 11:39am

If other people do your homework for you, you may pass the class, but not the job interview.

These are common problems used in job interviews. If you can't stand up and explain these problems with eight people, critically staring at you, then you're gonna be living with mom and dad, longer than you might like.

If you want to make it in the professional world, then you must master these basic concepts.

Sergey Alexandrovich Kryukov 15-Mar-12 20:19pm

Right, but... Sadly, such people often successfully pass job interview as well (in part because too many interviews are idiotic, but also because mad skills in answering questions the do not understand helps). And then more experienced developers get blame for the negative impact. How about some elementary self-preservation instinct? :-)
--SA

Member 8576081 16-Mar-12 6:09am

Same goes for you Mr SAKrykov. I hope you 'll stay away from my posts atleast..And learn some sense to deal with people..Being knowledeable doesnt give the license to talk rubbish about others.

Sergey Alexandrovich Kryukov 16-Mar-12 11:56am

Why? :-)

Sergey Alexandrovich Kryukov 16-Mar-12 12:01pm

By the way, please pay attention that I did not say a word personally about you (but you did make some conclusions personally about me).

As to my general statement, I can sign under each and every word I've written above. If you disagree with any, you are more then welcome criticize it or argue with me, but I would prefer if you address specific statements arguably. Your personal attach per se does not look impressive. If you have a point, please explain it.

Thank you.
--SA

Member 8576081 16-Mar-12 1:25am

What is your problem dude! If you cannot help, then simply stay away man..I am starting to work on tool to detect memory corruptions..I just wanted to understand the concepts from people who are more knowledgeable than me so that I get a better picture of the scenario...why do assume things and start commenting on people you dont know..What is wrong with you? You and this some SAKryukov keeps on poking your nose here and there instead of helping people..and if you cant help,fair enough ,just stay away man..Who cares about your idiotic opinions?

Sergey Alexandrovich Kryukov 16-Mar-12 12:05pm

Or I see. You are simply rude. This is not acceptable. You see, we just expressed some opinion, but you jumps to personal offense.

Anyway, I'm sorry to tell you that people will probably decide by themselves where to comment and where not. You hardly can affect it. Nobody can prohibit sharing any opinion, but offensive behavior can lead to ban, but don't worry, it does not usually happen.

--SA

Member 8576081 19-Mar-12 5:20am

"Sadly, such people often successfully pass job interview as well (in part because too many interviews are idiotic, " What do you mean by such people? Do you even know a single thing about me? What made you post such comments? And if you really thought it was some homework stuff you could have stated it in a single line that we do not solve assignments or this kind of stuff. finish. What made you post such comments. Even if I do not understand basic concepts. What is wrong with it. Is starting to learn wrong? Is'nt this site meant to provide an opportunity for people to learn? If you cannot help, dont pass unnecessary remarks and if you do be ready to face the replies.

Sergey Alexandrovich Kryukov 19-Mar-12 12:44pm

I've even started to write disclaimers "The text below is not related to OP" or something. I never mentioned you personally, and I never meant to discuss you in this text. I only reply other comment to other member related to certain sort of people. You know, it often happens that we discuss related matters. Let me assure you it had nothing to do with you or your questions on this page. Of course, as my post was a reason for your confusion and the following flame, I want to apologize.

By the way, I personally think that your questions make sense. Probably, you are trying to to get deeper understanding of the problem. (Well, you got some answers.)

But you should understand that it is no more than confusion. In any case, you should not be so rude. Pay attention, I did not try to offend or something you even in response to your baseless accusations on the other page.

--SA

Member 8576081 20-Mar-12 1:20am

Even I do not meant to be rude and why would I want to waste my time over such petty issues.I reacted only beacause this is the second time you are doing this on my post.Last time you posted something like " why do you think that anyone would reply after reading wierd statements like 'various possible syntax for memory leaks'". See I am here to learn and not to get into arguements. I felt bad reading your post and over reacted .Even I apologize for overreacting. I should not have gone to your other posts.But please think before you write as these things unnecessarily waste a lot of time and effort which we are already short of..

Sergey Alexandrovich Kryukov 20-Mar-12 2:05am

Great. Let's not waste time, agree on that.
Thank you.
--SA

michaelmel 15-Mar-12 20:40pm

Don't think it is a tricky question, probably a typo. I suspect in question #5 the esteemed professor meant to have the following ERRONEOUS code in line 2:
delete abc_ptr;
This would match the heading of the question #5.
The way it is written, e.g. delete [] abc_ptr; is correct.

Member 8576081 16-Mar-12 5:26am

Ouestion has been edited and the link of the article which i was referring to has been provided for those who refuse to help because they "SMELL" homework.

[no name] 16-Mar-12 5:42am

I have a bad sense of smell.

Member 8576081 16-Mar-12 6:11am

Atleast that does not stop you from helping others. :)

Member 8576081 16-Mar-12 6:24am

http://www.codeproject.com/Questions/346883/Scientific-Calculator-Project
Take a look at this. Here he(SAKryukov) is "SMELLING" homework and on this post he is providing his code where the person has specifically mentioned that he needs the code for his project . He is doing this for acknowledgement or promotion or what god knows.One thing is for sure that he certainly needs to be taught the right way to help others.Open any of his posts/answers and you'll see sarcasm in his replies. Mr SAKryukov you dont need to do this for the sake of codeproject MVP award. You have already recieved it and mentioned it on your site as well..What else do you want here?

nv3 16-Mar-12 18:19pm

I am also new here and hence neutral. What I can see from SAKryukov's messages in this forum is that he is always very eager to help. When you look at the questions in this forum, it is obvious that many youngsters are pasting their homework here. So I can understand the frustration of professionals that are devoting their time here. And, to be honest, I admire how patient they are even with kids that just want to get their homework done. This might not be the case in this post. But when I look at the website this member has quoted as origin of the question, I find all the explanations their -- at least for someone with a basic understanding of C. So the "smell" of homework is not totally unsubstancial. The least thing I would assume, however, is that any of the pro's are spending endless hours here to receive promotions or points.

Member 8576081 19-Mar-12 5:31am

Ok. I am totally fine with what you said. My point is that if he does not wanted to help , he must have stayed away.Simple. Who asked for his expert comments? And even he wanted to convey that "you should not pass your homework here" kinda thing. Do you think this is the right way to do it? I simply asked a question and had to face comments like "sadly such people pass interviews and stuff" .Do you think this is the right way to convey your message? who is he to decide that I dont deserve to pass an interview or I might be a pain in the neck for some senior developer? I dont understand how could you defend such unnecessary and rude remarks. Strange!

Member 8576081 19-Mar-12 5:42am

http://www.codeproject.com/Questions/348098/cplusplus-code-for-shortest-path-finderThis is what I mean. I hope you got my point. I am not arguing that you should start solving assignments being posted by kids.I just mean to say that you should respond and comment gracefully.

Sergey Alexandrovich Kryukov 19-Mar-12 12:48pm

@nv3:
Thank you very much for your good works.
In fact, you don't even have to find excuses.
By the way, I never ever mentioned that OP's (I mean Member 8576081 specifically) question was homework. Accusing me in that is pure lie.

And I don't think such rude behavior and such baseless accusations should be tolerated. I take responsibility for every word I write. In any case, "must stay away" stuff is unacceptable.

By the way, I already apologized for possible confusion. My post about people passing interviews never meant to have anything with OP, but that was a reason for confusion. But even this confusion should not give excuses for rudeness and personal accusations I received on this and other page OP referenced above.

--SA

Member 8576081 20-Mar-12 1:25am

Running away from your bad making excuses like "confusion" is also not acceptable. You'll get what you give . As simple as that.take care!

Sergey Alexandrovich Kryukov 20-Mar-12 1:56am

OK, fine. You don't need it. I never offended you, but you want to be rude. You are not welcome here then.
--SA

2 solutions

Solution 2

To avoid erroneous use of unassigned pointers all pointers should be set to NULL when declared. They should be set to NULL again when the memory they point to is freed.

The language specification does not require the compiler to do this.

If this is done then using the unassigned pointer will throw an exception.

For example #1 should be

C++

struct *abc_ptr = NULL;

#2 should be

C#

int *p = new int;
//code
delete p;
p = NULL;

and #5 should be

C++

void x()
{
  abc *abc_ptr = new abc[100];
  delete [] abc_ptr;
  abc_ptr = NULL;
}

In all the other examples the pointer should either point to valid memory or be set to NULL.

If the pointer is not set to NULL it could have any value and subsequent use of that pointer will have unpredictable results.

The is the cause of many subtle bugs.

Posted 15-Mar-12 13:25pm

User 59241

Updated 15-Mar-12 14:03pm

v4

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OriginalGriff · Accepted Answer · 2012-03-15T03:20:00

1) Using an address before memory is allocated and set
If you assign a pointer, that doesn't mean it points somewhere useful.
If you wanted to call your mother, would you just press buttons at random and talk to whoever answered?

2) Using a pointer which is already freed
If you delete a memory block, then any other memory allocation is free to use that area of memeory. Just because the pointer value has nit changed, does not mean that you can use the memory it points to!
If you sold your house, would you expect to still be allowed to go back there and sleep in "your" bedroom?

3. Freeing memory which has already been freed.
If it is already freed, then trying to free it again could cause problems - because the same block of memory would be in teh free list twice. So two different requests for memory could refer to the same physical memory.

4. Freeing memory which was not dynamically allocated

If you free memory which isn't allocated on the heap, then it is going to cause problems - the stack space will be reused by a different function, and any memory allocation which gets the freed block will refer to it at the same time...

5. Incorrect use of delete: The delete must match the use of new
If you free memory by calling it a different type to that which you allocated it, then it is quite possible that the memory freed is not the same size as the block that was allocated. As a result, either memory is no longer available to the heap when it should be, or the block freed could include memory which is not part of the allocation, but "owned" by a different memory block.