Thread: Delete keyword??

I am trying to delete a node in a linked list, and I get a seg fault like this:

Code:

Program received signal SIGSEGV, Segmentation fault.
0x0804da34 in SCLLRep::Merge (this=0x82499a0, set2=0x82499f0) at SCLL.cpp:187
187                     delete set2_NodeToDelete;
(gdb)

Declared like this:

Code:

	SCLLNode* set2_NodeToDelete = new SCLLNode();
	set2_NodeToDelete = set2->Getptr2last()->Getnextnode();

(Tried without the new keyword too)

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What could be a possible problem that would cause this to result in a seg fault?

The thing that confuses me is that it doesn't even execute anything. I just say "delete variable" and BAM, seg fault.

Probably you try to delete something twice, or you try to delete an invalid pointer.
But without any additional code, we can't say for certain.

Code:

set2_NodeToDelete = set2->Getptr2last()->Getnextnode();

In common linked lists, "next" after "last" is always NULL... If so, then you'd get a segfault if you try to delete that node, for sure.

The thing that confuses me is that it doesn't even execute anything. I just say "delete variable" and BAM, seg fault.

Yeah, but "delete" itself does "execute something", in the sense that it expands to:

Code:

this->~SCLLNode();
deallocate(this);

If "this" is not a valid pointer, it can go wrong in either of those two statements.

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Mats

Well, I doubt that the gcc implementation of delete is broken, so if you try to delete something and it crashes, then I'd say it's your code. Show us the code, and maybe we can lead you in the right direction even if we can't spot it directly.

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Mats

It would crash before delete if set2->Getptr2last returns NULL, since deleting NULL is perfectly defined... but since this (probably) isn't the case, then there must be something else wrong.
Perhaps an invalid pointer to freed storage or a pointer corrupted due to some memory corruption.

Originally Posted by Elysia

It would crash before delete if set2->Getptr2last returns NULL, since deleting NULL is perfectly defined... but since this (probably) isn't the case, then there must be something else wrong.
Perhaps an invalid pointer to freed storage or a pointer corrupted due to some memory corruption.

Good point - NULL itself is safe to delete and won't call the destructor. So this turns into the more subtle problem of "overwritten data" or "double delete", both of which can lead to "mysterious" problems.

Again, seeing the source would be beneficial.

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Mats

Unfortunately, unless you can reduce the problem to something smaller, you have to debug your 1000's of lines. Showing us 0.1% of the code will not tell us what is wrong. The pieces you show, are not wrong in themselves.

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Mats

Code:

	SCLLNode* set2_NodeToDelete = new SCLLNode();
	set2_NodeToDelete = set2->Getptr2last()->Getnextnode();

So first you assign to the variable the address of a new SCLLNode object.
Then you overwrite this address with whatever those calls return.
You leak the object you allocated.
You try to delete something entirely different.