Re: Delete a list item from a separate thread safe?

"Igor Tandetnik" <>
Tue, 4 Mar 2008 18:03:17 -0500
Bruce. <> wrote:

I've got a list that will be accessed by multiple threads. The list
will be protected by a critical section.

However, I'm wondering about this case. Say one thread is using an
iterator to step through the list. The stepping from one list item
to the next will be in a critical section (but not while using the
list item).
Say the loop is on the 3rd item when a separate thread deletes the
3rd item. The deletion will be done inside the same critical section.

Essentially this means thread 1 will be processing item 3 while it is
being deleted by thread 2. Then thread 1 will increment the iterator
to step from item 3 (now deleted) to item 4 (now 3).

Will the iterator in thread 1 still step correctly if the item it's
stepping from has been deleted?

Highly unlikely. The node will actually be deallocated, so the iterator
becomes a dangling pointer, pointing to a free (or perhaps even
reallocated and reused for something completely unrelated) memory block.
This will exhibit undefined behavior even before you try to increment
the iterator.

Formally, when an element is erased, all iterators referring to it
become invalid. The only action safe to perform on an invalid iterator
is to assign to it a value of some other iterator.
With best wishes,
    Igor Tandetnik

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