Re: Semantics of STL containers (std::map) in a multithreaded scenario

Pete Becker <>
Thu, 15 Jun 2006 11:43:01 -0400
Joe Seigh wrote:

Pete Becker wrote:

Dilip wrote:

Pete.. another poster (redfloyd?) also pointed out the same thing. I
am trying to wrap my head around this one. Just to make sure I
understand: there is no guarantee what can happen if you try to
perform any operation on a map when another operation is an interrupted
state, right? That is an incomplete operation leaves the internal
state of the map inconsistent. Just so that I can convince my
colleagues with some esoteric knowledge can you give me an example of
what _might_ be the state of a map when a thread performing an insert
operation is interrupted because its time-slice ran out and another
thread mistakenly tries to read from the same map?

Well, without looking at the code, one possibility is that one of the
tree nodes will have a child pointer that points to its parent.
Another is that some node will have multiple parents.

It's not a matter of what the map's internal structure might be, but
of whether there's anything you can make sense of. For example, if
writing the value of an int is not atomic, then it's possible to get a
time slice between writing two parts of its value; if another thread
then looks at the stored value, it will be nonsense.

unsigned int i = 0;

in one thread:
    i = 0xffffffff;
If it gets sliced between writing the upper and lower halves of the
value, then the other thread sees a value of 0xffff0000 or 0x0000ffff.
Neither one makes sense. Same problem with pointer updates, which
occur frequently when rebalancing a tree.

That's a known issue. The usual solution is to use atomic types
with the right guarantees, i.e. atomicity and acquire or release
semantics if needed. STL don't use atomic types so you don't know
if they can be used in this way since it's compiler and platform

Yes, that is the point of the example.

The more insideous problem is you can't just arbitrarily move things
around and expect that the proper semantics will hold.

It's essentially the same issue: if you don't have a guarantee of
atomicity, all bets are off. (Unless you have some other eplicit guarantee)


Pete Becker
Roundhouse Consulting, Ltd.

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