Re: std::deque Thread Saftey Situtation

peter koch <>
Sat, 30 Aug 2008 02:31:30 -0700 (PDT)
On 30 Aug., 04:35, NvrBst <> wrote:

Ahh thank you. I find examples a lot easier to follow, one question:

some_struct_type myStruct;
while(q.size() > 0) {
  myStruct = q.front();


This would imply the ".size()" doesn't have to be wrapped in the

Yes - that is an error with the code: it should be wrapped.


Most my confusion comes from my C# background (java, and then C# is
what I grew up with), and in C#'s Queue, the size property is stored
as an Int32 which, when reading, is an atomic operation on mostly all
processors. Meaning it even read it before another thread changes it,
or after; either case is fine (thread safe) if there are only two
threads, and each thread either pop's or push's, thus can't cause an
exception. This is because in C# the size doesn't get incremented
untill after the insertion is completed, so exceptions can't occure in
my situation.

This is not correct, and your C# code will not work unless there is
more action involved when reading the Int32 size. The reason is that
even though the size gets written after the data, it might not be seen
this way by another thread. Memory writes undergo a lot of steps
involving caches of different kinds, and the cacheline with the size
might be written to memory before the cache-line containing the data.

I was thinking ".front()" should also be thread safe for the same (C#-
Based) reasion, since it's a 32bit applications, and .front() should
be returning a pointer to the first element. Since the thread
calling .front() is the only thread who's removing elements, and since
this thread knows that ".size()" shows one element, then I would of
assumed ".front()" would be thread-safe as well.

You have the same problem here.



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