Re: std::deque Thread Saftey Situtation

NvrBst <>
Fri, 29 Aug 2008 19:35:43 -0700 (PDT)
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
CriticalSections? The reasion I put the locks/unlocks outside the
whileloop was solely for that reasion. I actually already have a
wraper like you listed "CriticalSection::Owner
sLock(m_Q[TCPIP].qLock);" which does what you outlined, but I failed
to realize I could put empty curly brackets around the section I want,
to force it out of scope :)

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.

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.

Since .size() was a method call instead of a property, I was thinking
there might be more to it in c++... Which I'm not confused on again:
".size()" ".front()" would both be thread-safe operations for a thread
thats removes elements (being the only thread that removes items)? In
C# Enqueue/Dequeue arn't thread safe, but again, depends on
implementations, which is why I asked here; I would be suprised if
they were, but thought maybe.

So in essance I can change it to the following if I wanted to fine
tune the thread-safty locking to it's bare minimium in my example (and
remove my try statment to boot, yay)?

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

Thanks, NB

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