Re: std::deque Thread Saftey Situtation

peter koch <>
Fri, 29 Aug 2008 16:07:24 -0700 (PDT)
On 30 Aug., 00:49, NvrBst <> wrote:

I've read a bit online seeing that two writes are not safe, which I
understand, but would 1 thread push()'ing and 1 thread pop()'ing be


Basically my situation is the follows:

--Thread 1--
1. Reads TCPIP Buffer
2. Adds Buffer to Queue (q.size() to check queue isn't full, and
3. Signals Reading Thread Event & Goes Back to Wait for More Messages

--Thread 2--
1. Waits for Signal
2. while(q.size() > 0) myStruct = q.front();
2a. processes myStruct (doesn't modify it but does copy some
information for another queue).
2b. delete[] myStruct.Buffer
2c. q.pop_front();
3. Goes back to waiting for single

I've run the app for a few hours (basically saturating the TCPIP
traffic) and there were no apparent problems. The "processes
myStruct" takes a while and I can't have the push_back(...) thread
locked while processing is working.

If it did not fail, you were simply lucky. You will surely run out of
luck some day.

I can add Critical Section locks around the ".size()", ".front()" and/
or ".pop_front()/.push_back()", but my first inclination is that this
is thread safe?

It is thread-safe only when you protect everything with a mutex (which
could very well be a windows CRITICAL_SECTION).


If Critical Sections are needed, would it just be fore the
".pop_front()/.push_back()"? or all member functions I'm using

On all of them.

Thanks. Any information would be greatly appreciated. These are the
only two threads/functions accessing the queue. I currently
implemented it with locking on all just to be safe, but would like to
remove the locking if it is not needed, or fine tune it.

Did you measure the performance without a lock? Presumably, it would
not really make a difference.


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