Re: [Boost-users] [BOOST THREAD] Threads Spawning Unexpectedly

Subject: Re: [Boost-users] [BOOST THREAD] Threads Spawning Unexpectedly
From: Terrimane Pritchett (shon.pritchett_at_[hidden])
Date: 2009-04-28 17:28:24

Hello everyone,

Sorry for the delay in coming back to you.  I managed to cause my
workstation to overheat and die yesterday and had to wait for a replacement.

I have managed to get my multi-threaded implementation to run without any
obvious problems.  Basically I have to make the following the following
actions atomic by rapping them in a function call:

1. Invoke an external library to pase my file give me some data in one big
blob
2. Instantiate boost::tokenizer to tokenize the data
3. Convert the data to usable types with boost::lexical_cast
4 Return the usable data to the caller of the function

I really cannot say with certainty what went wrong but I will offer my best
estimation.

I use an external library to parse my files.  The library relies on
std::stringstream to convert its internal types to byte strings.  I had
multiple threads calling the same functions from the library and given
std::stringstream is not thread safe there was opportunity for the data I
requested to get corrupted before my code ever touched it.

I use boost::lexical_cast and boost::tokenizer extensively both of which are
not guaranteed to be thread safe - because this is dependent upon the thread
safety guarantees or lack there of standard c/c++ library implementations
they are built on top of.  Particualy the standard does not require thread
safety for std::stringstream, std::locale or the cctype library.  So
potentially multiple threads all using boost::lexical_cast or
boost::tokenizer could have caused my data to be corrupted.

These are all only precursory findings - so take them for what you will.

The obvious solution was to make getting raw data and converting it to
something useable as atomic as possible and then imposing my own thread
safety measures myself.

I've been able to complete several passes of my test units without any
problems after making these changes for both debug and release builds.

On the issue of my extra threads - I can only say for every boost::thread I
spawn there is another thread spawned.  The companion thread is something
that spins around in the function "free" until the boost::thread completes.
I'm sure it's an implementation detail I shouldn't concern myself about
since the behavior is predicatable and doesn't appear to be causing my
application any adverse affects.  Now that my application runs to completion
without crashing I see these "extra" threads dissappear as my boost::threads
dissappear so I've probably been fretting over nothing.

If I've obviously misunderstood, mistepped, or misrepresented something
please correct me.

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