Wednesday, October 19, 2005

@#$% Threads

There are many different paradigms for concurrent programming and threads is the worst. We are working on a sophisticate threaded system. It uses supposedly standard technology, pthreads on Linux. The number and variety of problems is quite astonishing. Recently we have had some problems with thread termination.

One "little" problem is that part of the system links with a well known database management system. The database installs a thread exit handler which clean up whenever a thread exits. The problem is that not all out threads access the database system. When we cancel a thread that has not run any database code the database exit handler is called and because it has not been initialized, it SEGVs and brings down the whole system.

Another "little" problem is that I moved our code to a new system and could reliably get it to spin forever in the pthread_join procedure. As this comes just after a call to pthread_cancel, the conclusion is that the pthreads library on that system is not thread safe. The pthreads library is in the same location as it is on the system that works, however it is not exactly the same, which suggests that we have a duff pthreads library. After spending a couple of hours, I could not find out where the thread library on either system came from.

Neither of the problems is really a problem with threads, however they are typical of what a threads programmer has to deal with day to day. I have much to unload on real threads problems that we can look at other times. This is important topic as concurrent programming is the way of the future.

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