It's never lupus nor a compiler bug

While testing “Project: Sippy-Cup [1],” one of the components I was using for testing asserted [2]:

tpp.c:63: __pthread_tpp_change_priority: Assertion `new_prio == -1 || (new_prio >= __sched_fifo_min_prio && new_prio <= __sched_fifo_max_prio)' failed.

This is not good. This wasn't our code that called assert(), but pthreads [3].

Sigh.

The hardest part of debugging this was getting it to happen consistently (yes, it's one of those bugs [4]) to test out various hypotheses.

It wasn't neglecting to initialize a variable [5]. It wasn't a compiler bug (It's never lupus!) [6]. It turned out to be a abuse of the mutex API (Application Program Interface) [7], where one thread locks a mutex, and some other thread attempts to unlock the mutex.

=> [1] /boston/2014/03/05.1 | [2] http://pubs.opengroup.org/onlinepubs/009695399/functions/assert.html | [3] https://en.wikipedia.org/wiki/POSIX_Threads | [4] https://en.wikipedia.org/wiki/Race_condition | [5] http://stackoverflow.com/questions/21825291/threading-issues | [6] https://gcc.gnu.org/ml/gcc-bugs/2006-10/msg00805.html | [7] http://stackoverflow.com/questions/1105745/pthread-mutex-assertion-error

=> Gemini Mention this post | Contact the author