The parallel mode STL algorithms are currently not exception-safe, i.e. user-defined functors must not throw exceptions. Also, the order of execution is not guaranteed for some functions, of course. Therefore, user-defined functors should not have any concurrent side effects.

Since the current GCC OpenMP implementation does not support OpenMP parallel regions in concurrent threads, it is not possible to call a parallel STL algorithm in concurrent threads, either. It might work with other compilers, though.