Is a shared list between multiple threads a race condition?

Answer 1

STL containers are not intrinsically thread-safe. You need to protect access to them using a synchronization mechanism. The typical ones could be a semaphore or a mutex, with the mutex being in general the faster of the two.

Answer 2

Disclaimer: I know nothing about OpenMP specifically. However I can say that yes, two threads doing push_back (or any other modifying operation) on a list at the same time will cause problems, just the same as for a single variable.

I don't know what tools OpenMP gives you for protecting against this. Some common approaches to avoiding this problem:

1. Put a lock (eg a mutex) around operations on the variable.
2. Give each thread its own copy of the list, and have them remain independent. At the end of the process, as a separate step, you can merge the results from the different threads. (This is Map-Reduce, pretty much).

The second approach can give better results if you have many threads, and if it fits with your algorithm. Some algorithms can't be structured that way.

If you don't have many threads, depending on the size of your loop body, a simple lock might be the most effective solution.