2.1. What is Thread Safety?

Defining thread safety is surprisingly tricky. The more formal attempts are so complicated as to offer little practical guidance or intuitive understanding, and the rest are informal descriptions that can seem downright circular. A quick Google search turns up numerous “definitions” like these:

. . . can be called from multiple program threads without unwanted interactions between the threads.

. . .may be called by more than one thread at a time without requiring any other action on the caller’s part.