Generic methods for checking if a library/API is thread safe

Question

I received a library from an external developer in form of a well defined API(in C++ and Java). What can be some tests to check if the library is thread-safe ?

Answer 1

Basically you can't, it's more or less impossible to test for thread-safety.

And also if you don't have the author's guarantee that the library is thread-safe then they aren't going to fix threading issues, so future versions might be less thread-safe.

If you've got the source code, then you can investigate common thread-safety issues: shared state, locks etc. But if you've only got binaries, then the best you can hope is to show that the library is not thread-safe. Even then reproducing the problems reliably might be extremely difficult.