Threads

The YubiKey is effectively single-threaded. Although there really is no concept of a thread on a YubiKey, it is best to think of all operations on the YubiKey happening in the same thread.

Suppose you have a multithreaded application. Now suppose you make a connection to a YubiKey in one of those threads, and then make another connection in a separate thread. You will not get two connections operating independently. Any call to the YubiKey in one thread will affect the internal state, which will affect any call from the other thread.

Because of this, the SDK is built to be run in a single thread. You should write your code accordingly. Either make sure all interactions with the YubiKey are handled by one thread only, or use locks to guarantee only one thread at a time calls to the YubiKey.