Threading

Threads provide a system programmer with a means to perform parallel execution. Specifically, a thread is a unit of execution that the operating system schedules when it deems appropriate. Both C++ and POSIX provide APIs for working with threads, with the C++ APIs arguably being easier to work with.

It should be noted that, under the hood, C++ leverages the POSIX threads library (pthreads)—so, even though C++ provides a set of APIs for working with threads, in the end, POSIX threads are responsible for threading in all cases. 

The reason for this is simple. POSIX defines the interface a program leverages to talk to the operating system. In this case, if you wish to tell the operating system to create a thread, you must do so by leveraging the APIs defined by the operating system. If the operating system is POSIX-compliant, those interfaces are POSIX, regardless of any abstractions that might be put in place to make working with the APIs easier.