Multithreaded C++ into function blocks - PLCOpen

[EinarM]

Hi there
I'm an embedded software engineer mostly programming C/C++ in Linux and have very little knowledge of the PLC environment. 
Recently my attention was diverted towards integrating C/C++ (Linux software) into PLC and then presumably as a function block which is supported with something called PLCOpen. Is that right ?

How is multithread support in PLC in general ? and/or with the these function blocks or what ever might become instead when integrating/porting C/C++ code into PLC. 

The current situation is purely embedded Linux environment with open-source 3rd party userland that our code relies heavily on. How feasible is to port such environment over to PLC (given that the system supports it) ?

I've been looking into B&R Automation Studio 4 and having a hard time to establish concrete feeling on how difficult/easy it is to port the code. 

Hope anyone can give me a slight insight into this. 
Kind regards
Einar