I am trying to understand what happeneds when a native thread with stack size specified while creating it is attached to a JVM which has -Xss property set. More specifically are these two settings different or if they are same what happens will the native stack size be changed when attached to the JVM?
What happens ...
Nothing. Your thread continues to exist the way you created it. -Xss is for threads created by the JVM, possibly only for Java threads.
As a matter of fact in the threading APIs I am familiar with, there is no way to change a thread's stack size after creation.