when creating a thread we pass an entry point method/function , why should I have this method , what is the purpose of it?
OS needs to know where a new thread of execution starts. When using a high-level programming language, one does not specify an address of machine instructions in memory to be executed in the context of a new thread, but uses execution units defined in the language like functions or methods. If thread creation worked like fork and execution of a new thread started at the point of fork invocation, then both threads would have the same local variables that usually reside in stack. Even if there is a copy of the stack created for a new thread, both threads will run the same clean-up code when leaving scopes (e.g., in C++ a smart pointer will be freed twice). So when you specify a starting point for a new thread, you are sure it will allocate a stack-frame of its own and function's epilog won't be executed twice.