Just another example, among the infinite others: you fsycn a filedes in a thread, then write to the same fd from another thread, and write(2) will block. Tens o

Just another example, among the infinite others: you fsycn a filedes in a thread, then write to the same fd from another thread, and write(2) will block. Tens of semantical behaviors like this are NOT part of POSIX, but are part of real world systems. So anyway in order to write non trivial systems you have to understand the implementation of different system calls in different operating systems.