The definition of exactly what is a system call and what is not varies by platform. For example, some OSes may implement sleep() as a system call; others may im

The definition of exactly what is a system call and what is not varies by platform. For example, some OSes may implement sleep() as a system call; others may implement it using nanosleep() or even alarm() and sigwait(). Even fork() has been implemented as a library function, eg in cygwin. What method is used to implement low-level library functions like this should be considered an implementation detail.