One frequently used system call is gettimeofday(2).  This system call
       is called both directly by user-space applications as well as
       indirectly by the C library.  Think timestamps or timing loops or
       polling—all of these frequently need to know what time it is right
       now.  This information is also not secret—any application in any
       privilege mode (root or any unprivileged user) will get the same
       answer.  Thus the kernel arranges for the information required to
       answer this question to be placed in memory the process can access.
       Now a call to gettimeofday(2) changes from a system call to a normal
       function call and a few memory accesses.