nice(2)

NAME
     nice - change priority of a time-sharing process

SYNOPSIS
     #include <unistd.h>
     int nice(int incr);

DESCRIPTION
     nice allows a process in the time-sharing scheduling class to change its
     priority.  The sched_setscheduler and schedctl system calls are more
     general interfaces to scheduler functions.

     nice adds the value of incr to the nice value of the calling process.  A
     process's nice value is a non-negative number for which a more positive
     value results in lower CPU priority.

     A maximum nice value of 39 and a minimum nice value of 0 are imposed by
     the system.  (The default nice value is 20.)  Requests for values above
     or below these limits result in the nice value being set to the
     corresponding limit.

     EPERM         nice fails and does not change the nice value if incr is
                   negative or greater than 40 and the effective user ID of
                   the calling process is not super-user.

     EINVAL        nice fails if called by a process in a scheduling class
                   other than time-sharing.

SEE ALSO
     exec(2), setpriority(2), sched_setscheduler(2).  schedctl(2).
     csh(1), sh(1), nice(1).

DIAGNOSTICS
     Upon successful completion, nice returns the new nice value minus 20.
     Otherwise, a value of -1 is returned and errno is set to indicate the
     error.