nice - change process priority
nice(2) adds inc to the nice value for the calling pid. (A large nice value means a low priority.) Only the super- user may specify a negative increment, or priority increase.
On success, the new nice variable is returned. On failure -1 is returned. Since -1 is a valid return for nice(2), set errno(3) to 0 before calling nice(2), and check its value afterwards if nice(2) returns a value of -1.
SVr4, SVID EXT, AT
Older (g)libc (earlier than glibc 2.2.4) return 0 on success, and the new nice value should be found by calling getpriority(2).