nice(3)
Name
nice − set program priority
Syntax
int nice(incr)
int incr;
Description
The scheduling priority of the process is augmented by incr. Positive priorities get less service than normal. Priority 10 is recommended to users who wish to execute long-running programs without flack from the administration.
Negative increments are ignored except on behalf of the super-user. The priority is limited to the range −20 (most urgent) to 20 (least).
The priority of a process is passed to a child process by fork(.). For a privileged process to return to normal priority from an unknown state, nice should be called successively with arguments −40 (goes to priority −20 because of truncation), 20 (to get to 0), then 0 (to maintain compatibility with previous versions of this call).
Environment
In any mode, nice returns -1 and sets errno on an error. On success, the return value depends on the mode in which your program was compiled. In POSIX or System V mode, it is the new priority; otherwise, it is zero. Note that, in POSIX and System V mode, -1 can indicate either success or failure; errno must be used to determine which.