int nice(incr)
The nice value of the process is changed by incr. Positive nice values get less service than normal. See nice.1 for a discussion of the relationship of nice value and scheduling priority.
A nice value of 10 is recommended to users who wish to execute long-running programs without undue impact on system performance.
Negative increments are illegal, except when specified by the super-user. The nice value is limited to the range -20 (most urgent) to 19 (least). Requests for values above or below these limits result in the nice value being set to the corresponding limit.
The nice value of a process is passed to a child process by fork.2v For a privileged process to return to normal nice value from an unknown state, nice() should be called successively with arguments -40 (goes to nice value -20 because of truncation), 20 (to get to 0), then 0 (to maintain compatibility with previous versions of this call).
The maximum allowed value for incr is 40 (least urgent).
nice() returns:
nice() returns the new nice value on success. On failure, it returns -1 and sets errno to indicate the error.
The nice value is not changed if:
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