#include <unistd.h> int nice(int incr);
The nice() function allows a process to change its priority. The invoking process must be in a scheduling class that supports the nice().
The nice() function 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 greater positive value results in lower CPU priority.
A maximum nice value of (2 * NZERO) −1 and a minimum nice value of 0 are imposed by the system. NZERO is defined in <limits.h> with a default value of 20. Requests for values above or below these limits result in the nice value being set to the corresponding limit. A nice value of 40 is treated as 39.
Calling the nice() function has no effect on the priority of processes or threads with policy SCHED_FIFO or SCHED_RR.
Only a process with the {PRIV_PROC_PRIOCNTL} privilege can lower the nice value.
Upon successful completion, nice() returns the new nice value minus NZERO. Otherwise, −1 is returned, the process's nice value is not changed, and errno is set to indicate the error.
The nice() function will fail if:
The nice() function is called by a process in a scheduling class other than time-sharing or fixed-priority.
The incr argument is negative or greater than 40 and the {PRIV_PROC_PRIOCNTL} privilege is not asserted in the effective set of the calling process.
The priocntl(2) function is a more general interface to scheduler functions.
Since −1 is a permissible return value in a successful situation, an application wishing to check for error situations should set errno to 0, then call nice(), and if it returns −1, check to see if errno is non-zero.
See attributes(5) for descriptions of the following attributes:
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nice(1), exec(2), priocntl(2), getpriority(3C), attributes(5), privileges(5), standards(5)