#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.
Only a process with super-user privileges 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.
The inc argument is negative or greater than 40 and the effective user ID of the calling process is not super-user.
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.