DESCRIPTION

The scheduling priority of the process, process group, or user, as indicated by which and who is obtained with the getpriority() call and set with the setpriority() call. The process attribute dealt with by these system calls is the same attribute (also known as the "nice" value) that is dealt with by nice(2).

The value which is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and who is interpreted relative to which (a process identifier for PRIO_PROCESS, process group identifier for PRIO_PGRP, and a user ID for PRIO_USER). A zero value for who denotes (respectively) the calling process, the process group of the calling process, or the real user ID of the calling process.

The prio argument is a value in the range −20 to 19 (but see NOTES below). with −20 being the highest priority and 19 being the lowest priority. The default priority is 0; lower values give a process a higher scheduling priority.

The getpriority() call returns the highest priority (lowest numerical value) enjoyed by any of the specified processes. The setpriority() call sets the priorities of all of the specified processes to the specified value.

Traditionally, only a privileged process could lower the nice value (i.e., set a higher priority). However, since Linux 2.6.12, an unprivileged process can decrease the nice value of a target process that has a suitable RLIMIT_NICE soft limit; see getrlimit(2) for details.

RETURN VALUE

Since getpriority() can legitimately return the value −1, it is necessary to clear the external variable errno prior to the call, then check it afterward to determine if −1 is an error or a legitimate value. The setpriority() call returns 0 if there is no error, or −1 if there is.

ERRORS

In addition to the errors indicated above, setpriority() may fail if:

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first appeared in 4.2BSD).

NOTES

A child created by fork(2) inherits its parent's nice value. The nice value is preserved across execve(2).

The degree to which their relative nice value affects the scheduling of processes varies across UNIX systems, and, on Linux, across kernel versions. Starting with kernel 2.6.23, Linux adopted an algorithm that causes relative differences in nice values to have a much stronger effect. This causes very low nice values (+19) to truly provide little CPU to a process whenever there is any other higher priority load on the system, and makes high nice values (−20) deliver most of the CPU to applications that require it (e.g., some audio applications).

The details on the condition for EPERM depend on the system. The above description is what POSIX.1-2001 says, and seems to be followed on all System V-like systems. Linux kernels before 2.6.12 required the real or effective user ID of the caller to match the real user of the process who (instead of its effective user ID). Linux 2.6.12 and later require the effective user ID of the caller to match the real or effective user ID of the process who. All BSD-like systems (SunOS 4.1.3, Ultrix 4.2, 4.3BSD, FreeBSD 4.3, OpenBSD-2.5, ...) behave in the same manner as Linux 2.6.12 and later.

The actual priority range varies between kernel versions. Linux before 1.3.36 had −infinity..15. Since kernel 1.3.43, Linux has the range −20..19. On some other systems, the range of nice values is −20..20.

Including <sys/time.h> is not required these days, but increases portability. (Indeed, <sys/resource.h> defines the rusage structure with fields of type struct timeval defined in <sys/time.h>

BUGS

According to POSIX, the nice value is a per-process setting. However, under the current Linux/NPTL implementation of POSIX threads, the nice value is a per-thread attribute: different threads in the same process can have different nice values. Portable applications should avoid relying on the Linux behavior, which may be made standards conformant in the future.

SEE ALSO

nice(1), renice(1), fork(2), capabilities(7), sched(7)

Documentation/scheduler/sched-nice-design.txt in the Linux kernel source tree (since Linux 2.6.23)