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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
GETPRIORITY(2) Linux Programmer's Manual GETPRIORITY(2)
getpriority, setpriority - get/set program scheduling priority
#include <sys/time.h>
#include <sys/resource.h>
int getpriority(int which, id_t who);
int setpriority(int which, id_t who, int prio);
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. Attempts to set a priority outside this range are silently
clamped to the range. 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.
On success, getpriority() returns the calling thread's nice value,
which may be a negative number. On error, it returns -1 and sets
errno to indicate the cause of the error. Since a successful call to
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.
setpriority() returns 0 on success. On error, it returns -1 and sets
errno to indicate the cause of the error.
EINVAL which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.
ESRCH No process was located using the which and who values
specified.
In addition to the errors indicated above, setpriority() may fail if:
EACCES The caller attempted to set a lower nice value (i.e., a higher
process priority), but did not have the required privilege (on
Linux: did not have the CAP_SYS_NICE capability).
EPERM A process was located, but its effective user ID did not match
either the effective or the real user ID of the caller, and
was not privileged (on Linux: did not have the CAP_SYS_NICE
capability). But see NOTES below.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first
appeared in 4.2BSD).
For further details on the nice value, see sched(7).
Note: the addition of the "autogroup" feature in Linux 2.6.38 means
that the nice value no longer has its traditional effect in many
circumstances. For details, see sched(7).
A child created by fork(2) inherits its parent's nice value. The
nice value is preserved across execve(2).
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.
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>.)
C library/kernel differences
Within the kernel, nice values are actually represented using the
range 40..1 (since negative numbers are error codes) and these are
the values employed by the setpriority() and getpriority() system
calls. The glibc wrapper functions for these system calls handle the
translations between the user-land and kernel representations of the
nice value according to the formula unice = 20 - knice. (Thus, the
kernel's 40..1 range corresponds to the range -20..19 as seen by user
space.)
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.
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)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-12-12 GETPRIORITY(2)
Pages that refer to this page: renice(1), getrlimit(2), ioprio_set(2), nice(2), sched_rr_get_interval(2), sched_setaffinity(2), sched_setattr(2), sched_setparam(2), sched_setscheduler(2), syscalls(2), errno(3), proc(5), systemd.exec(5), capabilities(7), credentials(7), pid_namespaces(7), pthreads(7), sched(7)