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getpriority(2) System Calls Manual getpriority(2)
NAME
getpriority, setpriority - get/set program scheduling priority
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/resource.h>
int getpriority(int which, id_t who);
int setpriority(int which, id_t who, int prio);
DESCRIPTION
The scheduling priority of the process, process group, or user, as in-
dicated 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 be-
low), with -20 being the highest priority and 19 being the lowest pri-
ority. 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 spec-
ified value.
Traditionally, only a privileged process could lower the nice value
(i.e., set a higher priority). However, since Linux 2.6.12, an unpriv-
ileged 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
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 error.
Since a successful call to getpriority() can legitimately return the
value -1, it is necessary to clear errno prior to the call, then check
errno afterward to determine if -1 is an error or a legitimate value.
setpriority() returns 0 on success. On failure, it returns -1 and sets
errno to indicate the error.
ERRORS
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).
EINVAL which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.
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 capabil-
ity). But see NOTES below.
ESRCH No process was located using the which and who values specified.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4, 4.4BSD (these interfaces first appeared in 4.2BSD).
NOTES
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 cir-
cumstances. 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 Linux 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 effec-
tive 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.
C library/kernel differences
The getpriority system call returns nice values translated to the range
40..1, since a negative return value would be interpreted as an error.
The glibc wrapper function for getpriority() translates the value back
according to the formula unice = 20 - knice (thus, the 40..1 range re-
turned by the kernel corresponds to the range -20..19 as seen by user
space).
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 re-
lying 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)
Linux man-pages 6.04 2023-03-30 getpriority(2)
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