GETPRIORITY(2) Linux Programmer's Manual GETPRIORITY(2)
getpriority, setpriority - get/set program scheduling priority
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 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). The default priority is 0; lower priori-
ties cause more favorable scheduling.
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. Only the superuser may lower priorities.
Since getpriority() can legitimately return the value -1, it is neces-
sary 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.
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 lower a process priority, but did not
have the required privilege (on Linux: did not have the
CAP_SYS_NICE capability). Since Linux 2.6.12, this error occurs
only if the caller attempts to set a process priority outside
the range of the RLIMIT_NICE soft resource limit of the target
process; see getrlimit(2) for details.
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.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first
appeared in 4.2BSD).
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 ver-
sions. 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 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.
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 porta-
bility. (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)
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Linux 2015-07-23 GETPRIORITY(2)
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