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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, int who);
int setpriority(int which, int 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. Prio is a value in the range
-20 to 19 (but see the Notes below). The default priority is 0;
lower priorities 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
specified value. Only the superuser may lower priorities.
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.
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
capability). But see NOTES below.
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD),
POSIX.1-2001.
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. Within the kernel, nice values are actually
represented using the corresponding 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.
On some 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>.)
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)
Documentation/scheduler/sched-nice-design.txt in the Linux kernel
source tree (since Linux 2.6.23)
This page is part of release 3.51 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2013-02-12 GETPRIORITY(2)
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