| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON | The Linux Programming Interface |
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 only occurs 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>.)
nice(1), renice(1), fork(2), capabilities(7)
Documentation/scheduler/sched-nice-design.txt in the kernel source tree (since
Linux 2.6.23).
This page is part of release 3.32 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 2008-05-29 GETPRIORITY(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface