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GETPRIORITY(2) Linux Programmer's Manual GETPRIORITY(2)
NAME
getpriority, setpriority - get/set program scheduling priority
SYNOPSIS
#include <sys/time.h>
#include <sys/resource.h>
int getpriority(int which, int who);
int setpriority(int which, int who, int prio);
DESCRIPTION
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 setprior-
ity() call.
The value which is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and who is inter-
preted 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 pro-
cess, 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 priori-
ties of all of the specified processes to the specified value. Only the superuser
may lower priorities.
RETURN VALUE
Since getpriority() can legitimately return the value -1, it is necessary to clear
the external variable errno prior to the call, then check it afterwards to deter-
mine if -1 is an error or a legitimate value. The setpriority() call returns 0 if
there is no error, or -1 if there is.
ERRORS
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.
CONFORMING TO
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.
NOTES
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 ker-
nel, 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 repre-
sentations 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>.)
SEE ALSO
nice(1), fork(2), capabilities(7), renice(8)
Documentation/scheduler/sched-nice-design.txt in the kernel source tree (since
Linux 2.6.23).
COLOPHON
This page is part of release 3.22 of the Linux man-pages project. A description of
the project, and information about reporting bugs, can be found at http://www.ker-
nel.org/doc/man-pages/.
Linux 2008-05-29 GETPRIORITY(2)
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