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.\"     @(#)getpriority.2	6.9 (Berkeley) 3/10/91
.\"
.\" Modified 1993-07-24 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1996-07-01 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 1996-11-06 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 2001-10-21 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"    Corrected statement under EPERM to clarify privileges required
.\" Modified 2002-06-21 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"    Clarified meaning of 0 value for 'who' argument
.\" Modified 2004-05-27 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.TH GETPRIORITY 2 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
getpriority, setpriority \- get/set program scheduling priority
.SH SYNOPSIS
.B #include <sys/time.h>
.br
.B #include <sys/resource.h>
.PP
.BI "int getpriority(int " which ", id_t " who );
.br
.BI "int setpriority(int " which ", id_t " who ", int " prio );
.SH DESCRIPTION
The scheduling priority of the process, process group, or user, as
indicated by
.I which
and
.I who
is obtained with the
.BR getpriority ()
call and set with the
.BR 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
.BR nice (2).
.PP
The value
.I which
is one of
.BR PRIO_PROCESS ,
.BR PRIO_PGRP ,
or
.BR PRIO_USER ,
and
.I who
is interpreted relative to
.I which
(a process identifier for
.BR PRIO_PROCESS ,
process group
identifier for
.BR PRIO_PGRP ,
and a user ID for
.BR PRIO_USER ).
A zero value for
.I who
denotes (respectively) the calling process, the process group of the
calling process, or the real user ID of the calling process.
.PP
The
.I prio
argument is a value in the range \-20 to 19 (but see NOTES below).
with \-20 being the highest priority and 19 being the lowest priority.
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.
.PP
The
.BR getpriority ()
call returns the highest priority (lowest numerical value)
enjoyed by any of the specified processes.
The
.BR setpriority ()
call sets the priorities of all of the specified processes
to the specified value.
.PP
Traditionally, only a privileged process could lower the nice value
(i.e., set a higher priority).
However, since Linux 2.6.12, an unprivileged process can decrease
the nice value of a target process that has a suitable
.BR RLIMIT_NICE
soft limit; see
.BR getrlimit (2)
for details.
.SH RETURN VALUE
On success,
.BR getpriority ()
returns the calling thread's nice value, which may be a negative number.
On error, it returns \-1 and sets
.I errno
to indicate the cause of the error.
Since a successful call to
.BR getpriority ()
can legitimately return the value \-1, it is necessary
to clear the external variable
.I errno
prior to the
call, then check it afterward to determine
if \-1 is an error or a legitimate value.
.PP
.BR setpriority ()
returns 0 on success.
On error, it returns \-1 and sets
.I errno
to indicate the cause of the error.
.SH ERRORS
.TP
.B EINVAL
.I which
was not one of
.BR PRIO_PROCESS ,
.BR PRIO_PGRP ,
or
.BR PRIO_USER .
.TP
.B ESRCH
No process was located using the
.I which
and
.I who
values specified.
.PP
In addition to the errors indicated above,
.BR setpriority ()
may fail if:
.TP
.B 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
.B CAP_SYS_NICE
capability).
.TP
.B 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
.B CAP_SYS_NICE
capability).
But see NOTES below.
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008,
SVr4, 4.4BSD (these interfaces first appeared in 4.2BSD).
.SH NOTES
For further details on the nice value, see
.BR sched (7).
.PP
.IR 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 circumstances.
For details, see
.BR sched (7).
.PP
A child created by
.BR fork (2)
inherits its parent's nice value.
The nice value is preserved across
.BR execve (2).
.PP
The details on the condition for
.B 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 \fIwho\fP (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 \fIwho\fP.
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.
.PP
Including
.I <sys/time.h>
is not required these days, but increases portability.
(Indeed,
.I <sys/resource.h>
defines the
.I rusage
structure with fields of type
.I struct timeval
defined in
.IR <sys/time.h> .)
.\"
.SS 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
.BR setpriority ()
and
.BR 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
.IR "unice\ =\ 20\ \-\ knice" .
(Thus, the kernel's 40..1 range corresponds to the
range \-20..19 as seen by user space.)
.SH 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 relying on the Linux behavior,
which may be made standards conformant in the future.
.SH SEE ALSO
.BR nice (1),
.BR renice (1),
.BR fork (2),
.BR capabilities (7),
.BR sched (7)
.PP
.I Documentation/scheduler/sched-nice-design.txt
in the Linux kernel source tree (since Linux 2.6.23)