path: root/man2/getpriority.2

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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>
.\"
.\" FIXME Oct 2008: Denys Vlasenko is working on a PRIO_THREAD feature that
.\" is likely to get included in mainline; this will need to be documented.
.\"
.TH GETPRIORITY 2 2008-05-29 "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>
.sp
.BI "int getpriority(int " which ", int " who );
.br
.BI "int setpriority(int " which ", int " 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 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.
.I 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
.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.
Only the superuser may lower priorities.
.SH "RETURN VALUE"
Since
.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.
The
.BR setpriority ()
call returns 0 if there is no error, or
\-1 if there is.
.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 lower a process priority, but did not
have the required privilege (on Linux: did not have the
.B 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
.B RLIMIT_NICE
soft resource limit of the target process; see
.BR getrlimit (2)
for details.
.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"
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD),
POSIX.1-2001.
.SH NOTES
A child created by
.BR fork (2)
inherits its parent's nice value.
The nice value is preserved across
.BR 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
.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.
.LP
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
.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" .
.LP
On some systems, the range of nice values is \-20..20.
.LP
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> .)
.SH "SEE ALSO"
.BR nice (1),
.BR fork (2),
.BR capabilities (7),
.BR renice (1)
.sp
.I Documentation/scheduler/sched-nice-design.txt
in the kernel source tree (since Linux 2.6.23).