GETPRIORITY
Section: Linux Programmer's Manual (2)
Updated: 2014-08-19
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NAME
getpriority, setpriority - get/set program scheduling priority
SYNOPSIS
#include <sys/time.h>
#include <sys/resource.h>
int getpriority(int which, id_t who);
int setpriority(int which, id_t 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
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.
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 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.
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 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.
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.
On some other 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>.)
C library/kernel ABI 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
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.
(Thus, the kernels 40..1 range corresponds to the
range -20..19 as seen by user space.)
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.
SEE ALSO
nice(1),
renice(1),
fork(2),
capabilities(7),
sched(7)
Documentation/scheduler/sched-nice-design.txt
in the Linux kernel source tree (since Linux 2.6.23)
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- C library/kernel ABI differences
-
- BUGS
-
- SEE ALSO
-
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Time: 02:54:51 GMT, September 18, 2014