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
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 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.
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
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>.)
SEE ALSO¶
nice(1),
renice(1),
fork(2),
capabilities(7)
Documentation/scheduler/sched-nice-design.txt in the Linux kernel source
tree (since Linux 2.6.23)
COLOPHON¶
This page is part of release 3.44 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/.