getpriority, setpriority - get/set program scheduling priority
#include <sys/time.h>
#include <sys/resource.h>
int getpriority(int which
, id_t
who
);
int setpriority(int which
, id_t
who
, int
prio
);
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 process attribute dealt with by
these system calls is the same attribute (also known as the "nice"
value) that is dealt with by nice(2).
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.
The 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.
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.
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 RLIMIT_NICE soft limit; see getrlimit(2) for details.
On success, getpriority() returns the calling
thread's nice value, which may be a negative number. On error, it
returns -1 and sets errno
to indicate the cause of the
error.
Since a successful call to getpriority() can
legitimately return the value -1, it is necessary to clear the external
variable errno
prior to the call, then check errno
afterward to determine if -1 is an error or a legitimate value.
setpriority() returns 0 on success. On error, it
returns -1 and sets errno
to indicate the cause of the
error.
which
was not one of PRIO_PROCESS,
PRIO_PGRP, or PRIO_USER.
No process was located using the which
and who
values specified.
In addition to the errors indicated above, setpriority() may fail if:
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 CAP_SYS_NICE capability).
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.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first appeared in 4.2BSD).
For further details on the nice value, see sched(7).
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 sched(7).
A child created by fork(2) inherits its parent's nice value. The nice value is preserved across execve(2).
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.
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>
.)
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 kernel's 40..1 range corresponds
to the range -20..19 as seen by user space.)
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.
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)
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.