getpriority, setpriority — get/set program scheduling priority
#include <sys/time.h> #include <sys/resource.h>
int getpriority( |
int which, |
int who); |
int setpriority( |
int which, |
| int 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 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.
Since getpriority() can
legitimately return the value −1, it is necessary to
clear the external variable errno prior to the call, then check it
afterwards 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.
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 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.
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.
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>
nice(1), fork(2), capabilities(7), renice(1)
Documentation/scheduler/sched-nice-design.txt
in the kernel source tree (since Linux 2.6.23).