getpriority, setpriority - get/set program scheduling priority
int getpriority(int which, int who);
int setpriority(int which, int who, int
The scheduling priority of the process, process group, or user, as indicated by
is obtained with the getpriority
() call and
set with the setpriority
The value which
is one of PRIO_PROCESS
, and who
is interpreted relative to which
process identifier for PRIO_PROCESS
, process group identifier for
, and a user ID for PRIO_USER
). A zero value for
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.
() 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.
() 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
() 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.
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.
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
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
. 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
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.
is not required these days, but increases
portability. (Indeed, <sys/resource.h>
defines the rusage
structure with fields of type struct timeval
in the Linux kernel source
tree (since Linux 2.6.23)
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