Section: Linux Programmer's Manual (2)
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getpriority, setpriority - get/set program scheduling priority
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
is obtained with the
call and set with the
is one of
is interpreted relative to
(a process identifier for
and a user ID for
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.
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.
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
prior to the
call, then check it afterward to determine
if -1 is an error or a legitimate value.
call returns 0 if there is no error, or
-1 if there is.
was not one of
No process was located using the
In addition to the errors indicated above,
may fail if:
The caller attempted to lower a process priority, but did not
have the required privilege (on Linux: did not have the
Since Linux 2.6.12, this error occurs only if the caller attempts
to set a process priority outside the range of the
soft resource limit of the target process; see
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
But see NOTES below.
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD),
A child created by
inherits its parent's nice value.
The nice value is preserved across
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
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.
is not required these days, but increases portability.
structure with fields of type
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
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.)
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.
in the Linux kernel source tree (since Linux 2.6.23)
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- RETURN VALUE
- CONFORMING TO
- C library/kernel ABI differences
- SEE ALSO