получить / установить приоритет планирования программы (get/set program scheduling priority)
Имя (Name)
getpriority, setpriority - get/set program scheduling priority
Синопсис (Synopsis)
#include <sys/resource.h>
int getpriority(int which, id_t who);
int setpriority(int which, id_t who, int prio);
Описание (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 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.
Возвращаемое значение (Return value)
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 error.
Since a successful call to getpriority() can legitimately return
the value -1, it is necessary to clear 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 failure, it returns -1
and sets errno to indicate the error.
Ошибки (Error)
EACCES 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).
EINVAL which was not one of PRIO_PROCESS, PRIO_PGRP, or
PRIO_USER.
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.
ESRCH No process was located using the which and who values
specified.
Стандарты (Conforming to)
POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first
appeared in 4.2BSD).
Примечание (Note)
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.
C library/kernel 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 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.)
Ошибки (баги) (Bugs)
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.
Смотри также (See also)
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)
