GETPRIORITY
NAME
getpriority, setpriority - get/set program scheduling priority
SYNOPSIS
#include <sys/time.h>
#include <sys/resource.h>
I int getpriority(int which , int who );
I int setpriority(int which , int who , int prio );
DESCRIPTION
The scheduling priority of the process, process group, or user, as
indicated by
which
and
who
is obtained with the
R getpriority ()
call and set with the
R setpriority ()
call.
The value
which
is one of
R PRIO_PROCESS ,
R PRIO_PGRP ,
or
R PRIO_USER ,
and
who
is interpreted relative to
which
(a process identifier for
R PRIO_PROCESS ,
process group
identifier for
R PRIO_PGRP ,
and a user ID for
R 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
R getpriority ()
call returns the highest priority (lowest numerical value)
enjoyed by any of the specified processes.
The
R setpriority ()
call sets the priorities of all of the specified processes
to the specified value.
Only the superuser may lower priorities.
RETURN VALUE
Since
R 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 a -1 is an error or a legitimate value.
The
R setpriority ()
call returns 0 if there is no error, or
-1 if there is.
ERRORS
EINVAL
which
was not one of
R PRIO_PROCESS ,
R PRIO_PGRP ,
or
R PRIO_USER .
ESRCH
No process was located using the
which
and
who
values specified.
In addition to the errors indicated above,
R setpriority ()
may fail if:
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.
EACCES
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.
CONFORMING TO
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD),
POSIX.1-2001.
NOTES
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
R 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.
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
R setpriority ()
and
R 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
R 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
R <sys/time.h> .)
SEE ALSO
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