Originální popis anglicky:
getpriority, setpriority - get/set program scheduling priority
Návod, kniha: Linux Programmer's Manual
#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 20 (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 super-user 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 a -1 is an error or a legitimate value. The
setpriority call returns 0 if there is no error, or -1 if there is.
- EINVAL
- which was not one of PRIO_PROCESS,
PRIO_PGRP, or PRIO_USER.
- ESRCH
- No process was located using the which and
who values specified.
In addition to the errors indicated above,
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 (on
Linux systems) the caller did not have the CAP_SYS_NICE
capability.
- EACCES
- A non super-user attempted to lower a process
priority.
The details on the condition for EPERM depend on the system. The above
description is what SUSv3 says, and seems to be followed on all SYSV-like
systems. Linux requires the real or effective user ID of the caller to match
the real user of the process
who (instead of its effective user ID).
All BSD-like systems (SunOS 4.1.3, Ultrix 4.2, BSD 4.3, FreeBSD 4.3,
OpenBSD-2.5, ...) require the effective user ID of the caller to match the
real or effective user ID of the process
who.
The actual priority range varies between kernel versions. Linux before 1.3.36
had -infinity..15. Linux since 1.3.43 has -20..19, and the system call
getpriority returns 40..1 for these values (since negative numbers are error
codes). The library call converts N into 20-N.
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>.)
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD).
nice(1),
fork(2),
capabilities(7),
renice(8)
