Linux man pages : getpriority (2)
GETPRIORITY(2) Linux Programmer's Manual GETPRIORITY(2)
getpriority, setpriority - get/set program scheduling priority
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. 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.
ESRCH No process was located using the which and who values specified.
EINVAL Which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.
In addition to the errors indicated above, setpriority may fail if:
EPERM A process was located, but neither the effective nor the real
user ID of the caller matches its effective user ID.
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 effec-
tive 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 sys-
tem 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 porta-
bility. (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), renice(8)
BSD Man Page 2002-06-21 GETPRIORITY(2)