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.
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