getpriority - phpMan

GETPRIORITY(2) Linux Programmer’s Manual GETPRIORITY(2)

NAME
getpriority, setpriority - get/set program scheduling priority

SYNOPSIS
#include <sys/time.h>
#include <sys/resource.h>

int getpriority(int which, int who);
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 getpriority call and set with the setpriority
call. Which is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and who is inter-
preted 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 pro-
cess, 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.

RETURN VALUE
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 deter-
mine 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.

ERRORS
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.

NOTES
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>.)

CONFORMING TO
SVr4, 4.4BSD (these function calls first appeared in 4.2BSD).