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.\"     @(#)getpriority.2	8.1 (Berkeley) 6/4/93
.\"
.Dd April 13, 2012
.Dt GETPRIORITY 2
.Os
.Sh NAME
.Nm getpriority ,
.Nm setpriority
.Nd get/set program scheduling priority
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/resource.h
.Ft int
.Fn getpriority "int which" "id_t who"
.Ft int
.Fn setpriority "int which" "id_t who" "int prio"
.Sh DESCRIPTION
The scheduling
priority of the process, process group, or user, as indicated by
.Fa which
and
.Fa who
is obtained with the
.Fn getpriority
call and set with the
.Fn setpriority
call.
.Pp
The priority is maintained in a per-process basis and
affects scheduling of LWPs which belong to the process and use the
.Dv SCHED_OTHER
scheduling class.
.Pp
.Fa which
is one of
.Dv PRIO_PROCESS ,
.Dv PRIO_PGRP ,
or
.Dv PRIO_USER ,
and
.Fa who
is interpreted relative to
.Fa which
(a process identifier for
.Dv PRIO_PROCESS ,
process group
identifier for
.Dv PRIO_PGRP ,
and a user ID for
.Dv PRIO_USER ) .
A zero value of
.Fa who
denotes the current process, process group, or user.
.Fa prio
is a value in the range -20 to 20.
The default priority is 0;
numerically lower priority values cause more favorable scheduling.
A value of 19 or 20
will schedule a process only when nothing at priority \*(Le 0 is runnable.
.Pp
The
.Fn getpriority
call returns the highest priority (lowest numerical value)
enjoyed by any of the specified processes.
The
.Fn setpriority
call sets the priorities of all of the specified processes
to the specified value.
Only the super-user may lower priority values.
.Sh RETURN VALUES
Since
.Fn getpriority
can legitimately return the value \-1, it is necessary
to clear the external variable
.Va errno
prior to the
call, then check it afterward to determine
if a \-1 is an error or a legitimate value.
The
.Fn setpriority
call returns 0 if there is no error, or
\-1 if there is.
.Sh ERRORS
.Fn getpriority
and
.Fn setpriority
will fail if:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa which
was not one of
.Dv PRIO_PROCESS ,
.Dv PRIO_PGRP ,
or
.Dv PRIO_USER .
.It Bq Er ESRCH
No process was located using the
.Fa which
and
.Fa who
values specified.
.El
.Pp
In addition to the errors indicated above,
.Fn setpriority
will fail if:
.Bl -tag -width Er
.It Bq Er EACCES
A non super-user attempted to lower a process priority value.
.It Bq Er EPERM
A process was located, but neither its effective nor real user
ID matched the effective user ID of the caller.
.El
.Sh SEE ALSO
.Xr nice 1 ,
.Xr fork 2 ,
.Xr sched 3 ,
.Xr renice 8
.Sh HISTORY
The
.Fn getpriority
function call appeared in
.Bx 4.2 .