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.\"	$NetBSD: getpriority.2,v 1.4 1995/02/27 12:33:15 cgd Exp $
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.\"     @(#)getpriority.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt GETPRIORITY 2
.Os BSD 4
.Sh NAME
.Nm getpriority ,
.Nm setpriority
.Nd get/set program scheduling priority
.Sh SYNOPSIS
.Fd #include <sys/resource.h>
.Ft int
.Fo getpriority
.Fa "int which"
.Fa "id_t who"
.Fc
.Ft int
.Fo setpriority
.Fa "int which"
.Fa "id_t who"
.Fa "int prio"
.Fc
.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.  Additionally, the current thread can be set to background
state.
.Fa Which
is one of
.Dv PRIO_PROCESS ,
.Dv PRIO_PGRP ,
.Dv PRIO_USER ,
or
.Dv PRIO_DARWIN_THREAD ,
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, user, thread (for PRIO_DARWIN_THREAD).
.Fa prio
is a value in the range -20 to 20.  The default priority is 0;
lower priorities cause more favorable scheduling.
For PRIO_DARWIN_THREAD,
.Fa prio
is either 0 (to remove current thread from background status) or PRIO_DARWIN_BG
(to set current thread into background state).
.Pp
The
.Fn getpriority
call returns the highest priority (lowest numerical value)
enjoyed by any of the specified processes or for PRIO_DARWIN_THREAD
returns 0 when current thread is not in background state or 1
when the current thread is in background state.
The
.Fn setpriority
call sets the priorities of all of the specified processes
to the specified value.  When setting a thread into background
state the scheduling priority is set to lowest value, disk and
network IO are throttled.  Network IO will be throttled for any
sockets the thread opens after going into background state.  Any
previously opened sockets are not affected.
Only the super-user may lower priorities, but any thread can set
itself into background state.
.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
is not one of
.Dv PRIO_PROCESS ,
.Dv PRIO_PGRP ,
.Dv PRIO_USER ,
or
.Dv PRIO_DARWIN_THREAD .
.\" ==========
.It Bq Er EINVAL
.Fa Who
is not a valid process, process group, or user ID.
.\" ==========
.It Bq Er EINVAL
.Fa Who
is not 0 when
.Fa which
is PRIO_DARWIN_THREAD.
.\" ==========
.It Bq Er ESRCH
No process can be located using the 
.Fa which
and
.Fa who
values specified.
.El
.Pp
.Bl -tag -width Er
In addition to the errors indicated above,
.Fn setpriority
will fail if:
.\" ==========
.It Bq Er EACCES
A non super-user attempts to lower a process priority.
.\" ==========
.It Bq Er EPERM
A process is located,
but neither its effective nor real user ID
matches the effective user ID of the caller.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/resource.h>
.Pp
The include file
.In sys/types.h
is necessary.
.Pp
.Ft int
.br
.Fo getpriority
.Fa "int which"
.Fa "int who"
.Fc ;
.Pp
.Ft int
.br
.Fo setpriority
.Fa "int which"
.Fa "int who"
.Fa "int value"
.Fc ;
.Pp
The type of
.Fa who
has changed.
.Sh SEE ALSO
.Xr nice 1 ,
.Xr fork 2 ,
.Xr compat 5 ,
.Xr renice 8
.Sh HISTORY
The
.Fn getpriority
function call appeared in
.Bx 4.2 .