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

       getpriority, setpriority	- get/set program scheduling priority

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

       int getpriority(int which, id_t who);
       int setpriority(int which, id_t who, int	prio);

       The  scheduling priority	of the process,	process	group, or user,	as in-
       dicated 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  19  (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 spec-
       ified value.  Only the superuser	may lower priorities.

       Since  getpriority() can	legitimately return the	value -1, it is	neces-
       sary to clear the external variable errno prior to the call, then check
       it afterward to determine if -1 is an error or a	legitimate value.  The
       setpriority() call returns 0 if there is	no error, or -1	if there is.

       EINVAL which was	not one	of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.

       ESRCH  No process was located using the which and who values specified.

       In addition to the errors indicated above, setpriority()	may fail if:

       EACCES The caller attempted to lower a process priority,	 but  did  not
	      have  the	 required  privilege  (on  Linux:  did	not  have  the
	      CAP_SYS_NICE capability).	 Since Linux 2.6.12, this error	occurs
	      only  if	the  caller attempts to	set a process priority outside
	      the range	of the RLIMIT_NICE soft	resource limit of  the	target
	      process; see getrlimit(2)	for details.

       EPERM  A	 process  was located, but its effective user ID did not match
	      either the effective or the real user ID of the caller, and  was
	      not privileged (on Linux:	did not	have the CAP_SYS_NICE capabil-
	      ity).  But see NOTES below.

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

       A  child	created	by fork(2) inherits its	parent's nice value.  The nice
       value is	preserved across execve(2).

       The degree to which their relative nice value affects the scheduling of
       processes varies	across UNIX systems, and, on Linux, across kernel ver-
       sions.  Starting	with kernel 2.6.23, Linux adopted  an  algorithm  that
       causes  relative	differences in nice values to have a much stronger ef-
       fect.  This causes very low nice	values (+19) to	truly  provide	little
       CPU  to	a  process whenever there is any other higher priority load on
       the system, and makes high nice values (-20) deliver most of the	CPU to
       applications that require it (e.g., some	audio applications).

       The details on the condition for	EPERM depend on	the system.  The above
       description is what POSIX.1-2001	says, and seems	to be followed on  all
       System V-like  systems.	 Linux kernels before 2.6.12 required the real
       or effective user ID of the caller  to  match  the  real	 user  of  the
       process who (instead of its effective user ID).	Linux 2.6.12 and later
       require the effective user ID of	the caller to match the	real or	effec-
       tive  user  ID  of the process who.  All	BSD-like systems (SunOS	4.1.3,
       Ultrix 4.2, 4.3BSD, FreeBSD 4.3,	OpenBSD-2.5, ...) behave in  the  same
       manner as Linux 2.6.12 and later.

       The actual priority range varies	between	kernel versions.  Linux	before
       1.3.36 had -infinity..15.  Since	kernel 1.3.43,	Linux  has  the	 range
       -20..19.	 On some other systems,	the range of nice values is -20..20.

       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_.)

   C library/kernel ABI	differences
       Within the kernel, nice values are actually represented using the range
       40..1 (since negative numbers are error codes) and these	are the	values
       employed	 by  the  setpriority()	 and  getpriority() system calls.  The
       glibc wrapper functions for these system	calls handle the  translations
       between	the user-land and kernel representations of the	nice value ac-
       cording to the formula unice = 20 - knice.  (Thus,  the	kernels	 40..1
       range corresponds to the	range -20..19 as seen by user space.)

       According  to POSIX, the	nice value is a	per-process setting.  However,
       under the current Linux/NPTL implementation of POSIX threads, the  nice
       value  is a per-thread attribute: different threads in the same process
       can have	different nice values.	Portable applications should avoid re-
       lying  on the Linux behavior, which may be made standards conformant in
       the future.

       nice(1),	renice(1), fork(2), capabilities(7), sched(7)

       Documentation/scheduler/sched-nice-design.txt  in  the	Linux	kernel
       source tree (since Linux	2.6.23)

       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at

Linux				  2014-08-19			GETPRIORITY(2)


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