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

       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  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  afterwards  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

       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  only
	      occurs  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
       effect.	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.	 Within	the kernel, nice values	are actually represented using
       the corresponding range 40..1 (since negative numbers are error	codes)
       and  these  are	the values employed by the setpriority() and getprior-
       ity() system calls.  The	glibc wrapper functions	for these system calls
       handle  the  translations  between the user-land	and kernel representa-
       tions of	the nice value according to the	formula	unice =	20 - knice.

       On some 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_.)

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

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

       This  page  is  part of release 3.25 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found	at

Linux				  2008-05-29			GETPRIORITY(2)


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