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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(8)

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

       This page is part of release 3.22 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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