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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
       indicated by which and who is obtained with the getpriority() call  and
       set  with  the setpriority() call.  The process attribute dealt with by
       these system calls is the same attribute	 (also	known  as  the	"nice"
       value) that is dealt with by nice(2).

       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.

       The prio argument is a value in the range -20  to  19  (but  see	 NOTES
       below).	 with  -20  being the highest priority and 19 being the lowest
       priority.  Attempts to set a priority outside this range	 are  silently
       clamped	to  the range.	The default priority is 0; lower values give a
       process a higher scheduling priority.

       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.

       Traditionally,  only  a	privileged  process could lower the nice value
       (i.e., set a higher priority).  However, since Linux 2.6.12, an unpriv-
       ileged process can decrease the nice value of a target process that has
       a suitable RLIMIT_NICE soft limit; see getrlimit(2) for details.

       On success, getpriority() returns  the  calling	thread's  nice	value,
       which may be a negative number.	On error, it returns -1 and sets errno
       to indicate the cause of the error.  Since a successful call to getpri-
       ority()	can legitimately return the value -1, it is necessary 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.

       setpriority()  returns  0 on success.  On error, it returns -1 and sets
       errno to indicate the cause of the error.

       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 set a lower nice value (i.e.,  a	higher
	      process  priority),  but did not have the required privilege (on
	      Linux: did not have the CAP_SYS_NICE capability).

       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.

       POSIX.1-2001,   POSIX.1-2008,  SVr4,  4.4BSD  (these  interfaces	 first
       appeared in 4.2BSD).

       For further details on the nice value, see sched(7).

       Note: the addition of the "autogroup" feature  in  Linux	 2.6.38	 means
       that  the  nice value no longer has its traditional effect in many cir-
       cumstances.  For details, see sched(7).

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

       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.

       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 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
       according to the formula unice = 20 - knice.  (Thus, the kernel's 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
       relying	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)

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Linux				  2016-12-12			GETPRIORITY(2)