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Linux 2.6 - man page for setpriority (linux section 2)

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 specified value.  Only the superuser may lower priorities.

       Since  getpriority()  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.	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 capability).  But see NOTES below.

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

       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 versions.  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 ker-
       nels  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 effective user ID of the
       process who.  All BSD-like systems (SunOS 4.1.3, Ultrix 4.2, 4.3BSD,  FreeBSD  4.3,  Open-
       BSD-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 -infin-
       ity..15.  Since kernel 1.3.43, Linux has the range -20..19.  Within the kernel, nice  val-
       ues  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  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  for-
       mula 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 portability.  (Indeed,
       <sys/resource.h> defines the rusage structure with fields of type struct  timeval  defined
       in <sys/time.h>.)

       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: dif-
       ferent  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

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

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

       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

Linux					    2013-02-12				   GETPRIORITY(2)

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