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       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  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).   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.


       POSIX.1-2001,   POSIX.1-2008,  SVr4,  4.4BSD  (these  interfaces  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
       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 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
       effective 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
       portability.   (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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