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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


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