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       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  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 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  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
              capability).  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
       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.  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
       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.

       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>.)


       nice(1), fork(2), capabilities(7), renice(8)

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


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