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NAME

       getpriority, setpriority - get/set program scheduling priority

SYNOPSIS

       #include <sys/time.h>
       #include <sys/resource.h>

       int getpriority(int which, int who);
       int setpriority(int which, int who, int prio);

DESCRIPTION

       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.

RETURN VALUE

       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.

ERRORS

       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.

CONFORMING TO

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

NOTES

       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>.)

SEE ALSO

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

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

COLOPHON

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