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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 spec-
       ified value.  Only the superuser may lower priorities.

       Since  getpriority() can legitimately return the value -1, it is neces-
       sary 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

       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 capabil-
              ity).  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 ver-
       sions.  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  pro-
       cess  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 effec-
       tive  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 getprior-
       ity() system calls.  The glibc wrapper functions for these system calls
       handle  the  translations  between the user-land and kernel representa-
       tions 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 porta-
       bility.   (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(1)

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

       This  page  is  part of release 3.25 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux                             2008-05-29                    GETPRIORITY(2)

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