GETPRIORITY(2)                                      Linux Programmer's Manual                                     GETPRIORITY(2)



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 get-
       priority() 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 rep-
       resentations 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
       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)