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 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 call-
       ing 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

       This  page is part of release 3.27 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)