Ubuntu Manpage: getpriority, setpriority - get/set program scheduling priority

NAME

       getpriority, setpriority - get/set program scheduling priority

SYNOPSIS

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

       int getpriority(int which, id_t who);
       int setpriority(int which, id_t 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.  The prio argument is a value in the range
       -20 to 19 (but see 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 afterward 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 occurs only 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

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first appeared in 4.2BSD).

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. On some other 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>.)

C library/kernel differences
Within the kernel, nice values are actually represented using the 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. (Thus, the kernel's 40..1
range corresponds to the range -20..19 as seen by user space.)

BUGS

       According to POSIX, the nice value is a per-process  setting.   However,  under  the  current  Linux/NPTL
       implementation  of POSIX threads, the nice value is a per-thread attribute: different threads in the same
       process can have different nice  values.   Portable  applications  should  avoid  relying  on  the  Linux
       behavior, which may be made standards conformant in the future.

COLOPHON

       This  page  is  part  of  release  4.04  of  the  Linux man-pages project.  A description of the project,
       information  about  reporting  bugs,  and  the  latest  version  of  this   page,   can   be   found   at
       http://www.kernel.org/doc/man-pages/.

Linux                                              2015-07-23                                     GETPRIORITY(2)