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 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

       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>.)

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.

SEE ALSO

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

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

COLOPHON

       This  page  is  part  of  release 3.54 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/.