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 process  attribute  dealt  with  by
       these system calls is the same attribute (also known as the "nice" value) that is dealt with by nice(2).

       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).  with -20  being  the  highest
       priority  and  19  being the lowest priority.  Attempts to set a priority outside this range are silently
       clamped to the range.  The default priority is 0;  lower  values  give  a  process  a  higher  scheduling
       priority.

       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.

       Traditionally,  only  a  privileged  process  could  lower  the nice value (i.e., set a higher priority).
       However, since Linux 2.6.12, an unprivileged process can decrease the nice value of a target process that
       has a suitable RLIMIT_NICE soft limit; see getrlimit(2) for details.

RETURN VALUE

       On  success,  getpriority()  returns the calling thread's nice value, which may be a negative number.  On
       error, it returns -1 and sets errno to indicate the cause of the error.

       Since a successful call to getpriority() can legitimately return the value -1, it is necessary  to  clear
       the  external variable errno prior to the call, then check errno afterward to determine if -1 is an error
       or a legitimate value.

       setpriority() returns 0 on success.  On error, it returns -1 and sets errno to indicate the cause of  the
       error.

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 set a lower nice value (i.e., a higher process priority), but did not have
              the required privilege (on Linux: did not have the CAP_SYS_NICE capability).

       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

       For further details on the nice value, see sched(7).

       Note: the addition of the "autogroup" feature in Linux 2.6.38 means that the nice value no longer has its
       traditional effect in many circumstances.  For details, see sched(7).

       A  child  created  by  fork(2)  inherits  its  parent's  nice  value.  The nice value is preserved across
       execve(2).

       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.

       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.

SEE ALSO

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

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

COLOPHON

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