setpriority(2)

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 it
   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

   This page is part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.

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