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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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       found at https://www.kernel.org/doc/man-pages/.