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man:getpriority

GETPRIORITY(2) Linux Programmer's Manual GETPRIORITY(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 spec-
     ified value.
     Traditionally,  only  a  privileged  process could lower the nice value
     (i.e., set a higher priority).  However, since Linux 2.6.12, an unpriv-
     ileged 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 getpri-
     ority()  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 capabil-
            ity).  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  cir-
     cumstances.  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 effec-
     tive 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 porta-
     bility.   (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 4.16 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/.

Linux 2017-09-15 GETPRIORITY(2)

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