GETPRIORITY(2)             Linux Programmer's Manual            GETPRIORITY(2)

       getpriority, setpriority - get/set program scheduling priority

       #include <sys/time.h>
       #include <sys/resource.h>

       int getpriority(int which, id_t who);
       int setpriority(int which, id_t who, int prio);

       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

       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.

       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.

       POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (these interfaces first
       appeared in 4.2BSD).

       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.)

       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.

       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)

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       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                             2017-09-15                    GETPRIORITY(2)