getpriority(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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

NAME         top

       getpriority, setpriority - get/set program scheduling priority

SYNOPSIS         top

       #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         top

       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         top

       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 error.

       Since a successful call to getpriority() can legitimately return
       the value -1, it is necessary to clear 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 failure, it returns -1
       and sets errno to indicate the error.

ERRORS         top

       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         top

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

NOTES         top

       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         top

       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         top

       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         top

       This page is part of release 5.11 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                          2021-03-22                 GETPRIORITY(2)

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