This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       The getpriority() and setpriority() functions work with an offset nice value (nice value  -{NZERO}).  The
       nice  value  is  in  the  range  [0,2*{NZERO} -1], while the return value for getpriority() and the third
       parameter for setpriority() are in the range [-{NZERO},{NZERO} -1].

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee  document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                   GETPRIORITY(3POSIX)

       The  getpriority()  function  shall  obtain  the  nice  value  of  a process, process group, or user. The
       setpriority() function shall set the nice value of a process, process group, or user to value+{NZERO}.

       Target processes are specified by the values of the which and who arguments. The which  argument  may  be
       one  of  the following values: PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, indicating that the who argument is
       to be interpreted as a process ID, a process group ID, or an effective user ID, respectively. A  0  value
       for the who argument specifies the current process, process group, or user.

       The  nice value set with setpriority() shall be applied to the process. If the process is multi-threaded,
       the nice value shall affect all system scope threads in the process.

       If more than one process is specified, getpriority() shall return value {NZERO} less than the lowest nice
       value pertaining to any of the specified processes, and setpriority() shall set the nice values of all of
       the specified processes to value+{NZERO}.

       The default nice value is {NZERO}; lower nice values shall cause more  favorable  scheduling.  While  the
       range  of  valid  nice values is [0,{NZERO}*2-1], implementations may enforce more restrictive limits. If
       value+{NZERO} is less than the system's lowest supported nice value, setpriority()  shall  set  the  nice
       value to the lowest supported value; if value+{NZERO} is greater than the system's highest supported nice
       value, setpriority() shall set the nice value to the highest supported value.

       Only a process with appropriate privileges can lower its nice value.

       Any  processes  or  threads  using SCHED_FIFO or SCHED_RR shall be unaffected by a call to setpriority().
       This is not considered an error. A process which subsequently reverts to SCHED_OTHER need  not  have  its
       priority affected by such a setpriority() call.

       The effect of changing the nice value may vary depending on the process-scheduling algorithm in effect.

       Since getpriority() can return the value -1 upon successful completion, it is necessary to set errno to 0
       prior  to  a  call to getpriority().  If getpriority() returns the value -1, then errno can be checked to
       see if an error occurred or if the value is a legitimate nice value.

       The getpriority() and setpriority() functions shall fail if:

       ESRCH  No process could be located using the which and who argument values specified.

       EINVAL The value of the which argument was not recognized, or the value of the  who  argument  is  not  a
              valid process ID, process group ID, or user ID.

       In addition, setpriority() may fail if:

       EPERM  A  process  was located, but neither the real nor effective user ID of the executing process match
              the effective user ID of the process whose nice value is being changed.

       EACCES A request was made to change the nice value to a lower numeric value and the current process  does
              not have appropriate privileges.

       Thefollowingsectionsareinformative.

Usinggetpriority()
       The  following example returns the current scheduling priority for the process ID returned by the call to
       getpid().

           #include <sys/resource.h>
           ...
           int which = PRIO_PROCESS;
           id_t pid;
           int ret;

           pid = getpid();
           ret = getpriority(which, pid);

   Usingsetpriority()
       The following example sets the priority for the current process ID to -20.

           #include <sys/resource.h>
           ...
           int which = PRIO_PROCESS;
           id_t pid;
           int priority = -20;
           int ret;

           pid = getpid();
           ret = setpriority(which, pid, priority);

       None.

       getpriority, setpriority — get and set the nice value

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       None.

       Upon successful completion, getpriority() shall return an integer in the  range  -{NZERO}  to  {NZERO}-1.
       Otherwise, -1 shall be returned and errno set to indicate the error.

       Upon  successful  completion, setpriority() shall return 0; otherwise, -1 shall be returned and errno set
       to indicate the error.

nice(), sched_get_priority_max(), sched_setscheduler()

       The Base Definitions volume of POSIX.1‐2017, <sys_resource.h>

       #include <sys/resource.h>

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