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       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

       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


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