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           #include <sys/time.h>
           #include <sys/resource.h>
           int getpriority(int which, int who);
           int setpriority(int which, int 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  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.  Prio is a value in the range -20  to  19  (but
           see  the  Notes  below).   The  default priority is 0; lower priorities
           cause more favorable scheduling.
           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.  Only the superuser may lower priorities.


           Since  getpriority() can legitimately return the value -1, it is neces-
           sary 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.  The
           setpriority() call returns 0 if there is no error, or -1 if there is.


           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 lower a process priority, but did not
                  have  the  required  privilege  (on  Linux:  did  not  have  the
                  CAP_SYS_NICE capability).  Since Linux 2.6.12, this error occurs
                  only if the caller attempts to set a  process  priority  outside
                  the  range  of the RLIMIT_NICE soft resource limit of the target
                  process; see getrlimit(2) for details.
           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.


           applications that require it (e.g., some audio applications).
           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 pro-
           cess 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.
           The actual priority range varies between kernel versions.  Linux before
           1.3.36  had  -infinity..15.   Since  kernel 1.3.43, Linux has the range
           -20..19.  Within the kernel, nice values are actually represented using
           the  corresponding range 40..1 (since negative numbers are error codes)
           and these are the values employed by the  setpriority()  and  getprior-
           ity() system calls.  The glibc wrapper functions for these system calls
           handle the translations between the user-land  and  kernel  representa-
           tions of the nice value according to the formula unice = 20 - knice.
           On some systems, the range of nice values is -20..20.
           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>.)


           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)
           Documentation/scheduler/sched-nice-design.txt  in  the   Linux   kernel
           source tree (since Linux 2.6.23)

    Linux 2013-02-12 GETPRIORITY(2)


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