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



NAME
       getpriority, setpriority - get/set program scheduling priority

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

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

DESCRIPTION
       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  setprior-
       ity() call.

       The  value which is one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER, and who is inter-
       preted 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  pro-
       cess, 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 priori-
       ties  of all of the specified processes to the specified value.  Only the superuser
       may lower priorities.

RETURN VALUE
       Since getpriority() can legitimately return the value -1, it is necessary to  clear
       the  external  variable errno prior to the call, then check it afterwards to deter-
       mine if -1 is an error or a legitimate value.  The setpriority() call returns 0  if
       there is no error, or -1 if there is.

ERRORS
       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 only occurs 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 capability).  But see NOTES below.

CONFORMING TO
       SVr4, 4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.

NOTES
       A  child  created  by  fork(2) inherits its parent's nice value.  The nice value is
       preserved across execve(2).

       The degree to which their relative nice value affects the scheduling  of  processes
       varies  across  Unix systems, and, on Linux, across kernel versions.  Starting with
       kernel 2.6.23, Linux adopted an algorithm that causes relative differences in  nice
       values  to  have a much stronger effect.  This causes very low nice values (+19) to
       truly provide little CPU to a process whenever there is any other  higher  priority
       load  on  the  system,  and makes high nice values (-20) deliver most of the CPU to
       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 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.

       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  ker-
       nel,  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  getpriority()  system  calls.   The glibc wrapper functions for
       these system calls handle the translations between the user-land and kernel  repre-
       sentations 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 portability.
       (Indeed, <sys/resource.h> defines the rusage structure with fields of  type  struct
       timeval defined in <sys/time.h>.)

SEE ALSO
       nice(1), fork(2), capabilities(7), renice(8)

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-05-29                    GETPRIORITY(2)

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