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

       getpriority, setpriority - get/set program scheduling priority

       #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.   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 20 (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  specified
       value.  Only the super-user may lower priorities.

       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 determine if a -1 is an error or a legitimate value.  The
       setpriority call returns 0 if there is no error, or -1 if there is.

       ESRCH  No process was located using the which and who values specified.

       EINVAL Which was not one of PRIO_PROCESS, PRIO_PGRP, or PRIO_USER.

       In addition to the errors indicated above, setpriority may fail if:

       EPERM  A  process  was  located, but neither the effective nor the real
              user ID of the caller matches its effective user ID.

       EACCES A non super-user attempted to lower a process priority.

       The details on the condition for EPERM depend on the system.  The above
       description  is  what SUSv3 says, and seems to be followed on all SYSV-
       like systems.  Linux requires the real or  effective  user  ID  of  the
       caller to match the real user of the process who (instead of its effec-
       tive user ID).  All BSD-like systems (SunOS 4.1.3, Ultrix 4.2, BSD 4.3,
       FreeBSD  4.3,  OpenBSD-2.5,  ...)  require the effective user ID of the
       caller to match the real or effective user ID of the process who.

       The actual priority range varies between kernel versions.  Linux before
       1.3.36  had -infinity..15. Linux since 1.3.43 has -20..19, and the sys-
       tem call getpriority returns 40..1 for  these  values  (since  negative
       numbers are error codes).  The library call converts N into 20-N.

       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>.)

       SVr4, 4.4BSD (these function calls first appeared in 4.2BSD).

       nice(1), fork(2), renice(8)

BSD Man Page                      2002-06-21                    GETPRIORITY(2)