sched_setscheduler(3)					     Library Functions Manual					     sched_setscheduler(3)

NAME

       sched_setscheduler - Sets the scheduling policy and scheduling parameters of the specified process (P1003.1b)

LIBRARY

       Realtime Library (librt.so, librt.a)

SYNOPSIS

       #include <sched.h>

       int sched_setscheduler (
       pid_t pid,
       int policy,
       const struct sched_param *param);

PARAMETERS

       pid     Specifies  the  ID  of the process for which scheduling policy and priority is to be set. If pid is zero, the scheduling policy and
	       priority is set for the calling process.

       policy  Specifies the scheduling policy to be set (SCHED_FIFO, SCHED_RR, or SCHED_OTHER).

       *param  Specifies a pointer to a sched_param structure, which contains the scheduling parameters of the specified process.  Currently,  the
	       sched_param structure contains only a priority field.  The value of priority  in the param structure indicates the priority level.

DESCRIPTION

       The  sched_setscheduler	function  changes  the	scheduling  policy and priority of a process.  Changing the scheduling policy and priority
       ensures that an application can determine more effectively when a process will run.

       At run time, a process starts out with an initial priority of SCHED_PRIO_USER_MAX. A call to either the sched_setparam  or  sched_setsched-
       uler  function  can raise or lower the priority of a process.  If you raise the priority higher than the initial priority, the new priority
       becomes the maximum for the process. This higher maximum priority exists for the life of the process or until the priority is set to a new,
       higher priority through another call to the sched_setparam function. The maximum priority cannot be adjusted downward, but subsequent calls
       to the sched_setparam or sched_setscheduler functions can specify that a process run at a lower priority.

       Three scheduling policies are supported: two fixed-priority scheduling policies	(SCHED_FIFO  and  SCHED_RR)  and  one  timesharing  policy
       (SCHED_OTHER).  Under a fixed-priority scheduling policy, only the user sets and adjusts process priorities. Under a timesharing scheduling
       policy, the scheduler automatically adjusts priorities according to system resource usage and other factors.

       The scheduling policies supported by the realtime interface are as follows:

       SCHED_FIFO     Specifies a fixed-priority, first in-first out (FIFO) scheduling policy. Processes waiting at a specific priority level  are
		      selected from a process list that is ordered by the amount of time the processes have been on the process list without being
		      executed. Generally, the process at the head of the list has waited the longest time; the process at the tail  of  the  list
		      has waited the shortest time.

       SCHED_RR       Secifies a fixed-priority, round-robin (RR) scheduling policy.  Processes waiting at a specific priority level are scheduled
		      in much the same way as for SCHED_FIFO,  scheduling with the additional condition that the length of  time  that	a  process
		      executes is subject to a quantum.

       SCHED_OTHER    Specifies  the  standard	timesharing scheduling policy.	Processes are scheduled in much the same way as for the SCHED_FIFO
		      scheduling policy with the additional condition that the scheduler adjusts process priorities. Recalculation of process pri-
		      orities results in preemption.

       Setting	priorities in conjunction with a FIFO scheduling policy allows a critical process to run as soon as it is ready, for as long as it
       needs to run, because it will preempt other, lower-priority processes. This behavior is important in situations where scheduling a  process
       must be as fast and as precise as possible.

       Use the sched_get_priority_max and sched_get_priority_min functions to determine the maximum and minimum values allowed for each scheduling
       policy.	The value of the priority field in the sched_param structure pointed to by param can be any integer within the inclusive range for
       the current scheduling policy, as defined in <sched.h>.	Higher numerical values for param represent higher priorities.

       The scheduling policy of a process is inherited across fork and exec calls.

       An application designed for portability must initialize all fields of the sched_param structure before making the function call.

       You must have superuser privileges to call the sched_setscheduler function.

RETURN VALUES

       On a successful call to sched_setscheduler, the former scheduling policy of the process is returned. On an unsuccessful call, a value of -1
       is returned and errno is set to indicate that an error occurred and that the scheduling policy and parameters of the specified process  are
       unchanged.

ERRORS

       The sched_setscheduler function fails under the following conditions:

       [EINVAL]       An invalid value is specified for the policy argument or the param pointer is NULL.

       [EPERM]	      The  requesting  process	does  not  have  permission  to  set either the priority or the scheduling policy of the specified
		      process.

       [ESRCH]	      No process can be found corresponding to that specified by pid.

RELATED INFORMATION

       getpid(2), sched_getparam(3), sched_getscheduler(3), sched_setparam(3)

       Guide to Realtime Programming delim off

															     sched_setscheduler(3)