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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_setscheduler 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 priorities 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

Index for Section 3

Alphabetical listing for S

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