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sched_setscheduler(3)
NAME
  sched_setscheduler - Sets the scheduling policy and scheduling parameters
  of the specified process (P1003.1b)

SYNOPSIS
  #include <sched.h>

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

LIBRARY
  Realtime Library (librt.so, librt.a)

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.

SEE ALSO
  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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