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sched_setscheduler(3)
Contents
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sched_setscheduler - Sets the scheduling policy and
scheduling parameters of the specified process (P1003.1b)
#include <sched.h>
int sched_setscheduler ( pid_t pid, int policy, const
struct sched_param *param);
Realtime Library (librt.so, librt.a)
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.
Specifies the scheduling policy to be set
(SCHED_FIFO, SCHED_RR, or SCHED_OTHER). 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.
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: Specifies a fixed-priority, first infirst
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. 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. 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.
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.
The sched_setscheduler function fails under the following
conditions: An invalid value is specified for the policy
argument or the param pointer is NULL. The requesting
process does not have permission to set either the priority
or the scheduling policy of the specified process. No
process can be found corresponding to that specified by
pid.
getpid(2), sched_getparam(3), sched_getscheduler(3),
sched_setparam(3)
Guide to Realtime Programming
sched_setscheduler(3)
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