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The sched_setscheduler() function sets the scheduling policy and scheduling parameters of the process specified by pid to policy and the parameters specified in the sched_param structure pointed
to by param, respectively. The value of the sched_priority member in the sched_param structure is any integer within the inclusive priority range for the scheduling policy specified by policy. The sched_setscheduler() function ignores the other members of the sched_param structure. If the value of pid is negative, the behavior of the sched_setscheduler() function is unspecified.
The possible values for the policy parameter are defined in the header <sched.h> (see sched(3HEAD)):
If a process specified by pid exists and if the calling process has permission, the scheduling policy and scheduling parameters are set for the process whose process ID is equal to pid. The real or effective
user ID of the calling process must match the real or saved (from exec(2)) user ID of the target
process unless the effective user ID of the calling process is 0. See intro(2).
If pid is 0, the scheduling policy and scheduling parameters are set for the calling process.
To change the policy of any process to either of the real time policies SCHED_FIFO or SCHED_RR, the calling process must either have the SCHED_FIFO
or SCHED_RR policy or have an effective user ID of 0.
The sched_setscheduler() function is considered successful if it succeeds in setting the scheduling policy and scheduling parameters of the process specified by pid to the values specified by policy and the structure pointed
to by param, respectively.
The effect of this function on individual threads is dependent on the scheduling contention scope of the threads:
- For threads with system scheduling contention scope, these functions have no effect on their scheduling.
- For threads with process scheduling contention scope, the threads' scheduling policy and associated parameters will not be affected. However, the scheduling of these threads with respect to threads in other processes may be dependent on the scheduling parameters of their process, which
are governed using these functions.
The system supports a two-level scheduling model in which library threads are multiplexed on top of several kernel scheduled entities. The underlying kernel scheduled entities for the system contention scope threads will not be affected by these functions.
The underlying kernel scheduled entities for the process contention scope threads will have their scheduling policy and associated scheduling parameters changed to the values specified in policy and param, respectively. Kernel scheduled entities
for use by process contention scope threads that are created after this call completes inherit their scheduling policy and associated scheduling parameters from the process.
This function is not atomic with respect to other threads in the process. Threads are allowed to continue to execute while this function call is in the process of changing the scheduling policy and associated scheduling parameters for the underlying kernel scheduled entities used by the process
contention scope threads.
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