sched_setscheduler

Synopsis

#include <sched.h>

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

Status

Implemented

Conformance

IEEE Std 1003.1-2017

Description

The sched_setscheduler() function shall set 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 shall be any integer within the inclusive priority range for the scheduling policy specified by policy. If the value of pid is negative, the behavior of the sched_setscheduler() function is unspecified; Phoenix-RTOS returns -1 and sets errno to EINVAL.

The supported value of policy is SCHED_RR, which is defined in the <sched.h> header. The priority is a POSIX priority, see Scheduling priorities.

If a process specified by pid exists, and if the calling process has permission, the scheduling policy and scheduling parameters shall be set for the process whose process ID is equal to pid.

If pid is zero, the scheduling policy and scheduling parameters shall be set for the calling process.

The conditions under which one process has appropriate privileges to change the scheduling parameters of another process are implementation-defined. Phoenix-RTOS does not restrict changing the scheduling parameters of other processes.

The sched_setscheduler() function shall be 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.

This function is not atomic with respect to other threads in the process. Threads may 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.

Return value

Upon successful completion, the function shall return the former scheduling policy of the specified process, which is always SCHED_RR. If the sched_setscheduler() function fails to complete successfully, the policy and scheduling parameters shall remain unchanged, and the function shall return a value of -1 and set errno to indicate the error.

Errors

The sched_setscheduler() function shall fail if:

  • EINVAL - The value of the policy parameter is invalid, or one or more of the parameters contained in param is outside the valid range for the specified scheduling policy.

  • ENOTSUP - The value of the policy parameter is SCHED_FIFO or SCHED_OTHER.

  • EPERM - The requesting process does not have permission to set either or both of the scheduling parameters or the scheduling policy of the specified process.

  • ESRCH - No process can be found corresponding to that specified by pid.

POSIX deviations

IEEE Std 1003.1-2017 requires the supported values of policy to include SCHED_FIFO, SCHED_RR and SCHED_OTHER. Phoenix-RTOS supports only SCHED_RR, and sched_setscheduler() returns -1 and sets errno to ENOTSUP for SCHED_FIFO and SCHED_OTHER, an error that the standard does not define for it. The Phoenix-RTOS kernel schedules all threads with a single priority-based round-robin policy, which keeps the scheduler small and its behaviour predictable on resource-constrained real-time targets.