14. Scheduler Manager — RTEMS POSIX API Guide 5.4-rc2 (21st August 2026) documentation

# 14. Scheduler Manager

## 14.1. Introduction

The scheduler manager …

The directives provided by the scheduler manager are:

- sched_get_priority_min - Get Minimum Priority Value

- sched_get_priority_max - Get Maximum Priority Value

- sched_rr_get_interval - Get Timeslicing Quantum

- sched_yield - Yield the Processor

## 14.2. Background

### 14.2.1. Priority

In the RTEMS implementation of the POSIX API, the priorities range from the low priority of `sched_get_priority_min()` to the highest priority of `sched_get_priority_max()`. Numerically higher values represent higher priorities.

### 14.2.2. Scheduling Policies

The following scheduling policies are available:

SCHED_FIFO

  Priority-based, preemptive scheduling with no timeslicing. This is equivalent to what is called “manual round-robin” scheduling.

SCHED_RR

  Priority-based, preemptive scheduling with timeslicing. Time quantums are maintained on a per-thread basis and are not reset at each context switch. Thus, a thread which is preempted and subsequently resumes execution will attempt to complete the unused portion of its time quantum.

SCHED_OTHER

  Priority-based, preemptive scheduling with timeslicing. Time quantums are maintained on a per-thread basis and are reset at each context switch.

SCHED_SPORADIC

  Priority-based, preemptive scheduling utilizing three additional parameters: budget, replenishment period, and low priority. Under this policy, the thread is allowed to execute for “budget” amount of time before its priority is lowered to “low priority”. At the end of each replenishment period, the thread resumes its initial priority and has its budget replenished.

## 14.3. Operations

There is currently no text in this section.

## 14.4. Directives

This section details the scheduler manager’s directives. A subsection is dedicated to each of this manager’s directives and describes the calling sequence, related constants, usage, and status codes.

### 14.4.1. sched_get_priority_min - Get Minimum Priority Value

CALLING SEQUENCE:

    #include <sched.h>
    int sched_get_priority_min(
        int policy
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` to one of the following:

`EINVAL` | The indicated policy is invalid.

DESCRIPTION:

This routine return the minimum (numerically and logically lowest) priority for the specified `policy`.

NOTES:

NONE

### 14.4.2. sched_get_priority_max - Get Maximum Priority Value

CALLING SEQUENCE:

    #include <sched.h>
    int sched_get_priority_max(
        int policy
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` to one of the following:

`EINVAL` | The indicated policy is invalid.

DESCRIPTION:

This routine return the maximum (numerically and logically highest) priority for the specified `policy`.

NOTES:

NONE

### 14.4.3. sched_rr_get_interval - Get Timeslicing Quantum

CALLING SEQUENCE:

    #include <sched.h>
    int sched_rr_get_interval(
        pid_t            pid,
        struct timespec *interval
    );

STATUS CODES:

On error, this routine returns -1 and sets `errno` to one of the following:

`ESRCH` | The indicated process id is invalid.
`EINVAL` | The specified interval pointer parameter is invalid.

DESCRIPTION:

This routine returns the length of the timeslice quantum in the `interval` parameter for the specified `pid`.

NOTES:

The `pid` argument should be 0 to indicate the calling process.

### 14.4.4. sched_yield - Yield the Processor

CALLING SEQUENCE:

    #include <sched.h>
    int sched_yield( void );

STATUS CODES:

This routine always returns zero to indicate success.

DESCRIPTION:

This call forces the calling thread to yield the processor to another thread. Normally this is used to implement voluntary round-robin task scheduling.

NOTES:

NONE