24.17. Idle Task Configuration — RTEMS Classic API Guide 5.4-rc2 (21st August 2026) documentation

# 24.17. Idle Task Configuration

This section describes configuration options related to the idle tasks.

## 24.17.1. CONFIGURE_IDLE_TASK_BODY

CONSTANT:

  `CONFIGURE_IDLE_TASK_BODY`

OPTION TYPE:

  This configuration option is an initializer define.

DEFAULT VALUE:

  If BSP_IDLE_TASK_BODY is defined, then this will be the default value, otherwise the default value is `_CPU_Thread_Idle_body`.

VALUE CONSTRAINTS:

  The value of this configuration option shall be defined to a valid function pointer of the type `void *( *idle_body )( uintptr_t )`.

DESCRIPTION:

  The value of this configuration option initializes the IDLE thread body.

NOTES:

  IDLE threads shall not block. A blocking IDLE thread results in undefined system behaviour because the scheduler assume that at least one ready thread exists.

  IDLE threads can be used to initialize the application, see configuration option CONFIGURE_IDLE_TASK_INITIALIZES_APPLICATION.

## 24.17.2. CONFIGURE_IDLE_TASK_INITIALIZES_APPLICATION

CONSTANT:

  `CONFIGURE_IDLE_TASK_INITIALIZES_APPLICATION`

OPTION TYPE:

  This configuration option is a boolean feature define.

DEFAULT CONFIGURATION:

  If this configuration option is undefined, then the user is assumed to provide one or more initialization tasks.

DESCRIPTION:

  This configuration option is defined to indicate that the user has configured no user initialization tasks or threads and that the user provided IDLE task will perform application initialization and then transform itself into an IDLE task.

NOTES:

  If you use this option be careful, the user IDLE task cannot block at all during the initialization sequence. Further, once application initialization is complete, it shall make itself preemptible and enter an idle body loop.

  The IDLE task shall run at the lowest priority of all tasks in the system.

  If this configuration option is defined, then it is mandatory to configure a user IDLE task with the CONFIGURE_IDLE_TASK_BODY configuration option, otherwise a compile time error in the configuration file will occur.

  The application shall define exactly one of the following configuration options

  - CONFIGURE_RTEMS_INIT_TASKS_TABLE,

  - CONFIGURE_POSIX_INIT_THREAD_TABLE, or

  - CONFIGURE_IDLE_TASK_INITIALIZES_APPLICATION

  otherwise a compile time error in the configuration file will occur.

## 24.17.3. CONFIGURE_IDLE_TASK_STACK_SIZE

CONSTANT:

  `CONFIGURE_IDLE_TASK_STACK_SIZE`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is CONFIGURE_MINIMUM_TASK_STACK_SIZE.

VALUE CONSTRAINTS:

  The value of this configuration option shall satisfy all of the following constraints:

  - It shall be greater than or equal to a BSP-specific and application-specific minimum value.

  - It shall be small enough so that the IDLE task stack area calculation carried out by `<rtems/confdefs.h>` does not overflow an integer of type `size_t`.

DESCRIPTION:

  The value of this configuration option defines the task stack size for an IDLE task.

NOTES:

  In SMP configurations, there is one IDLE task per configured processor, see CONFIGURE_MAXIMUM_PROCESSORS.