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

# 24.9. Classic API Configuration

This section describes configuration options related to the Classic API.

## 24.9.1. CONFIGURE_MAXIMUM_BARRIERS

CONSTANT:

  `CONFIGURE_MAXIMUM_BARRIERS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Barriers that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects.

## 24.9.2. CONFIGURE_MAXIMUM_MESSAGE_QUEUES

CONSTANT:

  `CONFIGURE_MAXIMUM_MESSAGE_QUEUES`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Message Queues that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects. You have to account for the memory used to store the messages of each message queue, see CONFIGURE_MESSAGE_BUFFER_MEMORY.

## 24.9.3. CONFIGURE_MAXIMUM_PARTITIONS

CONSTANT:

  `CONFIGURE_MAXIMUM_PARTITIONS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Partitions that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects.

## 24.9.4. CONFIGURE_MAXIMUM_PERIODS

CONSTANT:

  `CONFIGURE_MAXIMUM_PERIODS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Periods that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects.

## 24.9.5. CONFIGURE_MAXIMUM_PORTS

CONSTANT:

  `CONFIGURE_MAXIMUM_PORTS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Ports that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects.

## 24.9.6. CONFIGURE_MAXIMUM_REGIONS

CONSTANT:

  `CONFIGURE_MAXIMUM_REGIONS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Regions that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects.

## 24.9.7. CONFIGURE_MAXIMUM_SEMAPHORES

CONSTANT:

  `CONFIGURE_MAXIMUM_SEMAPHORES`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Semaphore that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects.

  In SMP configurations, the size of a Semaphore Control Block depends on the scheduler count (see Configuration Step 3 - Scheduler Table). The semaphores using the Multiprocessor Resource Sharing Protocol (MrsP) need a ceiling priority per scheduler.

## 24.9.8. CONFIGURE_MAXIMUM_TASKS

CONSTANT:

  `CONFIGURE_MAXIMUM_TASKS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

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

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Tasks that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects.

  The calculations for the required memory in the RTEMS Workspace for tasks assume that each task has a minimum stack size and has floating point support enabled. The configuration parameter `CONFIGURE_EXTRA_TASK_STACKS` is used to specify task stack requirements ABOVE the minimum size required. See Reserve Task/Thread Stack Memory Above Minimum for more information about `CONFIGURE_EXTRA_TASK_STACKS`.

  The maximum number of POSIX threads is specified by CONFIGURE_MAXIMUM_POSIX_THREADS.

  A future enhancement to `<rtems/confdefs.h>` could be to eliminate the assumption that all tasks have floating point enabled. This would require the addition of a new configuration parameter to specify the number of tasks which enable floating point support.

## 24.9.9. CONFIGURE_MAXIMUM_TIMERS

CONSTANT:

  `CONFIGURE_MAXIMUM_TIMERS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

  - It may be defined through `rtems_resource_unlimited()` the enable unlimited objects for this object class, if the value passed to `rtems_resource_unlimited()` satisfies all other constraints of this configuration option.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API Timers that can be concurrently active.

NOTES:

  This object class can be configured in unlimited allocation mode, see Unlimited Objects.

## 24.9.10. CONFIGURE_MAXIMUM_USER_EXTENSIONS

CONSTANT:

  `CONFIGURE_MAXIMUM_USER_EXTENSIONS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is 0.

VALUE CONSTRAINTS:

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

  - It shall be greater than or equal to 0.

  - It shall be less than or equal to 65535.

  - It shall be less than or equal to a BSP-specific and application-specific value which depends on the size of the memory available to the application.

DESCRIPTION:

  The value of this configuration option defines the maximum number of Classic API User Extensions that can be concurrently active.

NOTES:

  This object class cannot be configured in unlimited allocation mode.