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

# 24.11. POSIX API Configuration

This section describes configuration options related to the POSIX API. Most POSIX API objects are available by default since RTEMS 5.1. The queued signals and timers are only available if RTEMS was built with the `--enable-posix` build configuration option.

## 24.11.1. CONFIGURE_MAXIMUM_POSIX_KEYS

CONSTANT:

  `CONFIGURE_MAXIMUM_POSIX_KEYS`

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 POSIX API Keys that can be concurrently active.

NOTES:

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

## 24.11.2. CONFIGURE_MAXIMUM_POSIX_KEY_VALUE_PAIRS

CONSTANT:

  `CONFIGURE_MAXIMUM_POSIX_KEY_VALUE_PAIRS`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is CONFIGURE_MAXIMUM_POSIX_KEYS * CONFIGURE_MAXIMUM_TASKS + CONFIGURE_MAXIMUM_POSIX_THREADS.

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 key value pairs used by POSIX API Keys that can be concurrently active.

NOTES:

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

  A key value pair is created by `pthread_setspecific()` if the value is not `NULL`, otherwise it is deleted.

## 24.11.3. CONFIGURE_MAXIMUM_POSIX_MESSAGE_QUEUES

CONSTANT:

  `CONFIGURE_MAXIMUM_POSIX_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 shall be small enough so that the RTEMS Workspace size 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 POSIX 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.

## 24.11.4. CONFIGURE_MAXIMUM_POSIX_QUEUED_SIGNALS

CONSTANT:

  `CONFIGURE_MAXIMUM_POSIX_QUEUED_SIGNALS`

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 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 RTEMS Workspace size calculation carried out by `<rtems/confdefs.h>` does not overflow an integer of type `uintptr_t`.

  - It shall be zero if the POSIX API is not enabled (e.g. RTEMS was built without the `--enable-posix` build configuration option). Otherwise a compile time error in the configuration file will occur.

DESCRIPTION:

  The value of this configuration option defines the maximum number of POSIX API Queued Signals that can be concurrently active.

NOTES:

  Unlimited objects are not available for queued signals.

  Queued signals are only available if RTEMS was built with the `--enable-posix` build configuration option.

## 24.11.5. CONFIGURE_MAXIMUM_POSIX_SEMAPHORES

CONSTANT:

  `CONFIGURE_MAXIMUM_POSIX_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 shall be small enough so that the RTEMS Workspace size 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 POSIX API Named Semaphores that can be concurrently active.

NOTES:

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

  Named semaphores are created with `sem_open()`. Semaphores initialized with `sem_init()` are not affected by this configuration option since the storage space for these semaphores is user-provided.

## 24.11.6. CONFIGURE_MAXIMUM_POSIX_SHMS

CONSTANT:

  `CONFIGURE_MAXIMUM_POSIX_SHMS`

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 RTEMS Workspace size 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 POSIX API Shared Memory objects that can be concurrently active.

NOTES:

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

## 24.11.7. CONFIGURE_MAXIMUM_POSIX_THREADS

CONSTANT:

  `CONFIGURE_MAXIMUM_POSIX_THREADS`

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`.

DESCRIPTION:

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

NOTES:

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

  This calculations for the required memory in the RTEMS Workspace for threads assume that each thread has a minimum stack size and has floating point support enabled. The configuration option CONFIGURE_EXTRA_TASK_STACKS is used to specify thread 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 Classic API Tasks is specified by CONFIGURE_MAXIMUM_TASKS.

  All POSIX threads have floating point enabled.

## 24.11.8. CONFIGURE_MAXIMUM_POSIX_TIMERS

CONSTANT:

  `CONFIGURE_MAXIMUM_POSIX_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.

  - It shall be zero if the POSIX API is not enabled (e.g. RTEMS was built without the `--enable-posix` build configuration option). Otherwise a compile time error in the configuration file will occur.

DESCRIPTION:

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

NOTES:

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

  Timers are only available if RTEMS was built with the `--enable-posix` build configuration option.

## 24.11.9. CONFIGURE_MINIMUM_POSIX_THREAD_STACK_SIZE

CONSTANT:

  `CONFIGURE_MINIMUM_POSIX_THREAD_STACK_SIZE`

OPTION TYPE:

  This configuration option is an integer define.

DEFAULT VALUE:

  The default value is two times the value of CONFIGURE_MINIMUM_TASK_STACK_SIZE.

VALUE CONSTRAINTS:

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

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

DESCRIPTION:

  The value of this configuration option defines the minimum stack size in bytes for every POSIX thread in the system.

NOTES:

  None.