This lists the technical data of CMSIS-FreeRTOS.

Limitations

The following list briefly describes the limitations and unsupported features of the CMSIS-RTOS2 wrapper for FreeRTOS:

Static memory allocation will only work if all memory (from attributes structure) is provided statically. In order to allocate object memory statically, you need to:
- provide the memory for control blocks and stack in the osThreadAttr_t structure for threads.
- provide the memory for control blocks and message data in the osMessageQueueAttr_t structure for memory queues.
- provide the memory for control blocks for other objects in the object's attributes structure.
Each timer object requires additional 8 bytes of memory:
- to allocate all memory statically, provide the memory for control block of size (sizeof(StaticTimer_t) + 8 bytes)
- otherwise, additional 8 bytes of dynamic memory will be used
osKernelSuspend and osKernelResume are not supported.
osThreadDetach, osThreadJoin() and attribute osThreadJoinable are not supported (osThreadNew returns NULL when osThreadJoinable attribute is specified).
osThreadGetStackSize is not implemented.
Event flags are limited to 24 bits.
osEventFlagsGetName is not implemented.
osEventFlagsWait cannot be called from an ISR.
Priority inherit protocol is used as default mutex behavior (osMutexNew creates priority inherit mutex object by default and ignores osMutexPrioInherit attribute when specified).
Robust mutex objects are not supported (osMutexNew returns NULL when osMutexRobust attribute is specified).
osMutexGetName is not implemented and always returns NULL.
osSemaphoreGetName is not implemented and always returns NULL.
osMessageQueueGetName is not implemented and always returns NULL.
osMessageQueuePut and osMessageQueueGet always ignore message priority.
Process Isolation (Functional Safety) functions are not implemented.

Validation suite results

CMSIS provides a CMSIS-RTOS2 validation suite that can be used to test a real-time operating system for compliance to the standard. The test suite has been executed successfully on the CMSIS-FreeRTOS implementation (see results).

The following table explains the exceptions:

Test Case	Result	Reason	Summary
TC_osKernelGetState_2	not executed	unsupported feature	Test attempts to call `osKernelGetState` after a `osKernelSuspend` call. osKernelSuspend is not implemented.
TC_osKernelLock_2	not executed	unsupported feature	Test attempts to call `osKernelLock` after a `osKernelSuspend` call. `osKernelSuspend` is not implemented.
TC_osKernelUnlock_2	not executed	unsupported feature	Test attempts to call `osKernelUnlock` after a `osKernelSuspend` call. `osKernelSuspend` is not implemented.
TC_osKernelSuspend_1	not executed	unsupported feature	Test validates `osKernelSuspend` which is not implemented.
TC_osKernelResume_1	not executed	unsupported feature	Test validates `osKernelResume` which is not implemented.
TC_osThreadNew_3	not executed	unsupported feature	Test attempts to create joinable thread using `osThreadJoinable` attribute. FreeRTOS does not support joinable threads.
TC_osThreadGetName_1	failed	deviation	Test attempt to retrieve a name on an unnamed thread. An empty string is returned instead of NULL pointer.
TC_osThreadGetState_3	not executed	unsupported feature	Test attempts to retrieve a state of a terminated joinable thread. FreeRTOS does not support joinable threads.
TC_osThreadDetach_1	not executed	unsupported feature	Test validates `osThreadDetach` which is not implemented.
TC_osThreadDetach_2	not executed	unsupported feature	Test validates `osThreadDetach` which is not implemented.
TC_osThreadJoin_1	not executed	unsupported feature	Test validates `osThreadJoin` which is not implemented.
TC_osThreadJoin_2	not executed	unsupported feature	Test validates `osThreadJoin` which is not implemented.
TC_osThreadJoin_3	not executed	unsupported feature	Test validates `osThreadJoin` which is not implemented.
TC_osThreadGetStackSize_1	not executed	unsupported feature	Test validates `osThreadGetStackSize` which is not implemented.
TC_ThreadReturn	not executed	unsupported feature	Test attempts to terminate a thread by just returning from a thread. FreeRTOS threads may not return.
TC_osEventFlagsSet_1	failed	deviation	Test attempts to set event flags by calling `osEventFlagsSet` multiple times without leaving ISR handler. To process ISR requests, FreeRTOS uses timer deamon which wakes-up after ISR execution.
TC_osEventFlagsClear_1	failed	deviation	Test attempts to clear event flags by calling `osEventFlagsClear` multiple times without leaving ISR handler. To process ISR requests, FreeRTOS uses timer deamon which wakes-up after ISR execution.
TC_osEventFlagsWait_1	failed	unsupported feature	Test attempts to wait for flags from ISR with zero timeout (try-semantic). FreeRTOS does not support such operation.
TC_osEventFlagsGetName_1	not executed	unsupported feature	Test validates `osEventFlagsGetName` which is not implemented.
TC_osMutexNew_4	not executed	unsupported feature	Test attempts to create a robust mutex. FreeRTOS implementation does not support robust mutexes.
TC_osMutexGetName_1	not executed	unsupported feature	Test validates `osMutexGetName` which is not implemented.
TC_MutexRobust	not executed	unsupported feature	Test attempts to validate robust mutex behavior. FreeRTOS implementation does not support robust mutexes.
TC_MutexOwnership	not executed	unsupported feature	Test attempts to release a mutex from a thread which is not the mutex owner. FreeRTOS implementation does not verify ownership on mutex release.
TC_osSemaphoreGetName_1	not executed	unsupported feature	Test validates `osSemaphoreGetName` which is not implemented.
TC_osMessageQueueGetName_1	not executed	unsupported feature	Test validates `osMessageQueueGetName` which is not implemented.

Troubleshooting

Users looking for help shall check subsection of the full FreeRTOS FAQ. It contains many useful information that also apply when using FreeRTOS in context of CMSIS-FreeRTOS.

Additionally, please take a look at the following:

Interrupts are disabled when main is called or before the kernel is started
Before the FreeRTOS kernel is started, threads (i.e. tasks) may be created together with other objects like mutexes, semaphores, message queues etc. When functions like xTaskCreate, xSemaphoreCreateMutex, xQueueCreate and others get called, they prevent interrupt handlers from calling FreeRTOS API functions in order to keep FreeRTOS kernel variables and state machine consistent (see also here Interrupts are not executing).
In cases when interrupts may be executed after object creation and before the FreeRTOS kernel is started they can be re-enabled:
portENABLE_INTERRUPTS();

Make sure that the interrupts you execute in such case do not call FreeRTOS API.