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xTaskCreateRestricted
[FreeRTOS-MPU Specific]
task. h
portBASE_TYPE xTaskCreateRestricted(
xTaskParameters *pxTaskDefinition,
xTaskHandle *pxCreatedTask );
Create a new Memory Protection Unit (MPU) restricted task and add it to the list of tasks that are ready to run.
xTaskCreateRestricted() is intended for use with FreeRTOS-MPU, the demo applications for which contain comprehensive and documented examples of xTaskCreateRestricted() being used.
- Parameters:
-
pxTaskDefinition Pointer to a structure that defines the task. The structure is described on this page. pxCreatedTask Used to pass back a handle by which the created task can be referenced.
- Returns:
- pdPASS if the task was successfully created and added to a ready list, otherwise an error code defined in the file projdefs.h
Tasks that include MPU support require even more parameters to create than those that don't. Passing each parameter to xTaskCreateRestricted() individually would be unwieldy so instead the structure xTaskParameters is used to allow the parameters to be configured statically at compile time. The structure is defined in task.h as:
typedef struct xTASK_PARAMTERS
{
pdTASK_CODE pvTaskCode;
const signed char * const pcName;
unsigned short usStackDepth;
void *pvParameters;
unsigned portBASE_TYPE uxPriority;
portSTACK_TYPE *puxStackBuffer;
xMemoryRegion xRegions[ portNUM_CONFIGURABLE_REGIONS ];
} xTaskParameters;
....where xMemoryRegion is defined as:
typedef struct xMEMORY_REGION
{
void *pvBaseAddress;
unsigned long ulLengthInBytes;
unsigned long ulParameters;
} xMemoryRegion;
Following is a description of each structure member:
- pvTaskCode to uxPriority
These members are exactly the same as the parameters to xTaskCreate() of the same name. In particular uxPriority is used to set both the priority of the task and the mode in which the task will execute. For example, to create a User mode task at priority 2 simply set uxPriority to 2, to create a Privileged mode task at priority 2 set uxPriority to ( 2 | portPRIVILEGE_BIT ).
- puxStackBuffer
Each time a task is switched in the MPU is dynamically re-configured to define a region that provides the task read and write access to its own stack. MPU regions must meet a number of constraints - in particular, the size and alignment of each region must both be equal to the same power of two value.
Standard FreeRTOS ports use pvPortMalloc() to allocate a new stacks each time a task is created. Providing a pvPortMalloc() implementation that took care of the MPU data alignment requirements would be possible but would also be complex and inefficient in its RAM usage. To remove the need for this complexity FreeRTOS-MPU allows stacks to be declared statically at compile time. This allows the alignment to be managed using compiler extensions and RAM usage efficiency to be managed by the linker. For example, if using GCC a stack could be declared and correctly aligned using the following code:
char cTaskStack[ 1024 ] __attribute__((align(1024));puxStackBuffer would normally be set to the address of the statically declared stack. As an alternative puxStackBuffer can be set to NULL - in which case pvPortMallocAligned() will be called to allocate the task stack and it is the application writers responsibility to provide an implementation of pvPortMallocAligned() that meets the alignment requirements of the MPU. - xMemoryRegions
xRegions is an array of xMemoryRegion structures, each of which defines a single user definable memory region for use by the task being created. The ARM Cortex-M3 FreeRTOS-MPU port defines portNUM_CONFIGURABLE_REGIONS to be 3.
The pvBaseAddress and ulLengthInBytes members are self explanatory as the start of the memory region and the length of the memory region respectively. ulParameters defines how the task is permitted to access the memory region and can take the bitwise OR of the following values:
portMPU_REGION_READ_WRITE portMPU_REGION_PRIVILEGED_READ_ONLY portMPU_REGION_READ_ONLY portMPU_REGION_PRIVILEGED_READ_WRITE portMPU_REGION_CACHEABLE_BUFFERABLE portMPU_REGION_EXECUTE_NEVER
Example usage (please refer to the FreeRTOS-MPU demo applications
for a much more complete and comprehensive example):
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