Task data. STMicroelectronics ST200-TOOLSET

OS21 Tasks

Note:

OS21 provides one word of task-data per task. This can be used by the application to store data which is specific to the task, but which needs to be accessed uniformly from multiple tasks.

This is typically used to store data which is required by a library, when the library can be used from multiple tasks but the data is specific to the task. For example, a library which manages an I/O channel may be called by multiple tasks, each of which has its own I/O buffers. To avoid having to pass an I/O descriptor into every call it could be stored in task-data.

Although only one word of storage is provided, this is usually treated as a pointer, which points to a user defined data structure which can be as large as required.

Two functions provide access to the task-data pointer: void* task_data_set (task_t* task, void* new_data); sets the task-data pointer of the task specified by task.

void* task_data (task_t* task); retrieves the task-data pointer of the task specified by task.

If task is NULL, both functions use the currently active task.

When a task is first created (including the root task), its task-data pointer is set to NULL (0).

For example: typedef struct {

char buffer[BUFFER_SIZE];

char* buffer_next;

char* buffer_end;

} ptd_t; char buffer_read(void)

{

ptd_t *ptd;

ptd = task_data(NULL);

if (ptd->buffer_next == ptd->buffer_end) {

... fill buffer ...

}

}

return *(ptd->buffer_next++); int main()

{

ptd_t *ptd;

task_t *task;

... create a task ...

ptd = memory_allocate(some_partition, sizeof(ptd_t));

}

ptd->buffer_next = ptd->buffer_end = ptd->buffer;

task_data_set(task, ptd); kernel_start()

must be called before

memory_allocate()

to prevent the function failing.

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