RTEMS C User's Guide
Per task variables are used to support global variables whose value may be unique to a task. After indicating that a variable should be treated as private (i.e. per-task) the task can access and modify the variable, but the modifications will not appear to other tasks, and other tasks' modifications to that variable will not affect the value seen by the task. This is accomplished by saving and restoring the variable's value each time a task switch occurs to or from the calling task.
The value seen by other tasks, including those which have not added the variable to their set and are thus accessing the variable as a common location shared among tasks, can not be affected by a task once it has added a variable to its local set. Changes made to the variable by other tasks will not affect the value seen by a task which has added the variable to its private set.
This feature can be used when a routine is to be spawned repeatedly as
several independent tasks. Although each task will have its own stack,
and thus separate stack variables, they will all share the same static and
global variables. To make a variable not shareable (i.e. a "global" variable
that is specific to a single task), the tasks can call
rtems_task_variable_add to make a separate copy of the variable
for each task, but all at the same physical address.
Task variables increase the context switch time to and from the tasks that own them so it is desirable to minimize the number of task variables. One efficient method is to have a single task variable that is a pointer to a dynamically allocated structure containing the task's private "global" data.
A critical point with per-task variables is that each task must separately request that the same global variable is per-task private.
RTEMS C User's Guide
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