RTEMS C User's Guide
RTEMS provides for a pointer to a user-defined data area for each extension set to be linked to each task's control block. This set of pointers is an extension of the TCB and can be used to store additional data required by the user's extension functions. It is also possible for a user extension to utilize the notepad locations associated with each task although this may conflict with application usage of those particular notepads.
The TCB extension is an array of pointers in the TCB. The index into the table can be obtained from the extension id returned when the extension is created:
index = rtems_get_index(extension_id);
The number of pointers in the area is the same as the number of user extension sets configured. This allows an application to augment the TCB with user-defined information. For example, an application could implement task profiling by storing timing statistics in the TCB's extended memory area. When a task context switch is being executed, the TASK_SWITCH extension could read a real-time clock to calculate how long the task being swapped out has run as well as timestamp the starting time for the task being swapped in.
If used, the extended memory area for the TCB should be allocated and the TCB extension pointer should be set at the time the task is created or started by either the TASK_CREATE or TASK_START extension. The application is responsible for managing this extended memory area for the TCBs. The memory may be reinitialized by the TASK_RESTART extension and should be deallocated by the TASK_DELETE extension when the task is deleted. Since the TCB extension buffers would most likely be of a fixed size, the RTEMS partition manager could be used to manage the application's extended memory area. The application could create a partition of fixed size TCB extension buffers and use the partition manager's allocation and deallocation directives to obtain and release the extension buffers.
RTEMS C User's Guide
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