RTEMS C User's Guide
The rtems_region_create directive creates a region with the
user-defined name. The user may select FIFO or task priority as
the method for placing waiting tasks in the task wait queue.
RTEMS allocates a Region Control Block (RNCB) from the RNCB free
list to maintain the newly created region. RTEMS also generates
a unique region ID which is returned to the calling task.
It is not possible to calculate the exact number of bytes available to the user since RTEMS requires overhead for each segment allocated. For example, a region with one segment that is the size of the entire region has more available bytes than a region with two segments that collectively are the size of the entire region. This is because the region with one segment requires only the overhead for one segment, while the other region requires the overhead for two segments.
Due to automatic coalescing, the number of segments in the region dynamically changes. Therefore, the total overhead required by RTEMS dynamically changes.
RTEMS C User's Guide
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