RTEMS C User's Guide
A region makes up a physically contiguous memory space with user-defined boundaries from which variable-sized segments are dynamically allocated and deallocated. A segment is a variable size section of memory which is allocated in multiples of a user-defined page size. This page size is required to be a multiple of four greater than or equal to four. For example, if a request for a 350-byte segment is made in a region with 256-byte pages, then a 512-byte segment is allocated.
Regions are organized as doubly linked chains of variable sized memory blocks. Memory requests are allocated using a first-fit algorithm. If available, the requester receives the number of bytes requested (rounded up to the next page size). RTEMS requires some overhead from the region's memory for each segment that is allocated. Therefore, an application should only modify the memory of a segment that has been obtained from the region. The application should NOT modify the memory outside of any obtained segments and within the region's boundaries while the region is currently active in the system.
Upon return to the region, the free block is coalesced with its neighbors (if free) on both sides to produce the largest possible unused block.
RTEMS C User's Guide
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