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     The _expand family of functions changes the size of a previously
     allocated memory block by attempting to expand or contract the
     block without moving its location in the heap. The <memblock>
     argument points to the beginning of the block. The <size> argument
     gives the new size of the block, in bytes. The contents of the
     block are unchanged up to the shorter of the new and old sizes.
 
     The <memblock> argument can also point to a block that has been
     freed, as long as there has been no intervening call to calloc,
     _expand, malloc, or realloc. If <memblock> points to a freed
     block, the block remains free after the call to _expand.
 
     The <segment> argument is the segment address of a based heap.
 
     In large data models (compact-, large-, and huge-model programs),
     _expand maps to _fexpand. In small data models (tiny-, small-, and
     medium-model programs), _expand maps to _nexpand.
 
     The various _expand functions change the size of the storage block
     in the data segments shown below:
 
     Function     Data Segment
 
     expand       Depends on data model of program
     _bexpand     Based heap specified by seg, or in all based heaps
                  if seg is zero
     _fexpand     Far heap (outside default data segment)
     _nexpand     Near heap (inside default data segment)
 
     Return Value
 
     The _expand family of functions returns a void pointer to the
     reallocated memory block. Unlike realloc, _expand cannot move a
     block to change its size. This means the <memblock> argument to
     _expand is the same as the return value if there is sufficient
     memory available to expand the block without moving it.
 
     The return value is NULL if there is insufficient memory available
     to expand the block to the given size without moving it. The
     _bexpand function returns _NULLOFF if insufficient memory is
     available. In this case, the item pointed to by <memblock> will
     have been expanded as much as possible in its current location.
 
     The storage space pointed to by the return value is guaranteed to
     be suitably aligned for storage of any type of object. The new
     size of the item can be checked with the msize function. To get a
     pointer to a type other than void, use a type cast on the return
     value.
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