9.7 Component_Size Clauses

Normally, the value specified in a component size clause must be consistent with the subtype of the array component with regard to size and alignment. In other words, the value specified must be at least equal to the size of this subtype, and must be a multiple of the alignment value.

In addition, component size clauses are allowed which cause the array to be packed, by specifying a smaller value. A first case is for component size values in the range 1 through 63. The value specified must not be smaller than the Size of the subtype. GNAT will accurately honor all packing requests in this range. For example, if we have:

type r is array (1 .. 8) of Natural;
for r'Component_Size use 31;

then the resulting array has a length of 31 bytes (248 bits = 8 * 31). Of course access to the components of such an array is considerably less efficient than if the natural component size of 32 is used. A second case is when the subtype of the component is a record type padded because of its default alignment. For example, if we have:

type r is record
  i : Integer;
  j : Integer;
  b : Boolean;
end record;

type a is array (1 .. 8) of r;
for a'Component_Size use 72;

then the resulting array has a length of 72 bytes, instead of 96 bytes if the alignment of the record (4) was obeyed.

Note that there is no point in giving both a component size clause and a pragma Pack for the same array type. if such duplicate clauses are given, the pragma Pack will be ignored.