6.12 Record Representation Clauses

Record representation clauses may be given for all record types, including types obtained by record extension. Component clauses are allowed for any static component. The restrictions on component clauses depend on the type of the component.

For all components of an elementary type, the only restriction on component clauses is that the size must be at least the 'Size value of the type (actually the Value_Size). There are no restrictions due to alignment, and such components may freely cross storage boundaries.

Packed arrays with a size up to and including 64 bits are represented internally using a modular type with the appropriate number of bits, and thus the same lack of restriction applies. For example, if you declare:

        type R is array (1 .. 49) of Boolean;
        pragma Pack (R);
        for R'Size use 49;

then a component clause for a component of type R may start on any specified bit boundary, and may specify a value of 49 bits or greater.

For packed bit arrays that are longer than 64 bits, there are two cases. If the component size is a power of 2 (1,2,4,8,16,32 bits), including the important case of single bits or boolean values, then there are no limitations on placement of such components, and they may start and end at arbitrary bit boundaries.

If the component size is not a power of 2 (e.g. 3 or 5), then an array of this type longer than 64 bits must always be placed on on a storage unit (byte) boundary and occupy an integral number of storage units (bytes). Any component clause that does not meet this requirement will be rejected.

Any aliased component, or component of an aliased type, must have its normal alignment and size. A component clause that does not meet this requirement will be rejected.

The tag field of a tagged type always occupies an address sized field at the start of the record. No component clause may attempt to overlay this tag. When a tagged type appears as a component, the tag field must have proper alignment

In the case of a record extension T1, of a type T, no component clause applied to the type T1 can specify a storage location that would overlap the first T'Size bytes of the record.

For all other component types, including non-bit-packed arrays, the component can be placed at an arbitrary bit boundary, so for example, the following is permitted:

        type R is array (1 .. 10) of Boolean;
        for R'Size use 80;
     
        type Q is record
           G, H : Boolean;
           L, M : R;
        end record;
     
        for Q use record
           G at 0 range  0 ..   0;
           H at 0 range  1 ..   1;
           L at 0 range  2 ..  81;
           R at 0 range 82 .. 161;
        end record;

Note: the above rules apply to recent releases of GNAT 5. In GNAT 3, there are more severe restrictions on larger components. For non-primitive types, including packed arrays with a size greater than 64 bits, component clauses must respect the alignment requirement of the type, in particular, always starting on a byte boundary, and the length must be a multiple of the storage unit.