Ada Reference Manual — Legal Information

13.2 Pragma Pack

A pragma Pack specifies that storage minimization should be the main criterion when selecting the representation of a composite type.

Syntax

The form of a pragma Pack is as follows:

pragma Pack(first_subtype_local_name);

Legality Rules

The first_subtype_local_name of a pragma Pack shall denote a composite subtype.

Static Semantics

A pragma Pack specifies the packing aspect of representation; the type (or the extension part) is said to be packed. For a type extension, the parent part is packed as for the parent type, and a pragma Pack causes packing only of the extension part.

Implementation Advice

If a type is packed, then the implementation should try to minimize storage allocated to objects of the type, possibly at the expense of speed of accessing components, subject to reasonable complexity in addressing calculations.

6.1/2

If a packed type has a component that is not of a by-reference type and has no aliased part, then such a component need not be aligned according to the Alignment of its subtype; in particular it need not be allocated on a storage element boundary.

The recommended level of support for pragma Pack is:

For a packed record type, the components should be packed as tightly as possible subject to the Sizes of the component subtypes, and subject to any record_representation_clause that applies to the type; the implementation may, but need not, reorder components or cross aligned word boundaries to improve the packing. A component whose Size is greater than the word size may be allocated an integral number of words.

For a packed array type, if the component subtype's Size is less than or equal to the word size, and Component_Size is not specified for the type, Component_Size should be less than or equal to the Size of the component subtype, rounded up to the nearest factor of the word size.