13.2 Pragma Pack
1
[A pragma Pack specifies
that storage minimization should be the main criterion when selecting
the representation of a composite type.]
Syntax
2
The form of
a pragma Pack is as follows:
3
pragma Pack(
first_subtype_local_name);
Legality Rules
4
The first_subtype_local_name
of a pragma Pack shall denote a composite
subtype.
Static Semantics
5
{representation pragma
(Pack) [partial]} {pragma,
representation (Pack) [partial]} {aspect
of representation (packing) [partial]} {packing
(aspect of representation)} {packed}
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.
5.a
Ramification: The only high level semantic
effect of a
pragma Pack is independent addressability
(see
9.10, “
Shared
Variables”).
Implementation Advice
6
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.a.1/2
Implementation Advice:
Storage allocated to objects of a packed
type should be minimized.
6.a
Ramification: A pragma
Pack is for gaining space efficiency, possibly at the expense of time.
If more explicit control over representation is desired, then a record_representation_clause,
a Component_Size clause, or a Size clause should be used instead of,
or in addition to, a pragma Pack.
6.1/2
{
AI95-00291-02}
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. 7
{recommended
level of support (pragma Pack) [partial]} The
recommended level of support for pragma Pack is:
8
- 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.
8.a
Ramification: The implementation can
always allocate an integral number of words for a component that will
not fit in a word. The rule also allows small component sizes to be rounded
up if such rounding does not waste space. For example, if Storage_Unit
= 8, then a component of size 8 is probably more efficient than a component
of size 7 plus a 1-bit gap (assuming the gap is needed anyway).
9
- 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.
9.a
Ramification: If a component subtype
is aliased, its Size will generally be a multiple of Storage_Unit, so
it probably won't get packed very tightly.
9.b/2
Implementation Advice:
The recommended level of support for
pragma Pack should be followed.
Wording Changes from Ada 95
9.c/2
{
AI95-00291-02}
Added clarification that pragma Pack can ignore
alignment requirements on types that don't have by-reference or aliased
parts. This was always intended, but there was no wording to that effect.
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