Pragma Pack applied to a record will pack the components to reduce wasted space from alignment gaps and by reducing the amount of space taken by components. We distinguish between package components and non-packable components. Components of the following types are considered packable:
All packable components occupy the exact number of bits corresponding to
Size value, and are packed with no padding bits, i.e. they
can start on an arbitrary bit boundary.
All other types are non-packable, they occupy an integral number of storage units, and are placed at a boundary corresponding to their alignment requirements.
For example, consider the record
type Rb1 is array (1 .. 13) of Boolean; pragma Pack (rb1); type Rb2 is array (1 .. 65) of Boolean; pragma Pack (rb2); type x2 is record l1 : Boolean; l2 : Duration; l3 : Float; l4 : Boolean; l5 : Rb1; l6 : Rb2; end record; pragma Pack (x2);
The representation for the record x2 is as follows:
for x2'Size use 224; for x2 use record l1 at 0 range 0 .. 0; l2 at 0 range 1 .. 64; l3 at 12 range 0 .. 31; l4 at 16 range 0 .. 0; l5 at 16 range 1 .. 13; l6 at 18 range 0 .. 71; end record;
Studying this example, we see that the packable fields
of length equal to their sizes, and placed at specific bit boundaries (and
not byte boundaries) to
eliminate padding. But
l3 is of a non-packable float type, so
it is on the next appropriate alignment boundary.
The next two fields are fully packable, so
minimally packed with no gaps. However, type
Rb2 is a packed
array that is longer than 64 bits, so it is itself non-packable. Thus
l6 field is aligned to the next byte boundary, and takes an
integral number of bytes, i.e. 72 bits.