Node: Alignment Clauses, Next: , Up: Representation Clauses and Pragmas

Alignment Clauses

GNAT requires that all alignment clauses specify a power of 2, and all default alignments are always a power of 2. The default alignment values are as follows:

An alignment clause may always specify a larger alignment than the default value, up to some maximum value dependent on the target (obtainable by using the attribute reference System'Maximum_Alignment). The only case in which it is permissible to specify a smaller alignment than the default value is in the case of a record for which a record representation clause is given. In this case, packable fields for which a component clause is given still result in a default alignment corresponding to the original type, but this may be overridden, since these components in fact only require an alignment of one byte. For example, given

       type v is record
          a : integer;
       end record;
       for v use record
          a at 0  range 0 .. 31;
       end record;
       for v'alignment use 1;

The default alignment for the type v is 4, as a result of the integer field in the record, but since this field is placed with a component clause, it is permissible, as shown, to override the default alignment of the record to a smaller value.