Packing data types

The pragma <* bytealignment(0) *> can be used to specify that the fields within a RECORD are to be packed. Currently this only applies to fields which are declared as subranges, ordinal types and enumerated types. Here is an example of how two subranges might be packed into a byte.

TYPE
   bits3c =  [0..7] ;
   bits3i = [-4..3] ;

   byte = RECORD
              <* bytealignment(0) *>
              x: bits3c ;
              <* bitsunused(2) *>
              y: bits3i ;
          END ;

Notice that the user has specified that in between fields x and y there are two bits unused.

Now the user wishes to create a record with byte numbers zero and one occupied and then an INTEGER32 field which is four byte aligned. In this case byte numbers two and three will be unused. The pragma bytealignment can be issued at the start of the record indicating the default alignment for the whole record and this can be overridden by individual fields if necessary.

   rec = RECORD
            <* bytealignment (1) *> ;
            a, b: byte ;
            x: INTEGER32 <* bytealignment(4) *> ;
         END ;

In the following example the user has specified that a record has two fields p and q but that there are three bytes unused between these fields.

   header = RECORD
               <* bytealignment(1) *>
               p: byte ;
               <* bytesunused(3) *>
               q: byte ;
            END ;

The pragma <* bytesunused(x) *> can only be used if the current field is on a byte boundary. There is also a SYSTEM pseudo procedure function TBITSIZE(T) which returns the minimum number of bits necessary to represent type T.

Another example of packing record bit fields is given below:

MODULE align21 ;

FROM libc IMPORT exit ;

TYPE
   colour = (red, blue, green, purple, white, black) ;

   soc = PACKEDSET OF colour ;

   rec = RECORD
            <* bytealignment(0) *>
            x: soc ;
            y: [-1..1] ;
         END ;

VAR
   r: rec ;
   v: CARDINAL ;
BEGIN
   v := SIZE(r) ;
   IF SIZE(r)#1
   THEN
      exit(1)
   END ;
   r.x := soc{blue} ;
   IF r.x#soc{blue}
   THEN
      exit(2)
   END
END align21.

Here we see that the total size of this record is one byte and consists of a six bit set type followed by a 2 bit integer subrange.

The GNU Modula-2 front end to GCC

Packing data types