Bit Fields

Bit Fields allow the packing of data in a structure. This is especially useful when memory or data storage is at a premium. Typical examples:

• Packing several objects into a machine word. e.g. 1 bit flags can be compacted - Symbol tables in compilers.
• Reading external file formats - non-standard file formats could be read in. E.g. 9 bit integers.

C lets us do this in a structure definition by putting :bit length after the variable. i.e.

Here the packed_struct contains 6 members: Four 1 bit flags f1..f3, a 4 bit type and a 9 bit funny_int.

C automatically packs the above bit fields together.

Access members as usual via:

pack.type = 7;

NOTE:

• Only lower bits will be assigned to an bit number. So type cannot take values larger than 15 (4 bits long).

   

• Bit fields are always converted to integer type for computation.

   

• You are allowed to mix ``normal'' types with bit fields.

   

• The unsigned definition is important - ensures that no bits are used as a flag.