Feb 10, 2008

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.

struct packed_struct {
unsigned int f1:1;
unsigned int f2:1;
unsigned int f3:1;
unsigned int f4:1;
unsigned int type:4;
unsigned int funny_int:9;
} pack;

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 as compactly as possible, provided that the maximum length of the field is less than or equal to the integer word length of the computer. If this is not the case then some compilers may allow memory overlap for the fields whilst other would store the next field in the next word (see comments on bit fiels portability below).

Access members as usual via:

pack.type = 7;

NOTE:

* Only n lower bits will be assigned to an n 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 $\pm$ flag.