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bit-fields                  Definition                 bit-fields




A bit-field is  a member of a structure or  union that is defined
to be  a cluster of  bits.  It provides  a way to  represent data
compactly.  For example, in the following structure


        struct example {
                int member1;
                long member2;
                unsigned int member3 :5;
        }


member3 is declared to be a bit-field that consists of five bits.
A  colon `:'  precedes the integral  constant that  indicates the
_w_i_d_t_h, or  the number of  bits in the bit-field.   Also, the bit-
field declarator  must include a type, which must  be one of int,
signed int, or unsigned int.

A bit-field that  is not given a name may  not be accessed.  Such
an object  is useful as  ``padding'' within an object  so that it
conforms to a template designed elsewhere.

A bit-field that is unnamed and  has a length of zero can be used
to force adjacent bit-fields into separate objects.  For example,
in the following structure


        struct example {
                int member1;
                int member2 :5;
                int :0;
                int member3 :5;
        };


the zero-length bit-field  forces member2 and member3 to be writ-
ten into separate objects.

Finally, it is illegal to take the address of a bit-field.

***** See Also *****

bit, bit map, byte, definitions

***** Notes *****

Because bit-fields  have many implementation-specific properties,
they are  not considered to  be highly portable.   Bit-fields use
minimal amounts of  storage, but the amount of computation needed
to  manipulate and  access them  may  negate this  benefit.  Bit-
fields  must  be  kept  in  integral-sized objects  because  many
machines  cannot directly  access a  quantity of  storage smaller
than a ``word'' (a word is generally used to store an int).



COHERENT Lexicon                                           Page 1

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