Question

I always thought I knew C very well until I saw something like this in another post:

struct foo {
    int x:1;
};

I would really like to know the purpose of the :1. Can anybody clue me in? Thanks.

Answer 1

bitfield.
x is 1 bit long.

Each field is accessed and manipulated
as if it were an ordinary member of a
structure. The keywords signed and
unsigned mean what you would expect,
except that it is interesting to note
that a 1-bit signed field on a two’s
complement machine can only take the
values 0 or -1. The declarations are
permitted to include the const and
volatile qualifiers.

The main use of bitfields is either to
allow tight packing of data or to be
able to specify the fields within some
externally produced data files. C
gives no guarantee of the ordering of
fields within machine words, so if you
do use them for the latter reason, you
program will not only be non-portable,
it will be compiler-dependent too. The
Standard says that fields are packed
into ‘storage units’, which are
typically machine words. The packing
order, and whether or not a bitfield
may cross a storage unit boundary, are
implementation defined. To force
alignment to a storage unit boundary,
a zero width field is used before the
one that you want to have aligned.

Be careful using them. It can require
a surprising amount of run-time code
to manipulate these things and you can
end up using more space than they
save.

Bit fields do not have addresses—you
can’t have pointers to them or arrays
of them.

http://publications.gbdirect.co.uk/c_book/chapter6/bitfields.html