C special syntax (adding ':' ) while defining a struct [duplicate]

Question

Possible Duplicate:
What does the : do in a struct declaration after a member

I would like to ask why the : character was added in this struct:

typedef union A
{
struct 
    {
        ubyte B:4;
         }
} struct_a;

Thanks in advance;

Answer 1

The :4 is putting a 4-bit limit on the variable. See Section 6.9 of Kernighan & Ritchie.