C special syntax (adding ':') when defining a structure

Question

Possible duplicate:
What it does: do in the structure declaration after the member

I would like to ask why the symbol was added in this structure::

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

Thank you in advance;

Answer 1

It declares a bit field with 4 bits.

Answer 2

:4 places a 4-bit constraint on the variable. See Section 6.9 of Kernighan and Richie.

Answer 3

He called the field a bit. In this case, he says that size B is 4 bits.