what does this mean in c int a:16;? [duplicate]

Question

Possible Duplicate:
What does 'unsigned temp:3' mean?

please what does this notation mean

int a:16;

I found it is code like this and it does compile.

struct name { int a:16; }

Answer 1

This is a bitfield.

This particular bitfield doesn't make much sense as you just could use a 16-bit type and you're wasting some space as the bitfield is padded to the size of int.

Usually, you are using it for structures that contain elements of bit size:

struct {
    unsigned nibble1 : 4;
    unsigned nibble2 : 4;
}

Answer 2

struct name { int a:16; }

It means a is defined as 16-bit memory space. The remaining bits (16 bits) from int can be used to defined another variable, say b, like this:

struct name { int a:16;  int b:16; }

So if int is 32-bit (4 bytes), then the memory of one int is divided into two variables a and b.

PS: I'm assuming sizeof(int) = 4 bytes, and 1 byte = 8 bits

Answer 3

 struct s
    {
     int a:1;
     int b:2;
     int c:7;
    };/*size of structure s is 4 bytes and not 4*3=12 bytes since all share the same space provided by int declaration for the first variable.*/
 struct s1
    {
     char a:1;
     };/*size of struct s1 is 1byte had it been having any more char _var:_val it would have    been the same.*/

Answer 4

It's a bitfield.

I've never seen a 16 bit bitfield; usually that's a short.

http://www.cs.cf.ac.uk/Dave/C/node13.html