Structure padding

[123music]

Hi All...

In C structures compiler will be padding extra bytes. How it is detected by the compiler?

struct
{
char a;
int b;
}

Here 3 bytes are padded (in VC++) after char a..
How compiler has detected it..How does it know that it has to add 3 bytes??

Can anyone give me an example how compiler exactly detects it..

Thanks...