sizeof struct

I have a struct with 2 members of type double and 1 member of type long.

The sizeof() function returns 24. Normaly it should be 20(2*8bytes for the doubles and 4 bytes for the long).

When I add a another member of type long sizeof() returns 32.

Another struct with 3 longs returns 12 with sizeof().

I suppose it has something to do with allignment and optimalisation? How can I avoid this.

Luc Vercruysse