You should also arrange data types so that they slot together nicely:
The size of sizeof(s1) is 16 bytes and the sizeof(s2) is 12 bytes, because the two 2-byte short values can fit into a single 4-byte "slot".Code:struct s1 { int i; short j; int k; short l; }; struct s2 { int i; int k; short j; short l; };
The "slot" theory often doesn't work, as Elysia noted, because shorts are often aligned to a larger byte interval.Originally Posted by Ushakal
A common rule of thumb is to declare the members in order of decreasing size (i.e. big ones first). That doesn't guarantee no padding but is often a reasonable step towards minimising the total amount of padding added.
Since alignment of variables on (processor dependent) word boundaries often has a performance benefit most compilers add padding by default. For this reason, the sizes you quoted in your example are not guaranteed.
