On Thu, 20 May 1999, Peter Lipa wrote:
<snip>
>
> struct Test_t {
> Char_t c;
> Short_t i;
> } testStruct;
>
> Naively (and rightly so) you would expect to get a sizeof(testStruct) = 3.
> This you will get e.g. with the gcc compiler and default flags.
<snip>
Your conclusions about the structure length being different than the sum
of the lengths of its constituent parts is of course correct, but I have
never seen a compiler that would allocate 3 bytes for testStruct. In
general (i.e. on all machines/compilers I have ever seen) the default
mapping of structures into memory follows three rules:
1) Structure elements are stored in memory in the order in which they
were defined.
2) Each atomic element (a simple variable or a member of a
sub-structure or array) is aligned on an offset that is a multiple
of its length.
3) The structure is padded to give a length that is a multiple of the
length of the longest atomic element.
Thus for your example structure, ALL compilers I have ever seen
(including gcc) will give sizeof(struct testStruct) == 4 (assuming
sizeof(Short_t)==2 and sizeof(Char_t) == 1).
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