[IRCServices Coding] GCC3

[Finny Merrill]

On Tue, 26 Feb 2002, Andrew Church wrote:

>      Just to clarify, my point is padding shouldn't be put in where it
> isn't needed; for example,
> 
> struct {
>     int8_t a, b, c;
> } baz;
> 
> should give sizeof(baz) == 3 (and it does in gcc 2.95.3).
odd...
> 
>   --Andrew Church
>     achurch at achurch.org
>     http://achurch.org/
> 
> >>>      Again, that's what I thought--compilers aren't supposed to pad more
> >>> than the largest type in the structure, and between structure members only
> >>> enough to align the next member to a multiple of its size.  I'm pretty sure
> >>> this is defined somewhere, and if not then it should be (see below).
> >>Not the largest type in the structure, the largest *type*.
> >>Most structures will pad to 32 bits on intel machines.
> >>
> >>like this:
> >>
> >>struct {
> >>  int8_t byte;
> >>  /* inserts 8 or 24 bits of padding here */
> >>  int16_t word;
> >>  /* inserts 16 bits of padding here */
> >>  int32_t dword;
> >>  /* no padding here */
> >>} something;
> >
> >     That's missing the point; you put a 32-bit type in there, which of
> >course means it will pad to 32 bits.  (And by your argument, it would have
> >to pad to at least the size of a double, not just an int32_t.)  What you're
> >saying would be something like:
> >
> >struct {
> >    int8_t byte;
> >    /* 24 bits of padding */
> >    int16_t word;
> >    /* 16 bits of padding */
> >} foo;  /* size = 64 bits */
> >
> >which is stupid because you have 32 bits of wasted space, when you could
> >just as easily and with no alignment problems (at least on any CPU I know
> >of) have done:
> >
> >struct {
> >    int8_t byte;
> >    /* 8 bits of padding */
> >    int16_t word;
> >} bar;  /* size = 32 bits */
> >
> >  --Andrew Church
> >    achurch at achurch.org
> >    http://achurch.org/
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