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14 <H1>[IRCServices Coding] GCC3
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17 TITLE=
"[IRCServices Coding] GCC3">achurch at achurch.org
19 <I>Tue Feb
26 15:
36:
30 PST
2002</I>
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34 <PRE> Just to clarify, my point is padding shouldn't be put in where it
35 isn't needed; for example,
41 should give sizeof(baz) ==
3 (and it does in gcc
2.95.3).
44 <A HREF=
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45 <A HREF=
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47 >>><i> Again, that's what I thought--compilers aren't supposed to pad more
48 </I>>>><i> than the largest type in the structure, and between structure members only
49 </I>>>><i> enough to align the next member to a multiple of its size. I'm pretty sure
50 </I>>>><i> this is defined somewhere, and if not then it should be (see below).
51 </I>>><i>Not the largest type in the structure, the largest *type*.
52 </I>>><i>Most structures will pad to
32 bits on intel machines.
54 </I>>><i>like this:
56 </I>>><i>struct {
57 </I>>><i> int8_t byte;
58 </I>>><i> /* inserts
8 or
24 bits of padding here */
59 </I>>><i> int16_t word;
60 </I>>><i> /* inserts
16 bits of padding here */
61 </I>>><i> int32_t dword;
62 </I>>><i> /* no padding here */
63 </I>>><i>} something;
65 </I>><i> That's missing the point; you put a
32-bit type in there, which of
66 </I>><i>course means it will pad to
32 bits. (And by your argument, it would have
67 </I>><i>to pad to at least the size of a double, not just an int32_t.) What you're
68 </I>><i>saying would be something like:
71 </I>><i> int8_t byte;
72 </I>><i> /*
24 bits of padding */
73 </I>><i> int16_t word;
74 </I>><i> /*
16 bits of padding */
75 </I>><i>} foo; /* size =
64 bits */
77 </I>><i>which is stupid because you have
32 bits of wasted space, when you could
78 </I>><i>just as easily and with no alignment problems (at least on any CPU I know
79 </I>><i>of) have done:
82 </I>><i> int8_t byte;
83 </I>><i> /*
8 bits of padding */
84 </I>><i> int16_t word;
85 </I>><i>} bar; /* size =
32 bits */
87 </I>><i> --Andrew Church
88 </I>><i> <A HREF=
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89 </I>><i> <A HREF=
"http://achurch.org/">http://achurch.org/
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