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14 <H1>[IRCServices Coding] GCC3
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17 TITLE=
"[IRCServices Coding] GCC3">griever at t2n.org
19 <I>Tue Feb
26 12:
01:
21 PST
2002</I>
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34 <PRE>On Tue,
26 Feb
2002, Trevor Talbot wrote:
36 ><i> On Monday, February
25,
2002, at
09:
48 PM, Andrew Church wrote:
38 </I>><i> >> Reordering is not permitted by the ANSI/ISO C standards.
40 </I>><i> > That's what I thought, but a whole bunch of people seemed to think
42 </I>><i> > 3.0 was doing just that.
44 </I>><i> It occurs to me that this has been talked about as a somewhat-useful
45 </I>><i> optimization. The idea is to reorder struct members for optimal padding.
46 </I>><i> But even if gcc did adopt this as an extension (I don't know if it does
48 </I>><i> not), it would have to be disabled by default, as several things (such as
49 </I>><i> ANSI offsetof()) would break.
51 </I>><i> > Again, that's what I thought--compilers aren't supposed to pad more
52 </I>><i> > than the largest type in the structure, and between structure members
54 </I>><i> > enough to align the next member to a multiple of its size. I'm pretty
56 </I>><i> > this is defined somewhere, and if not then it should be (see below).
58 </I>><i> >> Partly for this reason, mapping structs onto arbitrary data in
59 </I>><i> >> memory results in undefined behavior.
61 </I>><i> > It shouldn't, and if it did things would break all over the place.
62 </I>><i> > Suppose you have two compilers, one of which is used to compile a
63 </I>><i> > program,
64 </I>><i> > and the other of which is used to compile a library used by the program.
65 </I>><i> > Now suppose the program passes a pointer to a structure (say, a FILE *)
67 </I>><i> > the library. If the two compilers use different algorithms to pad
68 </I>><i> > structure members, guess what happens? Boom.
70 </I>><i> Actually, I was referring to things such as:
72 </I>><i> struct tag {
73 </I>><i> char type;
74 </I>><i> uint_32 value;
77 </I>><i> char map[
32];
78 </I>should be unsigned char.
80 </I>><i> /* load map with some data */
82 </I>><i> s = (struct tag *)map;
84 </I>><i> But your point about compilers is a valid one; I don't know about
85 </I>><i> POSIX (as someone else commented), but in general compilers adopt
86 </I>><i> the same alignment scheme for a particular architecture. After all,
87 </I>><i> the architecture is what requires certain alignments anyway :)
89 </I>><i> -- Quension
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