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  14    <H1>[IRCServices Coding] GCC3</H1>
  15     <B>Andrew Church</B>
  16     <A HREF="mailto:ircservices-coding%40ircservices.za.net?Subject=%5BIRCServices%20Coding%5D%20GCC3&In-Reply-To="
  17        TITLE="[IRCServices Coding] GCC3">achurch at achurch.org
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  19     <I>Tue Feb 26 14:48:57 PST 2002</I>
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  34 <PRE>&gt;<i>Reordering is not permitted by the ANSI/ISO C standards.
  35 </I>
  36      That's what I thought, but a whole bunch of people seemed to think GCC
  37 3.0 was doing just that.
  38
  39 &gt;&gt;<i> struct foo
  40 </I>&gt;&gt;<i> {
  41 </I>&gt;&gt;<i>     int_16 bar;
  42 </I>&gt;&gt;<i>     int_8 baz;
  43 </I>&gt;&gt;<i> };
  44 </I>&gt;&gt;<i>
  45 </I>&gt;&gt;<i> The compiler would append or prepend (depending on compiler)
  46 </I>&gt;&gt;<i> on an extra 40bits of data to align it in memory on each
  47 </I>&gt;&gt;<i> allocation.
  48 </I>&gt;<i>
  49 </I>&gt;<i>Under 32bit x86, it's likely to add 8 bits of padding to the end
  50 </I>&gt;<i>of the structure.  The alignment is for the width of the largest
  51 </I>&gt;<i>datatype.
  52 </I>
  53      Again, that's what I thought--compilers aren't supposed to pad more
  54 than the largest type in the structure, and between structure members only
  55 enough to align the next member to a multiple of its size.  I'm pretty sure
  56 this is defined somewhere, and if not then it should be (see below).
  57
  58 &gt;<i>Partly for this reason, mapping structs onto arbitrary data in
  59 </I>&gt;<i>memory results in undefined behavior.
  60 </I>
  61      It shouldn't, and if it did things would break all over the place.
  62 Suppose you have two compilers, one of which is used to compile a program,
  63 and the other of which is used to compile a library used by the program.
  64 Now suppose the program passes a pointer to a structure (say, a FILE *) to
  65 the library.  If the two compilers use different algorithms to pad
  66 structure members, guess what happens?  Boom.
  67
  68   --Andrew Church
  69     <A HREF="http://www.ircservices.za.net/mailman/listinfo/ircservices-coding">achurch at achurch.org</A>
  70     <A HREF="http://achurch.org/">http://achurch.org/</A>
  71
  72 </PRE>
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