software/RELEASES/ircservices/achurch.org/services/lists/ircservices-coding/2002/000349.html

   1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
   2 <HTML>
   3  <HEAD>
   4    <TITLE> [IRCServices Coding] GCC3
   5    </TITLE>
   6    <LINK REL="Index" HREF="index.html" >
   7    <LINK REL="made" HREF="mailto:ircservices-coding%40ircservices.za.net?Subject=%5BIRCServices%20Coding%5D%20GCC3&In-Reply-To=3c7b236e.01202%40achurch.org">
   8    <META NAME="robots" CONTENT="index,nofollow">
   9    <META http-equiv="Content-Type" content="text/html; charset=us-ascii">
  10    <LINK REL="Previous"  HREF="000330.html">
  11    <LINK REL="Next"  HREF="000350.html">
  12  </HEAD>
  13  <BODY BGCOLOR="#ffffff">
  14    <H1>[IRCServices Coding] GCC3</H1>
  15     <B>Trevor Talbot</B>
  16     <A HREF="mailto:ircservices-coding%40ircservices.za.net?Subject=%5BIRCServices%20Coding%5D%20GCC3&In-Reply-To=3c7b236e.01202%40achurch.org"
  17        TITLE="[IRCServices Coding] GCC3">quension at softhome.net
  18        </A><BR>
  19     <I>Tue Feb 26 08:09:34 PST 2002</I>
  20     <P><UL>
  21         <LI>Previous message: <A HREF="000330.html">[IRCServices Coding] GCC3
  22 </A></li>
  23         <LI>Next message: <A HREF="000350.html">[IRCServices Coding] GCC3
  24 </A></li>
  25          <LI> <B>Messages sorted by:</B>
  26               <a href="date.html#349">[ date ]</a>
  27               <a href="thread.html#349">[ thread ]</a>
  28               <a href="subject.html#349">[ subject ]</a>
  29               <a href="author.html#349">[ author ]</a>
  30          </LI>
  31        </UL>
  32     <HR>
  33 <!--beginarticle-->
  34 <PRE>On Monday, February 25, 2002, at 09:48  PM, Andrew Church wrote:
  35
  36 &gt;&gt;<i> Reordering is not permitted by the ANSI/ISO C standards.
  37 </I>&gt;<i>
  38 </I>&gt;<i>      That's what I thought, but a whole bunch of people seemed to think
  39 </I>&gt;<i> GCC
  40 </I>&gt;<i> 3.0 was doing just that.
  41 </I>
  42 It occurs to me that this has been talked about as a somewhat-useful
  43 optimization.  The idea is to reorder struct members for optimal padding.
  44 But even if gcc did adopt this as an extension (I don't know if it does
  45 or
  46 not), it would have to be disabled by default, as several things (such as
  47 ANSI offsetof()) would break.
  48
  49 &gt;<i>      Again, that's what I thought--compilers aren't supposed to pad more
  50 </I>&gt;<i> than the largest type in the structure, and between structure members
  51 </I>&gt;<i> only
  52 </I>&gt;<i> enough to align the next member to a multiple of its size.  I'm pretty
  53 </I>&gt;<i> sure
  54 </I>&gt;<i> this is defined somewhere, and if not then it should be (see below).
  55 </I>&gt;<i>
  56 </I>&gt;&gt;<i> Partly for this reason, mapping structs onto arbitrary data in
  57 </I>&gt;&gt;<i> memory results in undefined behavior.
  58 </I>&gt;<i>
  59 </I>&gt;<i>      It shouldn't, and if it did things would break all over the place.
  60 </I>&gt;<i> Suppose you have two compilers, one of which is used to compile a
  61 </I>&gt;<i> program,
  62 </I>&gt;<i> and the other of which is used to compile a library used by the program.
  63 </I>&gt;<i> Now suppose the program passes a pointer to a structure (say, a FILE *)
  64 </I>&gt;<i> to
  65 </I>&gt;<i> the library.  If the two compilers use different algorithms to pad
  66 </I>&gt;<i> structure members, guess what happens?  Boom.
  67 </I>
  68 Actually, I was referring to things such as:
  69
  70      struct tag {
  71          char type;
  72          uint_32 value;
  73      } *s;
  74
  75      char map[32];
  76
  77      /* load map with some data */
  78
  79      s = (struct tag *)map;
  80
  81 But your point about compilers is a valid one; I don't know about
  82 POSIX (as someone else commented), but in general compilers adopt
  83 the same alignment scheme for a particular architecture.  After all,
  84 the architecture is what requires certain alignments anyway :)
  85
  86 -- Quension
  87
  88
  89 </PRE>
  90
  91 <!--endarticle-->
  92     <HR>
  93     <P><UL>
  94         <!--threads-->
  95         <LI>Previous message: <A HREF="000330.html">[IRCServices Coding] GCC3
  96 </A></li>
  97         <LI>Next message: <A HREF="000350.html">[IRCServices Coding] GCC3
  98 </A></li>
  99          <LI> <B>Messages sorted by:</B>
 100               <a href="date.html#349">[ date ]</a>
 101               <a href="thread.html#349">[ thread ]</a>
 102               <a href="subject.html#349">[ subject ]</a>
 103               <a href="author.html#349">[ author ]</a>
 104          </LI>
 105        </UL>
 106
 107 </body></html>