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From: msb@dciem.UUCP (Mark Brader)
Newsgroups: net.lang.c
Subject: Re: Block Initialization; structs and sizeof
Message-ID: <1971@dciem.UUCP>
Date: Fri, 10-Oct-86 13:56:44 EDT
Article-I.D.: dciem.1971
Posted: Fri Oct 10 13:56:44 1986
Date-Received: Fri, 10-Oct-86 15:14:21 EDT
References: <586@calma.UUCP>
Reply-To: msb@dciem.UUCP (Mark Brader)
Distribution: net
Organization: NTT Systems Inc., c/o DCIEM, Toronto
Lines: 38
Summary: Do scalar structs have a trailing padding?

Suppose that, on your machine, ints are 4 bytes and must be aligned
on multiples of 4 bytes.  Chars, of course, are 1 byte.  Now declare:

	struct str {int i; char c;} arr[2], sca;
	char buf[32];

Now, in arr, each i member must go on a new 4-byte boundary, so there
is 3 bytes of padding after each c member.  So sizeof arr[0] is 8.

The most recent ANSI draft that I've examined specifies that if sizeof is
applied to a structure type, the result is the size of such a structure
including any trailing padding that would be necessary in an array of
such structures.  Therefore sizeof (struct str) is also 8 and, as far as
I know, sizeof sca is also 8.

However, the scalar struct sca is not required to have those 3 bytes
of padding.  This suggests that some compilers may place another
variable, say buf, at ((char *) &sca) + 5.  In this case a construct
such as the "bzero(&p, sizeof(p))" mentioned in Sin-Yaw Wang's article
The problem is that sca, the scalar struct, is NOT required to have
those 3 bytes of padding*.  Thus some compilers may decide to economize
on storage and place another variable, say buf, 5 bytes past the start
of sca.  And then, of course, something like the "bzero(&sca,sizeof sca);"
proposed in Sin-Yaw Wang's article (to which this is a followup) will
overwrite the first 3 elements of buf!

*K&R Appendix A does not speak of trailing padding at all; the most
 recent draft ANSI standard that I've examined does not speak of
 trailing padding in scalar structs.  I mentioned this to Larry Rosler,
 but I have not been able to check a later draft.  Of course, even if
 this is now covered, it doesn't affect current compilers.

ARE there any compilers out that don't always put trailing padding
on structs?  (Note: replies of the form "my compiler does put padding"
should not be posted unless there are a bunch of replies the other way!)

Mark Brader, utzoo!dciem!msb
#define	MSB(type)	(~(((unsigned type)-1)>>1))