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From: xsimon@its63b.ed.ac.uk (Simon Brown)
Newsgroups: comp.lang.c
Subject: Re: Arrays of Unknown Length in Structures (and sizeof)
Message-ID: <668@its63b.ed.ac.uk>
Date: Sat, 3-Oct-87 15:45:58 EDT
Article-I.D.: its63b.668
Posted: Sat Oct  3 15:45:58 1987
Date-Received: Wed, 7-Oct-87 06:22:34 EDT
References: <243@mit-prep.ARPA> <1044@ius1.cs.cmu.edu>
Reply-To: xsimon@its63b.ed.ac.uk (Simon Brown)
Organization: Computer Science Department, Edinburgh University
Lines: 55

In article <1044@ius1.cs.cmu.edu> edw@ius1.cs.cmu.edu (Eddie Wyatt) writes:
>> 	struct  STRUCT_FOO
>> 	{
>> 	  int  one;
>> 	  int  two;
>> 	  int  open[];
>> 	};
>> 
>> 	  foo_p = malloc(sizeof (struct STRUCT_FOO) + (7 * sizeof (int)));
>> 
>> 	  for (i = 0; i < 7; ++i)
>> 	   foo_p->open[i] = i * 100;
>> 
>> As far as I'm concerned, if this isn't kosher, it should be!
>
>   Let me give you a reason of why this should not be "kosher".
>  	    I don't think it is  required in C that the fields in a structure
>	    be phyically ordered in the same ordering they are declared.
>	    Hence any of the 6 permutations of the fields you describe
>	    are valid for phyical offset orderings (ie open could have
>	    an offset of 0, two could have an offset of 0 and one could
>	    have an offset of 0 + sizeof(int)).  This would wreak havoc!!
>

Umm, but what about the System V "msgbuf" type, used for the msg IPC system 
calls? This has type
	struct msgbuf {
		long mtype;
		char mtext[1];
	}
and is intended to be used as a template type, by malloc()'ing space of whatever
message buffer size is required, with something like
	msg = (struct msgbuf *)malloc(sizeof(struct msgbuf) + NBYTES);
which results in a message with bytes mtext[0] ... mtext[NBYTES].

If a compiler is free to alter the order of occurence of the fields of a
structure, then this is no longer portable. 
(Yes, I realize that this has nothing to do with the C language as such, but 
it can't be just ignored!).

(BTW, certainly I'd much prefer something like "char mtext[]" or even 
"char mtext[0]", but it seems that zero-sized types are not too popular with
compilers, more's the pity :-().

    %{
       Simon.
    %}
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