Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!usc!ucsd!ucbvax!SLIC.CELLBIO.DUKE.EDU!jit
From: jit@SLIC.CELLBIO.DUKE.EDU (Jit Keong Tan)
Newsgroups: comp.sys.sgi
Subject: (none)
Message-ID: <9011200509.AA03725@slic.cellbio.duke.edu>
Date: 20 Nov 90 05:09:25 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 73

Subject: Re:  cc compiler warning question.


#include <stdio.h>

typedef struct { int a;        int b; } TINY;
typedef struct { TINY two;     int c; } SMALL;
typedef struct { SMALL *one; int f; } BIG;
/*			^^^ corrected, I like to think 
in terms of linear address space instead of two-d array.
Besides *[] is an array of pointers. Since the size of the array
is not known yet, the structure size is unknown ---> it is illegal,
C compiler needs to know this. 
Whereas SMALL **one is the same size as SMALL *one; they only
differ in interpretation. You can declare it as SMALL **one but
I am quite sure this is not you want.
*/


typedef struct { SMALL **one; int f; } TBIG;


BIG *zero;
SMALL *sptr;

main()
{

	printf("sizeof(TBIG) %u bytes == sizeof(BIG) %u bytes\n",
		sizeof(TBIG),sizeof(BIG));
/*
>>>>{       zero = (BIG *) malloc( (sizeof(BIG) + sizeof(SMALL) * 2) );
>>>>		conceptually wrong, this is not you want 
*/
	if ( (zero = (BIG *) malloc(sizeof(BIG))) == NULL) {
		printf("MALLOC error\n");	/* Not enough memory */
		exit(1);
	}
	if (( sptr = (SMALL *) malloc(2*sizeof(SMALL))) == NULL){
		printf("Malloc SMALL: out of memory\n");
		exit(1);
	}
	zero->one = sptr;

	zero->f = 1;
	zero->one->c = 2;	/* No subscript is equivalent to [0] */
/* or zero->one[0].c = 2 if you like */

	(sptr+1)->c = 4;	/* Accessing the second element, C compiler
				will take care of the size increment. */


       printf("--- %d\n",zero->f);
       printf("--- %d %d\n",sptr->c,(sptr+1)->c);
	printf(" %d == %d == %d; %d == %d\n",
		sptr->c,  zero->one->c,	(*zero->one).c, 

	/* Not zero->one.c because zero->one is a pointer and
	you want to get the content of the pointer.
	*/
		(sptr+1)->c, zero->one[1].c);	/* should be equal */

	exit(0);
}
--------------------------------------------------------
Jit Keong Tan     | internet: jit@slic.cellbio.duke.edu
(919) 684-8098    | bitnet  : tan00001@dukemc.bitnet
--------------------------------------------------------
U.S. Mail:
Duke University Medical Center
Department Of Cell Biology
Box 3709
Durham, NC 27710