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From: gew@dnlunx.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: Problems with pointers
Message-ID: <265@dnlunx.UUCP>
Date: Tue, 29-Sep-87 05:20:23 EDT
Article-I.D.: dnlunx.265
Posted: Tue Sep 29 05:20:23 1987
Date-Received: Sat, 3-Oct-87 09:24:34 EDT
References: <9519@brl-adm.ARPA>
Organization: Dr Neher Laboratory (PTT)
Lines: 56
Summary: free() problem = not a problem at all


In article <9519@brl-adm.ARPA>, GLASGOW@NOSC-TECR.arpa writes:
) int    *p;
) 
) and by using:
) 
) p = (some_big_structure *)malloc(sizeof(some_big_structure));
) 
) I can get p pointing to a memory block of the correct size for the structre.
) 
) My problem arises when I try to release this memory back to the system. If
) I use free(), will it assume that p points to an integer and release only
) two bytes, or will it know that p is a larger structure and release the
) proper number of bytes.

free() uses size information left behind by malloc() or calloc()
(usually in a few bytes just preceding the memory allocated).
there is no way of deciding from the C code which size the allocated
object is, as you can in Pascal. So the answer is *GO*RIGHT*AHEAD*.
Using an integer pointer probably won't pose any problems, but generic
pointers can be declared like this:

union gptr {
	struct big *big_p;
	struct even_bigger *ebgr_p;
	struct huge *huge_p;
}

If you happen to know the type of object you're freeing, you might
even use

switch(ptrtype){
	case a_big_p:
		free(p.bigp);
		break;
	case an_ebgr_p:
		free(p.ebgr_p);
		break;
	case a_huge_p:
		free(p.huge_p);
		break;
	default:
		panic();
}

None of this will make any difference though on 'normal' computer architectures.

If you don't use the above strategy, use a char * pointer (this is malloc()s 
result type anyway)


-- 
| Ge' Weijers                            |  Disclaimer: the views expressed |
| PTT Dr. Neher Laboratories             |  are usually entirely my own,    |
| Leidschendam, the Netherlands          |  not my employers'. I don't own  |
| uucp:  ...!mcvax!dnlunx!gew            |  a cat to share them with.       |