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From: mike@BRL.ARPA (Mike Muuss)
Newsgroups: comp.unix.wizards
Subject: Re:  Problems with pointers
Message-ID: <9560@brl-adm.ARPA>
Date: Thu, 1-Oct-87 08:28:31 EDT
Article-I.D.: brl-adm.9560
Posted: Thu Oct  1 08:28:31 1987
Date-Received: Mon, 5-Oct-87 02:07:48 EDT
Sender: news@brl-adm.ARPA
Lines: 22

malloc(3) puts a small "header" in memory immediately before the
address of the pointer that it returns to you.  In there, it stores
the size of the block.  Thus, just handing the pointer back to free(3)
has the proper effect.

Here is some code that I often use to do this sort of thing:

/* Acquire storage for a given struct, eg, GETSTRUCT(ptr,structname); */
#define GETSTRUCT(p,str) \
	if( (p = (struct str *)malloc(sizeof(struct str)) == \
	    (struct str *)0 ) {\
		fprintf(stderr,"malloc str failure\n"); \
		exit(17); } \
	bzero( (char *)p, sizeof(struct str));

Memory acquired in this way is then released just by saying:

	free( (char *) p );

The cast is desirable as free(3) expects a char * argument.
	Best,
	 -Mike